Book of abstracts, "Jaszowiec" 2011 - Instytut Fizyki PAN

Book of abstracts, "Jaszowiec" 2011 - Instytut Fizyki PAN

Book of abstracts, "Jaszowiec" 2011 - Instytut Fizyki PAN


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This work relates to Department <strong>of</strong> the Navy Grant N62909-11-1-1076 issued by Office <strong>of</strong><br />

Naval Research Global. The United States Government has a royalty-free license throughout<br />

the world in all copyrightable material contained herein.<br />

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Skład własny (Ł. Cywi�ski, podzi�kowania dla T. Słupi�skiego)<br />

Druk i oprawa: Drukarnia Zalesie, 05-501 Piaseczno, ul. Norwida 10<br />


The 40 th “Jaszowiec” International School and Conference on the Physics <strong>of</strong><br />

Semiconductors continues the tradition <strong>of</strong> annual meetings <strong>of</strong> the international and Polish<br />

semiconductor communities. For the third time the event will be held in the mountain resort<br />

Krynica-Zdrój on June 25 th – July 1 st , <strong>2011</strong>. The first thirty seven meetings took place in the<br />

village Jaszowiec in another part <strong>of</strong> Polish mountains. For years, the name “Jaszowiec” has<br />

become the symbolic trademark <strong>of</strong> the conference and, therefore, it is still kept.<br />

The scope <strong>of</strong> the conference covers essentially all research fields in semiconductor<br />

physics, including technology, experiment, theory and modeling. During the present<br />

conference particular attention will be paid to the physics <strong>of</strong> semiconductor devices, and a<br />

special day-long symposium “Physics and modeling <strong>of</strong> devices” has been organized.<br />

The Conference is preceded by the School (tutorial session), addressed mainly to<br />

undergraduate and graduate students, and young scientists. The School will be held on June<br />

25 th -26 th . During the School outstanding scientists will give four lectures providing an<br />

introduction to important fields in semiconductor physics. The scientific program <strong>of</strong> the<br />

school and conference consists <strong>of</strong> 25 invited lectures, and 194 contributed papers (18 chosen<br />

for oral presentation and 176 posters). The School and Conference will be attended by<br />

approximately 250 participants.<br />

This booklet contains the detailed program and <strong>abstracts</strong> <strong>of</strong> all presentations that will<br />

be given during the event. The invited and contributed papers will be published in a special<br />

volume <strong>of</strong> Acta Physica Polonica A.<br />

The International School and Conference is organized by two institutes <strong>of</strong> the Polish<br />

Academy <strong>of</strong> Sciences: Institute <strong>of</strong> Physics and Institute <strong>of</strong> High Pressure Physics; two<br />

institutes <strong>of</strong> the Faculty <strong>of</strong> Physics, University <strong>of</strong> Warsaw: Institute <strong>of</strong> Experimental Physics<br />

and Institute <strong>of</strong> Theoretical Physics; Foundation “Pro Physica”, and Committee on Physics <strong>of</strong><br />

the Polish Academy <strong>of</strong> Sciences. The financial support <strong>of</strong> the organizing institutions as well<br />

as <strong>of</strong> the U.S. Army Forward Element Command-Atlantic, Research Division (USARFEC-A)<br />

and the U.S. Navy Office <strong>of</strong> Naval Research Global is gratefully acknowledged.<br />

Current information can be found on the conference web page:<br />

www.ifpan.edu.pl/jaszowiec .<br />

I would like to thank the members <strong>of</strong> the International Advisory Committee and the<br />

Program Committee for their contribution to the scientific program <strong>of</strong> the School and<br />

Conference. I would like also to express my gratitude to Łukasz Cywi�ski, the Secretary <strong>of</strong><br />

the Conference, to Jacek Szczytko, the Chairman <strong>of</strong> the School, as well as other members <strong>of</strong><br />

the Organizing Committee, Agnieszka J�drzejewska and Maciek Zaj�czkowski for their<br />

enthusiasm and excellent work.<br />

I wish all the participants <strong>of</strong> the meeting scientifically inspiring days and joyful social<br />

life. Have a pleasant stay in Krynica.<br />

Chairman <strong>of</strong> the Conference<br />

Czesław Skierbiszewski

40th _______________________________________________________________<br />

"Jaszowiec" <strong>2011</strong> International School and Conference on the Physics <strong>of</strong> Semiconductors<br />

Saturday, June 25 th , <strong>2011</strong><br />

9:20 -- 9:30 Jacek Szczytko – School opening address<br />


9:30 – 12:30 Detlef Hommel (University <strong>of</strong> Bremen, Germany)<br />

Modern technologies for semiconductor epitaxy and nano-processing<br />

15:00 – 18:00 Aldo Di Carlo (University <strong>of</strong> Rome “Tor Vergata”, Italy)<br />

Physical simulation <strong>of</strong> electronic devices<br />

19:00 Barbecue (on the terrace <strong>of</strong> the ‘Geovita’ hotel)<br />

Sunday, June 26 th , <strong>2011</strong><br />


9:30 – 12:30 Gerhard Abstreiter (Walter Schottky Institut and Institute for Advanced<br />

Study TU München, Germany)<br />

Physics and Technology <strong>of</strong> Arsenide Based Quantum Dots and Nanowires<br />

15:00 – 18:00 Debdeep Jena (University <strong>of</strong> Notre Dame, USA)<br />

Tutorial: Engineering <strong>of</strong> Electric Fields in Nitride-Based Semiconductors<br />

20:30 – 21:30 Concert – chamber music performed by young artists,<br />

laureates <strong>of</strong> international music competitions<br />

Angelika Kumi�ga (violin)<br />

Agata B�k (cello)<br />

Michał Kubarski (accordion)<br />

21:30 Welcoming glass <strong>of</strong> wine<br />


_______________________________________________________________<br />

40th "Jaszowiec" <strong>2011</strong> International School and Conference on the Physics <strong>of</strong> Semiconductors<br />

Monday, June 27 th , <strong>2011</strong><br />

8:50 -- 9:00 Czesław Skierbiszewski – Conference opening address<br />

INVITED TALKS (MoI1 … MoI4)<br />

9:00 – 10:00 Joerg Wrachtrup (University <strong>of</strong> Stuttgart, Germany)<br />

Coherent optical control <strong>of</strong> the NV center in diamond<br />

10:00 – 10:10 Break<br />

10:10 – 11:10 Tomasz Dietl (Institute <strong>of</strong> Physics, Polish Academy <strong>of</strong> Sciences)<br />

Understanding and exploiting magnetism <strong>of</strong> semiconductors<br />

11:10 – 11:40 C<strong>of</strong>fee break<br />

11:40 – 12:40 Łukasz Kłopotowski (Institute <strong>of</strong> Physics, Polish Academy <strong>of</strong> Sciences)<br />

Charging Effects in Self Assembled CdTe Quantum Dots<br />

12:40 – 12:50 Break<br />

12:50 – 13:50 Grzegorz S�k (Wrocław University <strong>of</strong> Technology, Poland)<br />

Optical properties <strong>of</strong> strongly in-plane asymmetric epitaxial<br />

nanostructures<br />


19:15 – 19:30 P. Utko, R. Ferone, I.V. Krive, R.I. Shekhter, M. Jonson, M. Monthioux,<br />

L. Noe, J. Nygård<br />

Coupling between electronic and vibrational excitations in carbon<br />

nanotubes filled with C60 fullerenes<br />

19:30 – 19:45 A. Urba�czyk, F.W M. van Otten, R. Nötzel<br />

Solid-state self assembly: a route to hybrid metal-semiconductor epitaxial<br />

nanostructures<br />

19:45 – 20:00 M. Galicka, P. Kacman, R. Buczko<br />

First-Principles Study <strong>of</strong> Doped III–V nanowires<br />

20:00 – 20:10 Break<br />

20:10 – 20:25 F. Schuster, P. Kopyt, P. Lukasik, W. Gwarek, D. Coquillat, F. Teppe,<br />

B. Giffard, W. Knap<br />

Terahertz Detectors Based on Low Cost 130 nm Silicon Field Effect<br />

Transistors<br />

20:25 – 20:40 P. Wojnar, E. Janik, S. Kret, A. Petrouchik, M. Goryca, T. Kazimierczuk,<br />

P. Kossacki, G. Karczewski, T. Wojtowicz<br />

Growth <strong>of</strong> optically active CdTe quantum dots in ZnTe nanowires<br />

20:40 – 20:55 J.M. Schneider, B.A. Piot, I. Sheikin, G.A. Goncharuk, P. Vasek,<br />

P. Svoboda, Z. Vyborny, L. Smrcka, M. Orlita, M. Potemski, D.K. Maude<br />

Electronic properties <strong>of</strong> graphite<br />

21:05 – 23:00 MONDAY POSTER SESSION (MoP1 … MoP60)<br />


40th _______________________________________________________________<br />

"Jaszowiec" <strong>2011</strong> International School and Conference on the Physics <strong>of</strong> Semiconductors<br />

Tuesday, June 28 th , <strong>2011</strong><br />

INVITED TALKS (TuI1 … TuI4)<br />

9:00 – 10:00�������Alex Greilich���(Technische Universität Dortmund, Germany)<br />

Optical control <strong>of</strong> one and two spins in interacting quantum dots<br />

10:00 – 10:10 Break<br />

10:10 – 11:10 Shaffique Adam (National Institute <strong>of</strong> Standards and Technology, USA)<br />

Graphene Transport Properties<br />

11:10 – 11:40 C<strong>of</strong>fee Break<br />

11:40 – 12:40 Arkadiusz Wójs (Wrocław University <strong>of</strong> Technology, Poland)<br />

Theoretical search for non-Abelian statistics in fractional quantum Hall<br />

states<br />

12:40 – 12:50 Break<br />

12:50 – 13:50 Dmitry Krizhanovskii (University <strong>of</strong> Sheffield, UK)<br />

Polariton condensation in dynamic acoustic lattices<br />


15:30 – 15:45 K. Nogajewski, K. Karpierz, M. Grynberg, W. Knap, R. Gaska, J. Yang,<br />

M.S. Shur, J. Łusakowski<br />

Resonant Terahertz Absorption by Magnetoplasmons in Grating-Gate<br />

GaN/AlGaN-based Field-Effect Transistors<br />

15:45 – 16:00 K. Milowska, M. Birowska, J.A. Majewski<br />

Structural and electronic properties <strong>of</strong> functionalized graphene<br />

16:00 – 16:15 O. Proselkov, W. Stefanowicz, S. Dobkowska, J. Sadowski, T. Dietl,<br />

M. Sawicki<br />

Analysis <strong>of</strong> the magnetic anisotropy in ultrathin GaMnAs<br />

16:15 – 16:25 Break<br />

16:25 – 16:40 M. Goryca, P. Plochocka, P. Wojnar, T. Kazimierczuk, M. Potemski,<br />

P. Kossacki<br />

Spin-lattice relaxation <strong>of</strong> a single Mn 2+ ion in a CdTe quantum dot<br />

16:40 – 16:55 T. Kazimierczuk, M. Goryca, P. Wojnar, A. Golnik, P. Kossacki<br />

Magnetophotoluminescence <strong>of</strong> CdTe/ZnTe Quantum Dots: G-factor and<br />

Diamagnetic Shift Variation in a Single Dot<br />

16:55 – 17:10 T. Jakubczyk, W. Pacuski, A. Golnik, C. Kruse, D. Hommel, H. Hilmer,<br />

R. Schmidt-Grund, J.A. Gaj<br />

Control over CdTe Quantum Dots Emission using ZnTe-based Micropillar<br />

Cavities<br />

17:30 – 19:15 TUESDAY POSTER SESSION (TuP1 … TuP58)<br />

20:00 Conference Banquet<br />


_______________________________________________________________<br />

40th "Jaszowiec" <strong>2011</strong> International School and Conference on the Physics <strong>of</strong> Semiconductors<br />

Wednesday, June 29 th , <strong>2011</strong><br />

INVITED TALKS (WeI1 … WeI4)<br />

9:00 – 10:00 Tomas Jungwirth (Institute <strong>of</strong> Physics, Academy <strong>of</strong> Sciences <strong>of</strong> the Czech<br />

Republic)<br />

Spintronics with antiferromagnetic materials<br />

10:00 – 10:10 Break<br />

10:10 – 11:10 Ewelina M. Hankiewicz (Universitaet Wuerzburg, Germany)<br />

Transport in topological insulators<br />

11:10 – 11:40 C<strong>of</strong>fee Break<br />

11:40 – 12:40 James Speck (University <strong>of</strong> California, Santa Barbara)<br />

Progress in Nonpolar and Semipolar GaN Materials and Devices<br />

12:40 – 12:50 Break<br />

12:50 – 13:50 Yasushi Nanishi (Ritsumeikan University, Japan, and Seoul National<br />

University, Korea)<br />

Recent Progress <strong>of</strong> DERI process for growth <strong>of</strong> InN and Related Alloys<br />


19:15 – 19:30 N. Gonzalez Szwacki, J. A. Majewski, T. Dietl<br />

Properties <strong>of</strong> TM pairs in the bulk and at the surface <strong>of</strong> GaN with and<br />

without Si or Mg codoping<br />

19:30 – 19:45 A. Bonanni, W. Stefanowicz, M. Sawicki, T. Devillers, B. Faina, T. Li,<br />

T.E. Winkler, D. Sztenkiel, A. Navarro-Quezada, M. Rovezzi, R. Jakieła,<br />

A. Meingast, G. Kothleitner, T. Dietl<br />

Experimental Probing <strong>of</strong> Exchange Interactions Between Localized Spins<br />

in a Dilute Magnetic Insulator (Ga,Mn)N<br />

19:45 – 20:00 M. Birowska, C. �liwa, K. Milowska, J.A. Majewski, T. Dietl<br />

Origin <strong>of</strong> uniaxial magnetic anisotropy in (Ga,Mn)As<br />

20:00 – 20:10 Break<br />

20:10 – 20:25 E. Calleja, A. Bengoechea-Encabo, S. Albert, M.A. Sanchez-García,<br />

F. Barbagini, E. Luna, A. Trampert, U. Jahn, P. Lefebvre<br />

Understanding the Selective Area Nucleation and Growth <strong>of</strong> GaN<br />

20:25 – 21:40 S.P. Łepkowski, I. Gorczyca<br />

Influence <strong>of</strong> composition and atomic arrangement on elastic properties <strong>of</strong><br />

wurtzite InGaN and InAlN alloys<br />

20:40 – 20:55 H. Turski, M. Siekacz, M. Sawicka, G. Cywi�ski, C. Cheze, S. Grzanka,<br />

Z. Wasilewski, P. Perlin, G. Muzioł, J. Pawłowska, I. Grzegory,<br />

C. Skierbiszewski<br />

InGaN laser diodes operating at 450-460 nm grown by RF-Plasma MBE<br />

21:05 – 23:00 WEDNESDAY POSTER SESSION (WeP1 … WeP58)<br />


40th _______________________________________________________________<br />

"Jaszowiec" <strong>2011</strong> International School and Conference on the Physics <strong>of</strong> Semiconductors<br />

Thursday, June 30 th , 2010<br />


"Physics and modeling <strong>of</strong> devices "<br />

INVITED TALKS (ThI1 …ThI9)<br />

9:00 – 9:30 Maciej Bugajski (Institute <strong>of</strong> Elektron Technology, Warsaw, Poland)<br />

Quantum Cascade Lasers<br />

9:30 – 10:00 Wladek Walukiewicz (Lawrence Berkeley National Laboratory,USA)<br />

New Concepts and Materials for Solar Power Conversion Devices<br />

10:00 – 10:10 Break<br />

10:10 – 10:40 Danek Elbaum (Institute <strong>of</strong> Physics, Polish Academy <strong>of</strong> Sciences)<br />

ZnO biosensing<br />

10:40 – 11:10 Katarzyna Holc (Institute <strong>of</strong> High Pressure Physics, Warsaw, Poland)<br />

Nitride laser diodes<br />

11:10 – 11:30 C<strong>of</strong>fee break<br />

11:30 – 12:00 Detlef Hommel (University <strong>of</strong> Bremen, Germany)<br />

Application <strong>of</strong> CdSe quantum dots for single photon emitters at room<br />

temperature<br />

12:00 – 12:30 Wojciech Knap (University <strong>of</strong> Montpellier, France)<br />

THz emitters based on GaN/AlGaN HEMTs<br />

12:30 -- 13:00 Tomasz Stobiecki (AGH University <strong>of</strong> Science and Technology, Kraków,<br />

Poland)<br />

Spin transfer torque in TMR and GMR nanostructures for spintronic<br />

devices<br />

15:00 – 15:30 Jan Dziuban (Wrocław University <strong>of</strong> Technology, Poland)<br />

Si-based MEMS devices<br />

15:30 – 16:00 Zbigniew Ku�nicki (Ecole Nationale Superiere de Physique, Illkirch-<br />

Graffenstaden)<br />

Giant photoconversion on silicon drived materials<br />

16:00 Czesław Skierbiszewski – Closing address<br />

19:00 Farewell dinner<br />


_______________________________________________________________<br />

40th "Jaszowiec" <strong>2011</strong> International School and Conference on the Physics <strong>of</strong> Semiconductors<br />


1. K.A. Kolwas, G. Grabecki, S. Trushkin, Ł. Cywi�ski, M. Aleszkiewicz, T. Dietl, G.<br />

Springholz, G. Bauer<br />

Nonlocal transport in PbTe/PbEuTe microstructures<br />

2. M. Szot, L. Kowalczyk, E. Smajek, B. Taliashvili, P. Dziawa, W. Kn<strong>of</strong>f, A. Reszka, V.<br />

Domukhovski, E. Łusakowska, P. Dłu�ewski, M. Wiater, T. Wojtowicz, T. Story, M.<br />

Bukała, R. Buczko, P. Kacman<br />

Optical transitions in PbTe/CdTe quantum wells grown by molecular beam epitaxy on<br />

GaAs (001) and BaF2 (111) substrates<br />

3. A. Woło�, A. Drabinska, M. Kaminska, G. Strzelecka, A. Hruban, A. Materna, M. Piersa,<br />

Z. Wilamowski<br />

Properties <strong>of</strong> three-dimensional topological insulators studied by microwave resonance<br />

spectroscopy<br />

4. A. Duzynska, A. Kaminska, H. Teisseyre, E. Przezdziecka, D. Dobosz, Z.R. Zytkiewicz,<br />

A. Kozanecki, J.D. Fidelus, A. Durygin, V. Drozd, R. Hrubiak, A. Suchocki<br />

Analysis <strong>of</strong> Optical Properties and Pressure Dependence <strong>of</strong> the Energy Gap <strong>of</strong> ZnO<br />

Layers, Bulk and Nano-Powders<br />

5. A. Radzvilavicius, E. Anisimovas<br />

Defect structure <strong>of</strong> two-dimensional Wigner crystals<br />

6. A. Korbecka, J.A. Majewski<br />

Gauge invariant computational scheme for heterostructures in magnetic field<br />

7. M. Bukała, P. Sankowski, R. Buczko, P. Kacman<br />

Modeling PbTe-based low dimensional structures<br />

8. A. Hruban, A. Materna, W. Dalecki, G. Strzelecka, M. Piersa, E. Jurkiewicz-Wegner, R.<br />

Diduszko, M. Romaniec, W. Orłowski<br />

Topological insulators – materials for fundamental researches and perspective<br />

applications<br />

9. M. Chwastyk, P.T. Ró�a�ski, M. Zieli�ski<br />

Atomistic calculation <strong>of</strong> screened Coulomb interactions in semiconductor<br />

nanostructures<br />

10. M. Birowska, K. Milowska, J.A. Majewski<br />

Van der Waals Density Functionals in Materials Science<br />

11. T. Gro�, E. Malicka, A.W. Pacyna, H. Duda, J. Krok-Kowalski<br />

Hopkinson-Like Effect in Single-Crystalline CdCr2Se4 Semiconductor<br />

12. Ł. Wachnicki, B.S. Witkowski, S. Gierałtowska, E. Janik, M. Godlewski, E. Guziewicz<br />

Optical and structural characterization <strong>of</strong> zinc oxide nanostructures obtained by<br />

Atomic Layer Deposition method<br />


40th _______________________________________________________________<br />

"Jaszowiec" <strong>2011</strong> International School and Conference on the Physics <strong>of</strong> Semiconductors<br />

13. V.V. Khomyak, M.I. Ilaschuk, O.A. Parfenyuk, V.V. Brus, Z.D. Kovalyuk<br />

Electrical properties <strong>of</strong> surface-barrier structures n-Zn1-xCdxO/ p-CdTe<br />

14. E.A. Wolska, D. Sibera, B.S. Witkowski, S.A. Yatsunenko, I. Pełech, U. Narkiewicz, M.<br />

Godlewski<br />

Photoluminescence and chromaticity properties <strong>of</strong> ZnO nanopowders obtained by a<br />

microwave solwothermal method<br />

15. T. Š�epka, D. Gregušová, R. Kúdela, Š. Gaži, V. Cambel<br />

Technology and testing the mechanical properties <strong>of</strong> the InGaP/GaAs/InGaP<br />

microcantilevers<br />

16. M.A. Borysiewicz, E. Dynowska, V. Kolkovsky, J. Dyczewski, E. Kami�ska, A.<br />

Piotrowska<br />

ZnO thin films <strong>of</strong> different crystalline structures grown on Si (100) substrates by<br />

reactive DC sputter deposition<br />

17. W. Kn<strong>of</strong>f, M.A. Pietrzyk, B.A. Orłowski, B. Taliashvili, T. Story, R.L. Johnson<br />

Comparison <strong>of</strong> the valence band <strong>of</strong> amorphous and crystalline GeTe and (Ge,Mn)Te<br />

layers<br />

18. A. Pietnoczka, R. Bacewicz, T. Słupi�ski, S.H. Wei, M. Jie, J. Antonowicz, T. Drobiazg<br />

Local Structure Study <strong>of</strong> GaAs:Te Using Te K-edge X-ray Absorption Fine Structure<br />

19. A. Taube, K. Korwin-Mikke, R. Mroczy�ski, S. Gierałtowska, A. Łaszcz, I. Pasternak,<br />

M. Sochacki, M. Zychowska, J. Dyczewski, M. Zdrojek, E. Dynowska, A. Piotrowska<br />

Thermal Stability <strong>of</strong> HfO2/SiO2 Gate Stack on Silicon Substrate<br />

20. M. Iwi�ska, D. Pier�ci�ska, K. Pier�ci�ski, A. Szerling, P. Karbownik, M. Bugajski<br />

Investigation <strong>of</strong> thermal properties <strong>of</strong> AlGaAs/GaAs Quantum Cascade Lasers by<br />

thermoreflectance spectroscopy<br />

21. P. Łach, M.G. Brik, I. Sildos, A. Kami�ska, A. Suchocki<br />

Luminescence properties <strong>of</strong> Sm 3+ in different phases <strong>of</strong> TiO2<br />

22. L.A. Karachevtseva, S.Ya. Kuchmii, O.A. Lytvynenko, F.F. Sizov, O.J. Stronska, A.L.<br />

Stroyuk<br />

Investigation <strong>of</strong> electro-optical properties <strong>of</strong> 2D macroporous silicon<br />

23. G.V. Lashkarev, A.M. Yaremko, V.A. Karpyna<br />

Investigation <strong>of</strong> multi-phonon excitations in ZnO textured crystalline films by Raman<br />

spectroscopy<br />

24. M.L. Peres, V.A. Chitta, D.K. Maude, N.F. Oliveira Jr., P.H. Rappl, A.Y. Ueta, E.<br />

Abram<strong>of</strong><br />

Rashba effect in n-type PbTe/Pb1-xEuxTe Quantum Wells<br />


_______________________________________________________________<br />

40th "Jaszowiec" <strong>2011</strong> International School and Conference on the Physics <strong>of</strong> Semiconductors<br />

25. I.I. Shtepliuk, G.V. Lashkarev, O.Yu. Khyzhun, V.V. Khomyak, V.I. Lazorenko, I.I.<br />

Tim<strong>of</strong>eeva, L.A. Klochkov, B. Kowalski, A. Reszka<br />

Enhancement the intensity <strong>of</strong> ultraviolet luminescence <strong>of</strong> ZnO film upon doping<br />

isovalent impurity <strong>of</strong> cadmium<br />

26. V.V. Brus, Z.D. Kovalyuk, P.D. Maryanchuk<br />

Optical Properties <strong>of</strong> TiO2 – MnO2 Thin Films Prepared by the Electron-Beam<br />

Evaporation Technique<br />

27. S. Gierałtowska, Ł. Wachnicki, B.S. Witkowski, T.A. Krajewski, M. Godlewski, E.<br />

Guziewicz<br />

Properties <strong>of</strong> high-k oxides grown by Atomic Layer Deposition method for transparent<br />

electronics<br />

28. E. Maci��ek, T. Gro�, A.W. Pacyna, T. Mydlarz, J. Krok-Kowalski<br />

High Spin-Low Spin Transitions in Cu0.2Co0.76Cr1.83Se4 Semiconductor<br />

29. D. Shevchenko, J. Mickevi�ius, G. Tamulaitis, N. Starzhinskiy, K. Katrunov, V.<br />

Ryzhikov<br />

Photoluminescence Study <strong>of</strong> ZnSe Scintillating Crystals Doped with Isovalent<br />

Tellurium and Oxygen<br />

30. K. Olender, T. Wosi�ski, A. M�kosa, P. Dłu�ewski, V. Kolkovsky, G. Karczewski<br />

Native Deep-Level Defects in MBE-Grown p-Type CdTe<br />

31. P. Urbanowicz, E. Tomaszewicz, T. Gro�, H. Duda, Z. Kukuła<br />

Residual Paramagnetism at High-Temperature CuEu2W2O10 and Cu3Eu2W4O18<br />

Semiconductors<br />

32. M.V. Radchenko, G.V. Lashakrev, M.E. Bugaiova, V.I. Sichkovskyi, V.I. Lazorenko,<br />

L.A. Krushynskaya, Y.A. Stelmakh, W. Kn<strong>of</strong>f, T. Story<br />

Ferromagnetic nanocomposite Co-Al2O3 as a spintronic material with engineered<br />

magnetic properties<br />

33. D.M. Bercha, K.E. Glukhov, M. Sznajder, S.A. Bercha<br />

The role <strong>of</strong> electron subsystem in the phase transition process in a layered CuInP2S6<br />

crystal<br />

34. A..I. Dmitriev<br />

Van der Waals surface <strong>of</strong> InSe as a standard nanorelief in the metrology <strong>of</strong><br />

nanoobjects<br />

35. A. Kaminska, C.G. Ma, M.G. Brik, A. Kozanecki, M. Bo�kowski, E. Alves, A. Suchocki<br />

Crystal field analysis <strong>of</strong> the Yb 3+ energy level scheme in III-V semiconductors<br />

36. J. Weszka, M. Doma�ski, J. Jurusik, B. Hajduk, M. Chwastek, H. Bednarski<br />

Studies <strong>of</strong> Electric Transport Properties <strong>of</strong> Single Layer Devices Based on the<br />

Polyazomethine Thin Films<br />


40th _______________________________________________________________<br />

"Jaszowiec" <strong>2011</strong> International School and Conference on the Physics <strong>of</strong> Semiconductors<br />

37. R. Jarimavi�iu-te.-Žvalioniene., I. Prosy�evas, Ž. Kaminskiene., S. Lapinskas<br />

Optical properties <strong>of</strong> black silicon with precipitated silver and gold nanoparticles<br />

38. I.P. Koziarskyi, P.D. Marianchuk, E.V. Maistruk, D.P. Koziarskyi<br />

Optical Properties <strong>of</strong> Crystals (3HgSe)0.5(In2Se3)0.5, (3HgS)0.5(In2S3)0.5,<br />

(3HgS)0.5(Al2S3)0.5, (3HgSe)0.5(Al2Se3)0.5, Doped with Mn or Fe<br />

39. N.S. Yurtsenyuk<br />

Self-Compensation Mechanism in Semi-Insulating CdMnTe:Sn Crystals Intended for<br />

X/�-Ray Detectors<br />

40. J. Krok-Kowalski, G. Władarz, T. Gro�, H. Duda, A.W. Pacyna, K. Nikiforov, P. Rduch<br />

Influence <strong>of</strong> Cu, Ga and Au Dopants and Technology Conditions on the Magnetic<br />

Interactions in HgCr2Se4 Single Crystals<br />

41. A.I. Ievtushenko, G.V. Lashkarev, V.I. Lazorenko, Z.J. Horvath, M.G. Dusheyko<br />

The Photodetectors with Vertical Integration Based on ZnO Films<br />

42. A.I. Ievtushenko, G.V. Lashkarev, V.I. Lazorenko, O.Y. Khyzhun, L.O. Klochkov, O.I.<br />

Bykov, V.M. Tkach, V.A. Baturin, A.Y. Karpenko<br />

Features <strong>of</strong> the Properties for Nitrogen Doping and Al-N Codoping <strong>of</strong> ZnO Films<br />

43. A.I. Ievtushenko, G.V. Lashkarev, V.I. Lazorenko, L.O. Klochkov, O.I. Bykov, O.M.<br />

Kutsay, S.P. Starik, V.A. Baturin, A.Y. Karpenko<br />

Influence <strong>of</strong> Oxygen Pressure on the Properties <strong>of</strong> AZO Films<br />

44. H. Duda, E. Malicka, T. Gro�, A. G�gor, R. Sitko, J. Krok-Kowalski, P. Rduch<br />

Thermoelectric Power <strong>of</strong> CuCrxVySe4 p-Type Spinel Semiconductors<br />

45. H. Duda, P. Rduch, E. Malicka, T. Gro�, A. G�gor<br />

Critical Behaviour <strong>of</strong> the 3D-Heisenberg Ferromagnetic<br />

46. J.M. Sajkowski, M.A. Pietrzyk, D. Dobosz, M. Stachowicz, A. Droba, E. Przezdziecka,<br />

A. Wierzbicka, A. Kozanecki<br />

Optical properties <strong>of</strong> ZnO/ZnMgO single quantum wells grown by molecular beam<br />

epitaxy<br />

47. B.A. Orłowski, S.P. Dziawa, A. Reszka, K. Gas, S. Mickievicius, S. Thiess, W. Drube<br />

Electronic structure <strong>of</strong> CdTe/PbEuTe/CdTe<br />

48. D. Ziółkowska, K.P. Korona, M. Kami�ska, S.H. Wu, M.S. Chen<br />

Raman Spectroscopy <strong>of</strong> LiFePO4 and Li3V2(PO4)3 Cathode Materials for Lithium-Ion<br />

Battery Applications<br />

49. M. Hosatte, M.Ł. Basta, B.S. Witkowski, Z.T. Ku�nicki, M. Godlewski<br />

Multi-interface layered P-doped silicon structures for third generation photovoltaics<br />

50. M. Stachowicz, E. Przezdziecka, D. Dobosz, M.A. Pietrzyk, J.M. Sajkowski, A. Droba,<br />

A. Wierzbicka, A. Kozanecki<br />

Optical Properties <strong>of</strong> Thin ZnO Layers Grown by MBE<br />


_______________________________________________________________<br />

40th "Jaszowiec" <strong>2011</strong> International School and Conference on the Physics <strong>of</strong> Semiconductors<br />

51. K. Gas, E. Dynowska, E. Janik, A. Kami�ska, S. Kret, J.F. Morhange, I. Pasternak, M.<br />

Wiater, W. Zaleszczyk, R. Hołyst, E. Kami�ska, T. Wojtowicz, W. Szuszkiewicz<br />

ZnO-based Nanotubes Obtained by the Oxidation <strong>of</strong> ZnTe and ZnTe/Zn Nanowires<br />

52. D. �ak, W. Nakwaski<br />

Composition-dependent thermal resistance <strong>of</strong> multilayered structures taking into<br />

account phonon reflection, scattering and tunneling<br />

53. A. Reszka, B.A. Orłowski, M.A. Pietrzyk, A. Szczerbakow, S. Mickievicius, S.<br />

Balakauskas, R.L. Johnson<br />

Pb1-xGexTe surface with Sm 2+ and Sm 3+ doping<br />

54. H. Bednarski, J. Weszka, M. Doma�ski, V. Cozan<br />

Studies <strong>of</strong> Optical Properties <strong>of</strong> Protonated Poliazomethine Thin Films<br />

55. A.V. Atrashchenko, V.P. Ulin, V.P. Evtikhiev<br />

The reasons <strong>of</strong> destruction <strong>of</strong> nanoporous GaAs matrix fabricated by electrochemical<br />

etching<br />

56. E. Malicka, T. Gro�, A.W. Pacyna, H. Duda, J. Krok-Kowalski<br />

Effect <strong>of</strong> Substitution <strong>of</strong> Ti for Cd in CdCr2Se4 p-Type Semiconductor<br />

57. K. Dybko , M. Szot, T. Story, G. Karczewski, S. Schreyeck, C. Schumacher, K. Brunner<br />

and L. W. Molenkamp<br />

Thermoelectric power in epitaxial Bi2Se3/Si(111) layers<br />

58. M. Zapalska and T. Doma�ski<br />

Diamagnetism <strong>of</strong> the pre-paired electronic systems: the flow equation study<br />

59. N. Gonzalez-Szwacki, J.A. Majewski<br />

Quantum Monte Carlo vs. Density Functional Methods for the prediction <strong>of</strong> relative<br />

energies <strong>of</strong> small Si-C clusters<br />

60. M. Koba and P. Szczepa�ski<br />

Analysis <strong>of</strong> a Spatial Hole Burning Effect in a Square and Triangular Lattice Photonic<br />

Crystal Laser<br />


40th _______________________________________________________________<br />

"Jaszowiec" <strong>2011</strong> International School and Conference on the Physics <strong>of</strong> Semiconductors<br />


1. T. Smole�ski, Ł. Cywi�ski, M. Goryca, P. Kossacki<br />

Exciton and Mn spin dynamics in a nonresonantly coupled pair <strong>of</strong> (Cd,Mn)Te<br />

quantum dots<br />

2. J. Kobak, W. Pacuski, T. Kazimierczuk, J. Suffczy�ski, T. Jakubczyk, A. Golnik,<br />

P. Kossacki, M. Nawrocki, J.A. Gaj, C. Kruse, D. Hommel<br />

Three-dimensional anisotropy studies <strong>of</strong> CdTe quantum dots<br />

3. K. Grodecki, W. Strupi�ski, A. Wysmołek, R. St�pniewski, J.M. Baranowski<br />

Probing electron concentration in epitaxial graphene using Raman spectroscopy<br />

4. M. Zieli�ski, G.W. Bryant, N. Malkova, J. Sims, W. Jaskolski, J. Aizpurua<br />

Dynamical control <strong>of</strong> excitonic fine structure with nanomechanical strain<br />

5. K. Roszak, T. Novotný<br />

Non-Markovian noise at the Fermi-edge singularity in quantum dots<br />

6. M. Czapkiewicz, J. Wróbel, V. Kolkovsky, P. Nowicki, M. Aleszkiewicz, M. Wiater,<br />

T. Wojtowicz<br />

Transport and Spin Properties <strong>of</strong> CdTe/CdMgTe Quantum Point Contacts<br />

7. M. Kozub, P. Machnikowski<br />

The Role <strong>of</strong> Strong Coupling in the Superradiance <strong>of</strong> Ensembles <strong>of</strong> Quantum Dots<br />

8. M. Szymura, Ł. Kłopotowski, P. Wojnar, K. Fronc, T. Kazimierczuk, G. Karczewski,<br />

T. Wojtowicz<br />

Photoluminescence Linewidth Analysis <strong>of</strong> Single CdMnTe Quantum Dots<br />

9. M. Koperski, T. Kazimierczuk, M. Goryca, A. Golnik, J.A. Gaj, M. Nawrocki,<br />

P. Wojnar, P. Kossacki<br />

Statistical Study <strong>of</strong> the Inter-Dot Excitation Transfer in CdTe/ZnTe Quantum Dots.<br />

10. Ł. Marcinowski, M. Krzy�osiak, K. Roszak, P. Machnikowski, R. Buczko, J. Mostowski<br />

Phonon influence on the weak measurement <strong>of</strong> double quantum dot spin states.<br />

11. P. Kaczmarkiewicz, A. Musiał, G. S�k, P. Podemski, J. Misiewicz, P. Machnikowski<br />

Influence <strong>of</strong> Carrier Trapping on the Optical Properties <strong>of</strong> InAs/InP Quantum Dashes<br />

12. K. Dziatkowski, D. Ratchford, T. Hartsfield, X. Li, Y. Gao, Z. Tang<br />

CdSe/ZnS Colloidal Quantum Dots with Alloyed Core/Shell Interfaces:<br />

a Photoluminescence Dynamics Study<br />

13. K. Kukli�ski, Ł. Kłopotowski, K. Fronc, P. Wojnar, T. Wojciechowski, M. Czapkiewicz,<br />

J. Kossut, G. Karczewski, T. Wojtowicz<br />

Spectroscopy <strong>of</strong> Indirect Excitons in Vertically Stacked CdTe Quantum Dot Structures<br />


_______________________________________________________________<br />

40th "Jaszowiec" <strong>2011</strong> International School and Conference on the Physics <strong>of</strong> Semiconductors<br />

14. W. Abdussalam, A. Sitek, P. Machnikowski<br />

Collective spontaneous emission from pairs <strong>of</strong> quantum dots: the role <strong>of</strong> coupling and<br />

system geometry<br />

15. K. Korzekwa, P. Machnikowski<br />

Tunneling transfer protocol in a quantum dots chain immune to inhomogeneity<br />

16. A. Musial, G. Sek, A. Marynski, P. Podemski, J. Andrzejewski, J. Misiewicz, A. Loeffler,<br />

S. Hoefling, S. Reitzenstein, J.P. Reithmaier, A. Forchel<br />

Thermal Quenching <strong>of</strong> Photoluminescence from epitaxial InGaAs/GaAs Quantum<br />

Dots with High Lateral Aspect Ratio<br />

17. P. Schillak, G. Czajkowski<br />

Excitonic Magnetoabsorption <strong>of</strong> Cylindrical Quantum Disks<br />

18. P. Karwat, A. Sitek, P. Machnikowski<br />

Spontaneous emission from double quantum dots: collective effects and carrier-<br />

phonon kinetics<br />

19. P. Łach, A. Reszka, G. Karczewski, P. Wojnar, T. Wojtowicz, A. Kami�ska, A. Suchocki<br />

Cathodoluminescence studies <strong>of</strong> the II – VI semiconducting quantum dots grown by<br />

molecular beam epitaxy<br />

20. E. Zielony, E. Popko, Z. Gumienny, P. Kamyczek, A. Henrykowski, J. Jacak, G.<br />

Karczewski<br />

Raman spectroscopy <strong>of</strong> CdTe/ZnTe quantum dots<br />

21. J. Bara�ski, T. Doma�ski<br />

Quantum interference in charge transport via the quantum dots coupled between<br />

the metallic and superconducting leads<br />

22. H. Bednarski, J. Spałek<br />

Bound magnetic polaron molecule in diluted magnetic semiconductors within the<br />

Heitler-London approximation<br />

23. M. Zału�ny, A. Kozłowski<br />

Intersubband polaritons in strongly pumped microcavities<br />

24. M.S. Mukhin, Y.V. Terent'ev, L.E. Golub, M.O. Nestoklon, B.Y. Meltser,<br />

A.N. Semenov, V.A. Solov'ev, A.A. Sitnikova, A.A. Toropov, S.V. Ivanov<br />

Magneto-Optical Studies <strong>of</strong> Narrow Band-Gap Heterostructures with Type II Quantum<br />

Dots InSb in an InAs Matrix<br />

25. K.H. Gawarecki, P. Machnikowski<br />

Phonon-assisted tunneling between hole states in double quantum dots<br />

26. I. Bragar, K. Gawarecki, P. Machnikowski<br />

Intermediate band formation for electrons and holes in an inhomogeneous chain <strong>of</strong><br />

quantum dots<br />


40th _______________________________________________________________<br />

"Jaszowiec" <strong>2011</strong> International School and Conference on the Physics <strong>of</strong> Semiconductors<br />

27. O. Rancova, E. Anisimovas<br />

Modelling <strong>of</strong> 1D-2D structural transitions in small Yukawa clusters<br />

28. P. Kowalski, P. Machnikowski<br />

Efficient Coulomb mixing between single- and biexciton states in InAs nanocrystals<br />

29. K. Gołasa, M. Molas, J. Borysiuk, Z.R. Wasilewski, A. Babi�ski<br />

Strongly disordered wetting layer in the InAs/GaAs self-assembled quantum dots<br />

system<br />

30. M. Molas, K. Gołasa, J. Łusakowski, T. Wojtowicz, A. Babi�ski<br />

Optical transformation <strong>of</strong> zero-dimensional confinement in the CdTe/CdMgTe multiple<br />

quantum wells<br />

31. M. Kozub, A. Musiał, G. S�k, J. Misiewicz, V. Zürbig, J.P. Reithmaier<br />

Optical Properties <strong>of</strong> InGaAs Quantum Dots on (100) GaAs Substrates Formed by<br />

Droplet Epitaxy<br />

32. M. Molas, K. Gołasa, T. Kazimierczuk, J. Łusakowski, T. Wojtowicz, A. Babi�ski<br />

Magnetooptical study <strong>of</strong> excitons confined in potential fluctuations in the<br />

CdTe/CdMgTe quantum well<br />

33. A. Ballester, J.M. Escartín, J.L. Movilla, M. Pi, J. Planelles<br />

Mixed Correlation Phases In Elongated Quantum Dots<br />

34. J. Ebeling, T. Aschenbrenner, S. Figge, D. Hommel<br />

Simulation and Realization <strong>of</strong> Photonic Crystals in Light Emitting Devices<br />

35. E. Zielony, E. Popko, Z. Gumienny, G. Karczewski<br />

Electro-optical characterization <strong>of</strong> Ti/Au-ZnTe Schottky diodes with CdTe quantum<br />

dots<br />

36. J. Jadczak, L. Bryja, A. Wójs, G. Bartsch, D.R. Yakovlev, M. Bayer, M. Potemski<br />

Magneto-photoluminescence studies <strong>of</strong> many body scattering processes in two-<br />

dimensional hole gas<br />

37. H. Videlier, N. Dyakonova, F. Teppe, C. Consejo, W. Knap, J. Lusakowski,<br />

D. Tomaszewski, J. Marczewski, P. Grabiec<br />

Spin related effect in Si-MOSFETs THz Photoresponse<br />

38. N. Dyakonova, A. ElFatimy, Y. Meziani, D. Coquillat, W. Knap, F. Teppe, P. Buzatu,<br />

L. Varani, H. Marinchio, J. Torres, P. Nouvel<br />

THz emission related to hot plasmons and plasma wave instability in field effect<br />

transistors<br />

39. M. Białek, M. Czapkiewicz, K. Fronc, J. Wróbel, V. Umansky, M. Grynberg,<br />

J. Łusakowski<br />

A Comb-Like Shape <strong>of</strong> the Cyclotron Resonance Line in GaAs/AlGaAs<br />

Heterostructure<br />


_______________________________________________________________<br />

40th "Jaszowiec" <strong>2011</strong> International School and Conference on the Physics <strong>of</strong> Semiconductors<br />

40. J. Przybytek, M. Gryglas-Borysiewicz, M. Baj<br />

Impurity-Related Noise in Si �-doped Single-Barrier GaAs/AlAs/GaAs Resonant<br />

Tunneling Devices<br />

41. F. Teppe, C. Consejo, J. Torres, B. Chenaud, P. Solignac, Z.R. Wasilewski, M. Zholudev,<br />

N. Dyakonova, D. Coquillat, A. El Fatimy, W. Knap<br />

Current Driven Terahertz Detection <strong>of</strong> Quantum Cascade Laser Emission by Plasma<br />

Waves in Nano-Transistors<br />

42. G. K�pisty, K. Grodecki, W. Strupi�ski, A. Wysmołek, R. St�pniewski, J.M. Baranowski<br />

Influence <strong>of</strong> SiC substrate orientation on epitaxial graphene quality studied by Raman<br />

spectroscopy<br />

43. M. Syperek, B. Bujko, J. Jadczak, M. Kubisa, L. Bryja, J. Misiewicz<br />

Fermi edge and band-to-band recombination dynamics in n-doped GaAs/AlGaAs<br />

quantum well<br />

44. M. Kozubal, M. Gryglas-Borysiewicz, J. Przybytek, J. Borysiuk, M. Baj, R. St�pniewski,<br />

A. Wysmołek, W. Strupi�ski, J. Baranowski<br />

Magnetotransport Studies <strong>of</strong> Graphene Exposed to Water Vapours<br />

45. K. Grodecki, J.A. Błaszczyk, A. Dominiak, W. Strupi�ski, A. Wysmołek,<br />

R. St�pniewski, A. Drabi�ska, M. Sochacki, J.M. Baranowski<br />

Interaction <strong>of</strong> epitaxial graphene with SiC substrates studied by Raman spectroscopy<br />

46. K. Nogajewski, H. Boukari, P. Kopyt, W. Gwarek, T. Wojtowicz, H. Mariette,<br />

M. Grynberg, J. Łusakowski<br />

Antenna-Equipped Field Effect Transistors on CdTe/CdMgTe Quantum Wells as<br />

Terahertz Detectors<br />

47. A.D. Chegodaev, D.K. Loginov<br />

Effect <strong>of</strong> homogeneous electric field on exciton dispersion in wide quantum well<br />

48. V.Ya. Roshko<br />

Theoretical Analysis <strong>of</strong> Optical Losses in CdS/CdTe Solar Cells<br />

49. M.Kubisa, K.Ryczko and J.Misiewicz<br />

Anisotropy <strong>of</strong> B=0 spin splitting <strong>of</strong> holes in symmetric GaAs/Ga(1-x)AlxAs quantum<br />

wells<br />

50. J. Szczytko, P. Arcade, E. Papis, A. Bara�ska, B. Pi�tka, J. Łusakowski<br />

Terahertz Properties <strong>of</strong> Gold Layers on GaAs<br />

51. P. Sznajder, B. Pi�tka, J. Szczytko, J. Łusakowski<br />

Towards Optically Tunable Terahertz Plasmonic Detectors<br />

52. I. Grigelionis, K. Fobelets, B. Vincent, J. Mitard, B. DeJaeger, E. Simoen, D. Jaworski,<br />

J. Łusakowski<br />

Mobility <strong>of</strong> Holes in Nanometer Ge-on-Si p-type Metal-Oxide-Semiconductor Field-<br />

Effect Transistors at Low Temperatures<br />


40th _______________________________________________________________<br />

"Jaszowiec" <strong>2011</strong> International School and Conference on the Physics <strong>of</strong> Semiconductors<br />

53. Paweł Utko, Morten H. Madsen, Trine Berthing, Sara Bonde, Claus B. Sorensen,<br />

Karen L. Martinez, Jesper Nygard<br />

Vertical InAs nanowires as biological probes<br />

54. M. �ciesiek, T. Jakubczyk, W. Pacuski, A. Golnik, P. Kossacki, C. Kruse, D. Hommel<br />

Towards better light-confinement in pillar cavities<br />

55. M. Rawski, J. �uk, A. Drozdziel, K. Pyszniak, M. Turek<br />

Investigation <strong>of</strong> ion-implanted SiC properties by means <strong>of</strong> a light absorption technique<br />

56. P. Kamyczek, E. Placzek-Popko, Łukasz Gelczuk, Maria D�browska-Szata<br />

SiC Schottky barrier diode studied by admittance spectroscopy<br />

57. P. Sitarek, K. Ryczko, J. Misiewicz, D. Reuter, A. Wieck<br />

Optical transitions between confined and unconfined states in p-type asymmetric<br />

GaAs/InGaAs/AlGaAs QW structures<br />

58. K. Tahy, W. S. Hwang, J.L. Tedesco, R.L. Myers Ward, P.M. Campbell, C.R. Eddy Jr.,<br />

D.K. Gaskill, H. Xing, A. Seabaugh, D. Jena<br />

Sub-10 nm Epitaxial Graphene Nanoribbon FETs<br />


_______________________________________________________________<br />

40th "Jaszowiec" <strong>2011</strong> International School and Conference on the Physics <strong>of</strong> Semiconductors<br />


1. C. �liwa, T. Dietl<br />

Thermodynamic and thermoelectric properties <strong>of</strong> (Ga,Mn)As<br />

2. M. Sobanska, K. Klosek, Z.R. Zytkiewicz, J. Borysiuk, A. Wierzbicka, A. Reszka, E.<br />

Lusakowska<br />

Plasma-assisted molecular beam epitaxy <strong>of</strong> GaN on Si (111) substrates<br />

3. J. Plaziak, J. Higersberger, J.A. Majewski<br />

Role <strong>of</strong> Interface Carrier Transport in Electrical Characteristics <strong>of</strong> Light Emitting<br />

Devices Based on Heterostructures<br />

4. A. Łusakowski, P. Bogusławski, W. Kn<strong>of</strong>f, T. Story<br />

Influence <strong>of</strong> crystal structure and hole concentration on magnetic anisotropy <strong>of</strong><br />

GeMnTe<br />

5. I. Ulfat, L. Ilver, J. Sadowski<br />

High-resolution study <strong>of</strong> valence band maximum for (GaMn)As<br />

6. J. Suffczy�ski, K. Gałkowski, P. Ka�mierczak, J. Papierska, M. Furman, W. Pacuski,<br />

A. Golnik, A.M. Witowski, J.A. Gaj, W. Stefanowicz, M. Sawicki, M. Łukasiewicz,<br />

E. Guziewicz, M. Godlewski<br />

Low temperature grown (Zn,Co)O studied in the band-gap spectral region<br />

7. J. Levrat, G. Rossbach, A. Dussaigne, H. Teisseyre, I. Grzegory, M. Bockowski,<br />

T. Suski, R. Butte, N. Grandjean<br />

Nonpolar III-nitride microcavities for polariton lasing<br />

8. W. Bardyszewski, S.P.. Łepkowski<br />

Symmetry <strong>of</strong> the top valence band states in GaN/AlGaN quantum wells and its<br />

influence on the polarization <strong>of</strong> emitted light<br />

9. D. Sztenkiel, T. Dietl<br />

Tunnelling effects in (Ga,Mn)As based heterostructures<br />

10. M. Lopuszynski, J.A. Majewski<br />

Modelling <strong>of</strong> Ordering Phenomena in Nitride Semiconducting Alloys<br />

11. M. Welna, R. Kudrawiec, M. Motyka, J. Misiewicz, R. Kucharski, M. Zaj�c, R.<br />

Doradzi�ski, R. Dwili�ski<br />

Infrared Spectroscopy <strong>of</strong> GaN Crystals Obtained by Ammonothermal Method<br />

12. M. Latkowska, R. Kudrawiec, G. S�k, J. Misiewicz, J. Ibán~ez, M. Henini,<br />

M. Hopkinson<br />

Micro-photoluminescence <strong>of</strong> GaInNAs layers: Thermal quenching <strong>of</strong> individual<br />

exciton lines<br />

13. M. Fandrich, G. Kunert, T. Aschenbrenner, S. Figge, C. Kruse, D. Hommel<br />


40th _______________________________________________________________<br />

"Jaszowiec" <strong>2011</strong> International School and Conference on the Physics <strong>of</strong> Semiconductors<br />

Nitride based sensors with Ga- and N-polarity<br />

14. V.Kh. Le, K. Swiatek, M. Pawlowski, R.R. Galazka<br />

Zinc vacancy induced ferromagnetic interaction in semimagnetic semiconductor<br />

(Zn,Mn)Te<br />

15. S. Dobkowska, W. Stefanowicz, O. Proselkov, R. �uberek, J. Sadowski, T. Dietl,<br />

M. Sawicki<br />

Magnetic Properties <strong>of</strong> (Ga,Mn)As near Metal-Isolator Transition<br />

16. M. Sakowicz, N. Gauthier, R. Leonelli, C. Silva, H. Nguyen, K. Cui, S. Zhang, Z. Mi<br />

Time-Resolved Photoluminescence <strong>of</strong> InGaN/GaN Dot-in-a-Wire Heterostructures on<br />

Si(111)<br />

17. M. Bajda, F. Dybała, A. Bercha, W. Trzeciakowski, J.A. Majewski<br />

Wide range wavelength tuning <strong>of</strong> InGaAsP/InP laser diodes<br />

18. M. Gryglas-Borysiewicz, J. Przybytek, M. Baj, A. Kwiatkowski, P. Juszy�ski, D. Wasik,<br />

P. Dziawa, J. Sadowski<br />

Transport properties <strong>of</strong> GaMnAs layers<br />

19. C. Chèze, M. Sawicka, M. Siekacz, H. Turski, A. Feduniewicz-Zmuda, G. Cywinski, B.<br />

Grzywacz, S. Grzanka, I. Dziecielewski, B. Lucznik, M. Bockowski, C. Skierbiszewski<br />

Group III-nitrides growth on N-polar substrates<br />

20. J. Binder, K.P. Korona, J. Borysiuk, M. Kaminska, M. Baeumler, K. Köhler, L. Kirste<br />

Absorption and Emission Properties <strong>of</strong> Light Emitting Diode Structures Containing<br />

GaInN/GaN QWs<br />

21. B.J. Kowalski, R. Nietuby�, J. Sadowski<br />

Mn contribution to the valence band <strong>of</strong> Ga1-xMnxSb<br />

22. M. Baranowski, M. Latkowska, M. Syperek, R. Kudrawiec, J. Misiewicz, T. Sarmiento,<br />

J.S. Harris<br />

Time resolved photoluminescence studies for GaInNAsSb quantum wells emitting at<br />

1.3 µm<br />

23. A. Kafar, J. Goss, S. Sta�czyk, R. Czernecki, M. Leszczynski, T. Suski, P. Wi�niewski,<br />

P. Perlin<br />

InGaN laser diodes with passive absorber section<br />

24. W. Szuszkiewicz, F. Ott, J.Z. Domagała, E. Dynowska, J. Sadowski<br />

Evidence <strong>of</strong> a new magnetic order in short-period (Ga,Mn)As/GaAs SLs<br />

25. Z. Wi�niewski, K. Izdebska, P. Sybilski, Z.R. �ytkiewicz, M. Soba�ska, K. Kłosek,<br />

A. Reszka, B.J. Kowalski, A. Suchocki<br />

Application <strong>of</strong> n-GaN layers grown by MBE for light-induced water splitting and<br />

hydrogen generation<br />

26. M. Sarzynski, J. Goss, M. Leszczynski, A. Reszka, B. Kowalski, P. Perlin, T. Suski<br />


_______________________________________________________________<br />

40th "Jaszowiec" <strong>2011</strong> International School and Conference on the Physics <strong>of</strong> Semiconductors<br />

Broad optical gain spectrum in III-N quantum structures<br />

27. M.I. Łukasiewicz, A. Cabaj, M. Godlewski, E. Guziewicz, A. Wittlin, M. Jaworski,<br />

A. Woło�, Z. Wilamowski<br />

Microwave techniques investigations <strong>of</strong> (Zn,Co)O films grown by Atomic Layer<br />

Deposition<br />

28. O. Yastrubchak, T. Andrearczyk, J. Sadowski, J. �uk, T. Wosi�ski<br />

Band-structure analysis from photoreflectance spectroscopy in (Ga,Mn)As<br />

29. K. Klosek, M. Sobanska, Z.R. Zytkiewicz, H. Teisseyre, E. Lusakowska, A. Wierzbicka,<br />

P. Nowakowski, L. Klopotowski<br />

Influence <strong>of</strong> nitrogen plasma parameters on growth <strong>of</strong> GaN by plasma-assisted<br />

molecular beam epitaxy<br />

30. T.A. Krajewski, P. Stallinga, E. Zielony, P. Kruszewski, K. Go�ci�ski, Ł. Wachnicki,<br />

S. Figge, D. Hommel, E. Guziewicz, M. Godlewski<br />

Deep defects in ZnO/GaN heterostructure analyzed by the admittance spectroscopy<br />

31. T.A. Krajewski, A.J. Zakrzewski, G. Łuka, Ł. Wachnicki, S. Gierałtowska,<br />

B.S. Witkowski, P. Kruszewski, E. Łusakowska, R. Jakieła, E. Guziewicz, M. Godlewski<br />

Electrical characterization <strong>of</strong> the ZnO-based Schottky diodes for possible sensor<br />

applications<br />

32. T. Zakrzewski, P. Bogusławski<br />

Ab initio calculations <strong>of</strong> transition metal impurity levels in III-V semiconductors<br />

33. P. Juszy�ski, D. Wasik, M. Baj, J. Przybytek, M. Gryglas-Borysiewicz, J. Sadowski<br />

Anisotropy <strong>of</strong> GaMnAs thin film. Planar and Anomalous Hall measurements.<br />

34. G. Staszczak, A. Khachapuridze, S. Grzanka, R. Piotrzkowski, R. Czernecki, P. Perlin,<br />

T. Suski<br />

Interplay between internal and external electric field studied by photoluminescence in<br />

InGaN/GaN light emitting diodes<br />

35. M. Baranowski, M. Latkowska, R. Kudrawiec, M. Syperek, J. Misiewicz,<br />

G.S. Karthikeyan, S. Jong-In, K.L. June<br />

Influence <strong>of</strong> antimony on the optical quality <strong>of</strong> GaInN:Sb multi quantum wells<br />

36. J. Sadowski, A. Siusys, P. Dziawa, A. Reszka, B.J. Kowalski<br />

Properties <strong>of</strong> Mn-doped GaAs Nanowires and GaAs/(Ga,Mn)As Core-Shell Nanowire<br />

Structures Grown by MBE on GaAs(111)B Substrates<br />

37. I.A. Kowalik, M.A. Nin~o, A. Locatelli, T. OnurMente�, A. Navarro-Quezada,<br />

M. Rovezzi, A. Bonanni, T. Dietl, D. Arvanitis<br />

Room temperature nano-magnetism <strong>of</strong> (Ga,Fe)N films: element specific spectroscopy<br />

and microscopy<br />

38. L. Marona, S. Grzanka, P. Wisniewski, T. Suski, M. Leszczynski, R. Czernecki,<br />

M. Bockowski, S.P. Najda, P. Perlin<br />


40th _______________________________________________________________<br />

"Jaszowiec" <strong>2011</strong> International School and Conference on the Physics <strong>of</strong> Semiconductors<br />

The non-radiative recombination rate in InGaN laser diodes during the aging<br />

39. L. Janicki, M. Gladysiewicz, R. Kudrawiec, M. Rudzi�ski, R. Kucharski, M. Zaj�c,<br />

J. Misiewicz, W. Strupinski, R. Doradzi�ski, R. Dwili�ski<br />

Electromodulation Spectroscopy <strong>of</strong> GaN/AlGaN Quantum Wells Grown on Polar and<br />

Non-polar GaN Substrates Obtained by Ammonothermal Method<br />

40. E.P. Smakman, R. vanVoornveld, J.G. Keizer, J.K. Garleff, P.M. Koenraad<br />

Spin-Polarized STM with Fe-Coated W Tips and Bulk Cr Tips<br />

41. B.S. Witkowski, Ł. Wachnicki, E. Guziewicz, M. Godlewski<br />

Cathodoluminescence measurements at liquid helium temperature <strong>of</strong> monocrystalline<br />

ZnO layers<br />

42. T. Andrearczyk, I. Krogulec, T. Wosi�ski, T. Figielski, A. M�kosa, Z. Tkaczyk,<br />

J. Wróbel, J. Sadowski<br />

Towards electrically controllable read-write devices based on ferromagnetic<br />

semiconductors<br />

43. J.B. Gosk, Z. Werner, C. Pochrybniak, M. Barlak, J. Szczytko, A. Twardowski<br />

Magnetic properties <strong>of</strong> Mn-ion implanted and plasma pulse treated Si<br />

44. G. Muzioł, M. Siekacz, H. Turski, C. Skierbiszewski<br />

Simulations <strong>of</strong> optical modes in InGaN based laser diodes operating at 455 nm<br />

45. M. Woi�ska, K. Madrak, J. Szczytko, J. Gosk, A. Majh<strong>of</strong>er, D. Pociecha, E. Górecka,<br />

A. Twardowski<br />

Monte-Carlo simulations and magnetic studies <strong>of</strong> ferromagnetic nanocomposites<br />

46. J. Szczytko, J. Szydlowska, N. Gonzalez Szwacki, K. Dziatkowski, P. Gizi�ski, A. Kaim,<br />

A. Twardowski<br />

Di-TEMPO amine for molecular spintronics – model <strong>of</strong> p-shell magnetism<br />

47. M. Baranowski, M. Latkowska, R. Kudrawiec, J. Misiewicz<br />

Hopping excitons in GaInNAs alloys: Radiative versus non-radiative recombination at<br />

various temperatures<br />

48. J.Z. Domagala, B. Lucznik, H. Teisseyre, M. Bockowski, I. Grzegory<br />

Structural characterization <strong>of</strong> the nonpolar substrate grown by multistep hydride vapor<br />

phase epitaxy.<br />

49. Ł. Gluba, O. Yastrubchak, H. Krzy�anowska, J.Z. Domagała, T. Andrearczyk, J. �uk,<br />

J. Sadowski, T. Wosi�ski<br />

Investigation <strong>of</strong> fundamental parameters <strong>of</strong> low doped (Ga,Mn)As epitaxial layers<br />

50. P. Kamyczek, E. Popko, Z. Gumienny, E. Zielony, S.Grzanka, R. Czernecki, T. Suski<br />

Admittance spectroscopy in GaN p-n junction<br />

51. Pawel Kempisty, Pawel Strak, Stanislaw Krukowski<br />


_______________________________________________________________<br />

40th "Jaszowiec" <strong>2011</strong> International School and Conference on the Physics <strong>of</strong> Semiconductors<br />

Role <strong>of</strong> hydrogen in the ammonia based growth <strong>of</strong> GaN and InGaN - ab initio study<br />

52. S.P. Lepkowski, W. Bardyszewski, H. Teisseyre<br />

Effect <strong>of</strong> the built-in strain on the in-plane optical anisotropy <strong>of</strong> m-plane GaN/AlGaN<br />

quantum wells<br />

53. J. Papierska, J.-G. Rousset, A. Golnik, W. Pacuski, M. Nawrocki, J. A. Gaj,<br />

J. Suffczynski, I. Kowalik, W. Stefanowicz, M. Sawicki, T. Dietl, A. Navarro-Quezada,<br />

B. Faina, Tian Li, A. Bonanni<br />

Magnetooptical properties <strong>of</strong> (Ga,Fe)N layers<br />

54. Maria Ptasi�ska, Jacek Piechota, Jakub Sołtys, Stanisław Krukowski<br />

Gas Phase Reactions during GaN Growth by MOVPE Method – Ab initio Study<br />

55. Konrad Sakowski, Leszek Marcinkowski, Szymon Grzanka, Elzbieta Litwin-<br />

Staszewska, Stanislaw Krukowski<br />

Simulations <strong>of</strong> Gallium Nitride luminescent devices with extension <strong>of</strong> standard<br />

nonradiative recombination model<br />

56. Pawel Strak, Pawel Kempisty, Konrad Sakowski, Stanislaw Krukowski<br />

Density Functional Theory (DFT) simulations <strong>of</strong> the physical properties <strong>of</strong> AlN/GaN<br />

multiple quantum wells (MQWs)<br />

57. Z. R. Zytkiewicz, P. Dluzewski, J. Borysiuk, M. Sobanska, K. Klosek, B. Witkowski,<br />

P. Nowakowski, J. Dabrowski<br />

Plasma-assisted MBE growth and characterization <strong>of</strong> GaN nanocolumns on Si (111)<br />

substrates<br />

58. Zbigniew R. Zytkiewicz, Pawel Strak, Stanisław Krukowski<br />

Crossing Size Limits <strong>of</strong> Bulk III-V Crystals Feasible by Liquid Phase Electroepitaxy<br />


40th _______________________________________________________________<br />

"Jaszowiec" <strong>2011</strong> International School and Conference on the Physics <strong>of</strong> Semiconductors SaI1<br />

Modern technologies for wide-gap semiconductor epitaxy<br />

and nano-processing<br />

Detlef Hommel<br />

Institute <strong>of</strong> Solid State Physics, University <strong>of</strong> Bremen, Germany<br />

An overview will be given on the growth and application <strong>of</strong> wide bandgap<br />

semiconductors like group-III nitrides and group-II selenides and tellurides. It will be shown<br />

that different epitaxial methods have to be used for different materials (MBE for II-VI and<br />

mostly MOVPE for nitrides). In this respect homo- and heteroepitaxy will be compared as<br />

well.<br />

The growth mechanisms for CdSe and InGaN quantum dots differ significantly and<br />

both have not much common with the well known Stranski-Krastanov growth mode. This will<br />

be discussed in detail.<br />

As an important tool for nano-processing a focused ion beam (FIB) tool will be<br />

presented. Such a dual beam system with a Ga-source for nanostructuring, an electron source<br />

for imaging, an integrated e-beam lithography and sources for insulator and metal deposition<br />

on a nanometer scale allows to design novel device structures.<br />

Approaches to grow fully monolithic microcavities based on selenides, nitrides and<br />

tellurides will be presented. Using the FIB micropillars can be designed out <strong>of</strong> such planar<br />

structures with interesting physical properties. Introducing quantum dots into the cavity one<br />

can obtain efficient sources for single photon emission up to room temperature.<br />


SaI2 _______________________________________________________________<br />

40th "Jaszowiec" <strong>2011</strong> International School and Conference on the Physics <strong>of</strong> Semiconductors<br />

Physical simulation <strong>of</strong> electronic devices<br />

Aldo Di Carlo<br />

Dept. <strong>of</strong> Electronics Engineering, University <strong>of</strong> Rome “Tor Vergata”, Rome, Italy<br />

Physical Simulation is a fundamental step for the design and fabrication <strong>of</strong><br />

commercially available and research developed electronic devices.[1] Since ’80 several<br />

Technology Computer Aided Design (TCAD) tools has been developed to this end, starting<br />

from the seminal work <strong>of</strong> R. Dutton at Stanford University. Nowadays, the need for faster<br />

and better performing electronic devices with higher integration density has been driving<br />

almost since the dawn <strong>of</strong> semiconductor technology a steady trend <strong>of</strong> scaling down the device<br />

dimensions. This led to an incredible increase in computing power and memory capacity,<br />

quantified by the well-known Moore’s law. Due to this downscaling, the dimensions <strong>of</strong> the<br />

active region <strong>of</strong> conventional metal–oxide–semiconductor field-effect transistors (MOSFETs)<br />

have already reached the nanometer scale. This makes device simulation very challenging and<br />

new theoretical framework should be considered. [2]<br />

Hand in hand with device scaling, new devices exploiting quantum mechanical effects for<br />

their functioning have been proposed, e.g., transistors based on quantum wires, dots, or<br />

molecules. Quantum dots and wires have also gained much attention for their use in<br />

optoelectronic devices for lasing or single photon emission or as color tunable and white-light<br />

emitters. The electronic, optical, and transport properties <strong>of</strong> the active regions <strong>of</strong> such highly<br />

scaled or new devices cannot be described by classical or semiclassical theories. A fully<br />

quantum mechanical treatment has to be used instead. [3]<br />

In this lecture, I will review the methods for electronic device simulation starting from the<br />

common picture based on drift-diffusion equation up to quantum mechanical based<br />

approaches for both continuum and atomistic models. A review <strong>of</strong> available TCAD tools will<br />

be given.<br />

Fig. 1<br />

Simulation <strong>of</strong> GaN based nanocolumn LED.<br />

Electrostatic potential and current flow lines<br />

around the intrinsic part <strong>of</strong> the column.<br />

(a) The classical results.<br />

(b)The self-consistent Quantum/driftdiffusion<br />

results, showing also the contours <strong>of</strong> the electron<br />

and hole densities at half <strong>of</strong> the mean density<br />

inside the intrinsic region. (from Ref. 2)<br />

[1] S. Selberherr, Analysis and Simulation <strong>of</strong> Semiconductor Devices, Springer, (1984).<br />

[2] M. Auf der Maur et al. IEEE Trans. Electron Devices 58, 1425 (<strong>2011</strong>)<br />

[3] A. Pecchia and A. Di Carlo, Rep. Prog. Phys., vol. 67, no. 8, pp. 1497–1561, (2004).<br />


40th _______________________________________________________________<br />

"Jaszowiec" <strong>2011</strong> International School and Conference on the Physics <strong>of</strong> Semiconductors SuI1<br />

Physics and Technology <strong>of</strong> Arsenide Based Quantum Dots and Nanowires<br />

Gerhard Abstreiter<br />

Walter Schottky Institut and Institute for Advanced Study<br />

TU München, 85748 Garching<br />

Optical control <strong>of</strong> charges, spins, excitons and photons has been achieved in recent years in<br />

semiconductor nanostructures. They are therefore excellent candidates for demonstrating<br />

basic principles <strong>of</strong> quantum information technology. I will discuss the basic optical properties<br />

and recent results on electro-optic control <strong>of</strong> single self-assembled InGaAs quantum dots.<br />

This includes results on spin storage and spin lifetimes <strong>of</strong> electrons and holes [1, 2] in dot<br />

ensembles, as well as single charge and spin control and readout [3]. In the second part <strong>of</strong> my<br />

lecture I will also discuss recent results on MBE grown GaAs and InAs based nanowire<br />

hetero-structures with new functionalities [4 - 9].<br />

[1] M. Kroutvar et al., Nature 432, 81 (2004)<br />

[2] D. Heiss et al., Phys. Rev. B 76, 24136 (2007)<br />

[3] D. Heiss et al., Phys. Rev. B 82, 245316 (2010)<br />

[4] A. Fontcuberta i Morral et al., Small 4, 899 (2008) and APL 92, 063112 (2008)<br />

[5] M. Heigoldt et al., J. Mater. Chem. 19, 840 (2009)<br />

[6] D. Spirkoska et al., Phys. Rev. B 80, 245325 (2009)<br />

[7] I. Zardo et al., Phys. Rev. B 80, 245324 (2009)<br />

[8] S. Hertenberger et al., J. Appl. Phys. 108, 114316 (2010)<br />

[9] M. Soini et al., Appl. Phys. Letters 97, 263107 (2010)<br />


SuI2 _______________________________________________________________<br />

40th "Jaszowiec" <strong>2011</strong> International School and Conference on the Physics <strong>of</strong> Semiconductors<br />

Tutorial: Engineering <strong>of</strong> Electric Fields in Nitride-Based Semiconductors<br />

Debdeep Jena<br />

Associate Pr<strong>of</strong>essor <strong>of</strong> Electrical Engineering University <strong>of</strong> Notre Dame,<br />

Notre Dame, IN 46556 USA<br />

djena@nd.edu, http://www.nd.edu/~djena<br />

The built-in polarization fields in III-Nitride semiconductor heterostructures play<br />

fundamental and crucial roles in many electronic and optical phenomena exhibited by these<br />

semiconductors. In addition, they strongly influence the design and performance <strong>of</strong> electronic<br />

and optical devices fabricated from III-Nitride heterostructures. The goals <strong>of</strong> this tutorial are –<br />

1) To introduce the concept <strong>of</strong> electronic polarization in semiconductors<br />

2) To explore its influence on electronic and optical properties<br />

3) To explore details <strong>of</strong> how charge electrostatics and transport are affected<br />

4) To explore how optical transitions and emission properties are influenced,<br />

5) To apply them to electronic and optical devices <strong>of</strong> the present, and<br />

6) To discuss open problems, and what is possible in the future using polarization.<br />

Specifically, the concept <strong>of</strong> polarization will be traced down to its microscopic origin in<br />

charge dipoles in the highly polar III-Nitride semiconductor crystals. We will discuss how the<br />

macroscopic properties emerge from the microscopic picture. The effect <strong>of</strong> dipoles on charge<br />

transport, electrostatics, and optical transition matrix elements will be described. We will<br />

study how the fields emerging from the polarization can be used to design ‘traditional’ highperformance<br />

electronic and optical devices such as high electron mobility transistors<br />

(HEMTs), light-emitting diodes (LEDs) and lasers. The discussion will cover both the ‘good’<br />

and ‘bad’ effects <strong>of</strong> polarization in such devices.<br />

We will also discuss various “novel” device applications <strong>of</strong> polarization physics in III-<br />

Nitride semiconductor heterostructures. Some <strong>of</strong> these include polarization-induced Zenerinterband<br />

tunneling and p-type doping for applications in light emitting devices. The wellknown<br />

concept <strong>of</strong> “lattice-- matching” in III-V semiconductor heterostructures will be<br />

extended to include the idea <strong>of</strong> “Polarization-- matching” and the opportunities <strong>of</strong> such<br />

concepts will be discussed.<br />


40th _______________________________________________________________<br />

"Jaszowiec" <strong>2011</strong> International School and Conference on the Physics <strong>of</strong> Semiconductors MoI1<br />

��<br />

Coherent optical control <strong>of</strong> the NV center in diamond<br />

Jörg Wrachtrup<br />

3rd� Physics Institute and Research Center SCOPE, University <strong>of</strong> Stuttgart, D-70659<br />

Stuttgart, Germany<br />


MoI2 _______________________________________________________________<br />

40th "Jaszowiec" <strong>2011</strong> International School and Conference on the Physics <strong>of</strong> Semiconductors<br />

Understanding and exploiting magnetism <strong>of</strong> semiconductors<br />

Tomasz Dietl<br />

Laboratory for Cryogenic and Spintronic Research,<br />

Institute <strong>of</strong> Physics, Polish Academy <strong>of</strong> Sciences and<br />

Institute <strong>of</strong> Theoretical Physics, University <strong>of</strong> Warsaw<br />

Triggered by a theoretical model <strong>of</strong> ferromagnetism mediated by valence band<br />

holes put forward a dozen years ago, the search for compounds combining the resources<br />

<strong>of</strong> semiconductors and ferromagnets has evolved into an important and challenging field<br />

<strong>of</strong> materials science. This endeavour has been fuelled by continual demonstrations <strong>of</strong><br />

remarkable low-temperature functionalities found for ferromagnetic structures <strong>of</strong><br />

(Ga,Mn)As, p-(Cd,Mn)Te, are related materials -- now transferred to elemental<br />

ferromagnets -- as well as by abundant experimental observations and theoretical<br />

modellings <strong>of</strong> ferromagnetic features persisting up to high temperatures in a number <strong>of</strong><br />

magnetically doped semiconductors and oxides without any valence band holes or even<br />

in materials nominally undoped with transition metals.<br />

In the talk, a broad overview <strong>of</strong> recent breakthrough experimental and theoretical<br />

developments will be given emphasizing that, from the one hand, they disentangle<br />

controversies and puzzles accumulated over the last decade and, on the other, <strong>of</strong>fer new<br />

outstanding research prospects [1]. More specifically, it will be shown how element<br />

specific and spin sensitive contemporary nanocharacterization methods help to asses the<br />

real structure <strong>of</strong> materials down to the atomic scale and, thus, to build a sensible<br />

theoretical understanding <strong>of</strong> macroscopic properties, including the origin <strong>of</strong> low- and<br />

high-temperature ferromagnetism in semiconductors. Recent experiments that<br />

demonstrate the influence <strong>of</strong> the electric field and current on magnetism and the<br />

generation <strong>of</strong> current by time-dependent magnetization will be discussed in a broad<br />

context <strong>of</strong> spin physics and spintronic applications.<br />

[1] See, T. Dietl, “A ten-year perspective on dilute magnetic semiconductors and oxides”,<br />

Nature Mater. 9, 965 (2010).<br />


40th _______________________________________________________________<br />

"Jaszowiec" <strong>2011</strong> International School and Conference on the Physics <strong>of</strong> Semiconductors MoI3<br />

Charging Effects in Self Assembled CdTe Quantum Dots<br />

Łukasz Kłopotowski<br />

Institute <strong>of</strong> Physics, Polish Academy <strong>of</strong> Sciences al. Lotników 32/46, Warszawa, Poland<br />

Semiconductor quantum dots (QDs) are <strong>of</strong>ten referred to as artificial atoms. This<br />

analogy stems mostly from zero dimensional density <strong>of</strong> states hindering interactions between<br />

these nanostructures and the environment. In fact, QDs are more than atom-like<br />

structures: they can be easily incorporated into semiconductor devices enabling studies<br />

<strong>of</strong> properties impossible to achieve in atom particles. These phenomena are related to<br />

a possibility <strong>of</strong> controllable feeding the dots with charge carriers one by one: an effect<br />

resulting from a Coulomb blockade. Such deterministic charging not only allows to study<br />

effects related to Coulomb interaction between carriers, but has proved to be essential<br />

in discoveries <strong>of</strong> phenomena requiring a particular QD charge state: creation <strong>of</strong> dynamic<br />

nuclear polarization, coherent qubit operations, creation <strong>of</strong> Mahan excitons, to name just<br />

a few.<br />

Most studies on charge tunability <strong>of</strong> QDs were done in InGaAs system. On the other<br />

hand, II - VI QDs possess a few interesting advantages over their better known counterparts:<br />

they can be easily doped with magnetic ions and exhibit stronger excitonic<br />

interactions. Combining these features with a possibility to prepare a II - VI QD in<br />

a given charge state can result for instance in electrical control over magnetism on a<br />

nanoscale.<br />

In this talk, we report on our recent achievements in studies <strong>of</strong> charging effects in<br />

self assembled CdTe QDs [1]. By studying QD photoluminescence (PL), we show charge<br />

tunability in dots embedded in a field-effect structure. Exploiting the vertical electric<br />

field resulting from biasing the device, we study the quantum confined Stark effect to<br />

gain insight into Coulomb interactions and changes <strong>of</strong> charge distribution upon tuning<br />

the QD occupancy. In particular, we find that the electron wave function is stiffer than the<br />

hole wave function – an effect which we subsequently confirm with analysis <strong>of</strong> PL lifetimes<br />

<strong>of</strong> different QD occupancies. We demonstrate that the confinement in our dots is far from<br />

the strong confinement limit. This in turn results in Coulomb correlations dominating<br />

over confinement. A fingerprint <strong>of</strong> this phenomenon is a universal shape <strong>of</strong> the QD PL<br />

spectrum featuring the same transition sequence – an effect entirely different from the<br />

InGaAs system, where confinement conditions totally determine the PL spectrum. We<br />

conclude with presenting an outlook for future studies involving magnetically doped dots<br />

and different approaches for extending the tunability in this system.<br />

[1] Ł. Kłopotowski et al. Phys. Rev. B 83, 155319 (<strong>2011</strong>).<br />


MoI4 _______________________________________________________________<br />

40th "Jaszowiec" <strong>2011</strong> International School and Conference on the Physics <strong>of</strong> Semiconductors<br />

Optical properties <strong>of</strong> strongly in-plane asymmetric epitaxial nanostructures<br />

Grzegorz Sęk<br />

Institute <strong>of</strong> Physics, Wrocław University <strong>of</strong> Technology<br />

Wybrzeże Wyspiańskiego 27, 50-370 Wrocław, Poland<br />

Strongly laterally asymmetric nanostructures have already been proven advantageous in<br />

both laser applications, <strong>of</strong>fering higher gain and broader spectral tunability, or fundamental<br />

studies <strong>of</strong> solid state quantum electrodynamics (QED), due to enhanced exciton oscillator<br />

strength. On the other hand, such structures are believed to have selective and geometrydriven<br />

polarization properties. However, our spectroscopic measurements on self-assembled<br />

structures <strong>of</strong> two material systems but similar geometries (lateral aspect ratio <strong>of</strong> about 3-5)<br />

demonstrated significantly different properties.<br />

For InAs elongated structures (called dashes) on InP substrate the linear-polarizationresolved<br />

photoluminescence measurements have shown the degree <strong>of</strong> polarization (DOP) at<br />

the level <strong>of</strong> 30% for high excitation conditions and/or at elevated temperatures. A systematic<br />

increase <strong>of</strong> the DOP has been observed for the enlarged dash height and eventually exceeded<br />

90% for very tall structures called columnar quantum dashes. The dependence, tending to<br />

saturate for larger heights, has been reproduced theoretically by both analytical considerations<br />

assuming the heavy - light hole bands mixing as the primary factor responsible for the<br />

polarization properties and by full 8 band kp calculations including strain. Unexpectedly<br />

however, low temperature single dot study revealed exciton to biexciton lifetimes ratio <strong>of</strong><br />

about 2 and biexciton binding energy below 0.5 meV, both together characteristic for the<br />

strong confinement regime. In addition, rather small exciton fine structure splitting in the<br />

range <strong>of</strong> 60 – 200 µeV has been detected, comparable to values reported for common selfassembled<br />

dots. All these facts suggest the occurrence <strong>of</strong> an additional exciton localization<br />

within the structures, e.g. on local size or content fluctuations, causing a decreased<br />

confinement anisotropy. This has further been supported by an observed decrease <strong>of</strong> the DOP<br />

down to about 10% at low temperatures and low excitation. The calculated temperature<br />

dependence <strong>of</strong> the DOP reproduced this feature very well after taking into account a<br />

fluctuation <strong>of</strong> the dash cross-sectional size. The latter confirmed that the low temperature<br />

emission originates from excitons trapped on potential fluctuations with the localization<br />

energy <strong>of</strong> about 5 meV.<br />

Quite an opposite behavior has been found for In0.3Ga0.7As/GaAs extended quantum<br />

dots. On one hand, single dot study, QED experiments on a dot inside a pillar microresonator<br />

and time-resolved spectroscopy have shown large oscillator strengths and weakened quantum<br />

confinement manifested in a small value <strong>of</strong> the exciton to biexciton lifetime ratio (below 1)<br />

and a decreased (as compared to standard quantum dots) exciton radiative lifetime (~ 300 ps).<br />

Further, we observed no fingerprints <strong>of</strong> any localization effects in the polarization-resolved<br />

experiments versus temperature and excitation, which agrees with a very smooth morphology<br />

<strong>of</strong> these structures when compared to the InP-based ones. The DOP is only about 6% despite<br />

the shape asymmetry and similar light hole contribution to valence band states as in InP-based<br />

structures (based on 8 band kp calculations). This is caused by a shallow electron confining<br />

potential making them (and hence the entire exciton) to experience less anisotropic<br />

confinement potential. Furthermore, exciton localization within the wetting layer enhancing<br />

their radiative lifetime to almost 1 ns has been observed. This appeared to be responsible for<br />

strong temperature sensitivity <strong>of</strong> the wetting layer - quantum dot energy transfer efficiency<br />

and the appearance <strong>of</strong> a low-energy sideband emission accompanying the biexciton, being a<br />

fingerprint <strong>of</strong> its interaction with excitons from the wetting layer.<br />


40th _______________________________________________________________<br />

"Jaszowiec" <strong>2011</strong> International School and Conference on the Physics <strong>of</strong> Semiconductors MoO1<br />

Coupling between electronic and vibrational excitations in<br />

carbon nanotubes filled with C60 fullerenes<br />

Pawel Utko 1 , Raffaello Ferone 2,3 , Ilya V. Krive 3,4 , Robert I. Shekhter 3 , Mats<br />

Jonson 3,5,6 , Marc Monthioux 7 , Laure Noé 7 , and Jesper Nyg˚ard 1<br />

1 Niels Bohr Institute & Nano-Science Center, University <strong>of</strong> Copenhagen,<br />

DK-2100 Copenhagen, Denmark<br />

2 Department <strong>of</strong> Physics, Lancaster University, Lancaster LA1 4YB, UK<br />

3 Department <strong>of</strong> Physics, University <strong>of</strong> Gothenburg, SE-412 96 Göteborg, Sweden<br />

4 B. I. Verkin Institute for Low Temperature Physics and Engineering,<br />

61103 Kharkov, Ukraine<br />

5 School <strong>of</strong> Engineering and Physical Sciences, Heriot-Watt University,<br />

Edinburgh EH14 4AS, UK<br />

6 School <strong>of</strong> Physics, Konkuk University, Seoul 143-701, Korea<br />

7 CEMES-CNRS, B.P. 94347, F-31055 Toulouse Cedex 4, France<br />

Filling single wall carbon nanotubes with C60 fullerenes yields hybrid structures [1],<br />

the so-called C60 fullerene peapods, which can <strong>of</strong>fer enhanced functionality with respect<br />

to empty tubes. Prospective nanoelectronic applications <strong>of</strong> such composites include data<br />

storage devices, single electron transistors and spin-qubit arrays for quantum computing.<br />

However, the extent to which the electronic properties <strong>of</strong> peapods are affected by<br />

entrapped fullerenes is still unclear. Even though theoretical investigations predict dramatic<br />

changes in the electronic band structure <strong>of</strong> the nanotube, the experimental results<br />

remain equivocal.<br />

Here, we investigate the effect <strong>of</strong> the C60 fullerenes on the low-temperature electron<br />

transport via peapod quantum dots [2] operated in the regime <strong>of</strong> weak electronic coupling<br />

Γ between energy states <strong>of</strong> the dot and its electrical leads. Compared with empty nanotubes,<br />

we find an abnormal temperature dependence <strong>of</strong> Coulomb blockade oscillations,<br />

indicating the presence <strong>of</strong> a nanoelectromechanical coupling between electronic states <strong>of</strong><br />

the nanotube and mechanical vibrations <strong>of</strong> fullerenes. This provides a useful method to<br />

detect the C60 presence and to probe the interplay between electrical and mechanical<br />

excitations in peapods, which thus emerge as a new class <strong>of</strong> nanoelectromechanical systems.<br />

We note that other types <strong>of</strong> devices for which a coupling to mechanical vibrations<br />

plays a crucial role in electrical transport are suspended nanotubes, as well as molecular<br />

conductors and transistors.<br />

[1] B. W. Smith, M. Monthioux, and D. E. Luzzi, Nature 396, 323 (1998).<br />

[2] P. Utko, R. Ferone, I. V. Krive, R. I. Shekhter, M. Jonson, M. Monthioux, L. Noé,<br />

and J. Nyg˚ard, Nature Commun. 1, 37 (2010).<br />


MoO2 _______________________________________________________________<br />

40th "Jaszowiec" <strong>2011</strong> International School and Conference on the Physics <strong>of</strong> Semiconductors<br />

Solid-state self assembly: a route to hybrid metal-semiconductor epitaxial<br />

nanostructures<br />

Adam Urba�czyk, Frank W.M. van Otten and Richard Nötzel<br />

COBRA Research Institute, Departament <strong>of</strong> Applied Physics, Eindhoven Univeristy <strong>of</strong><br />

Technology, The Netherlands<br />

�<br />

�<br />

Recently there has been a lot <strong>of</strong> research interest in metallic nanostructures for<br />

confining light at deeply subwavelength dimensions utilizing surface plasmons. Such<br />

plasmonic nanostructures are currently finding applications in fields <strong>of</strong> sensing and energy<br />

conversion. Combining them with optically active media such as semiconductor quantum dots<br />

(QDs) could lead to even more exciting applications in the field <strong>of</strong> nanophotnics and<br />

nanoelectronics, where the metal-QD hybrid structures could be used as ultrasmall light<br />

sources or switching elements. In order to obtain such hybrid structures in reproducible and<br />

scalable manner one has to control the metal-QD separation with single nanometer precision<br />

owing to extreme light confinement due to surface plasmon resonance (SPR). Recently our<br />

group demonstrated a promising approach to solve this problem by utilizing solid-state selfassembly.<br />

Here we report alignment <strong>of</strong> epitaxial nanocrystals <strong>of</strong> Ag and In on isolated InAs<br />

QDs and one dimensional QD arrays obtained by molecular beam epitaxy (MBE). By<br />

changing the process conditions the SPR wavelength <strong>of</strong> the metal nanocrystals can be easily<br />

tuned over wide range in order to match the emission wavelength <strong>of</strong> the QDs.<br />

Photoluminescence (PL) measurements reveal large enhancement <strong>of</strong> the emitted light<br />

intensity as compared to a reference structure without metal. What is more, in those hybrid<br />

nanostructures PL polarization is modified, evidencing electromagnetic character <strong>of</strong> the<br />

interaction between the metal nanocrystals and the QDs.<br />

�<br />

Figure 1. Atomic force microscope image <strong>of</strong> a Ag nanocrystas aligned on a single near-surface<br />

InAs QD and a reference structure with bare near surface QDs.<br />


40th _______________________________________________________________<br />

"Jaszowiec" <strong>2011</strong> International School and Conference on the Physics <strong>of</strong> Semiconductors MoO3<br />

First-Principles Study <strong>of</strong> Doped III–V nanowires<br />

M. Galicka 1 , P. Kacman 1 and R. Buczko 1<br />

1 Institute <strong>of</strong> Physics PAS, al. Lotników 32/46, 02-668 Warsaw, Poland<br />

GaAs and InAs nanowires (NWs) are considered as one <strong>of</strong> the most interesting structures for<br />

nanoelectronics and nanophotonics. Their potential application in novel electronic devices<br />

requires, however, controllable p-type and n-type conductivity. The specific growth conditions<br />

for NWs, their crystal structure and orientation <strong>of</strong> their side facets lead sometimes to different<br />

incorporation <strong>of</strong> dopants than known for planar layers. For example, GaAs epitaxial layers are<br />

typically p-type doped with Be and n-type doped with Si ions. In contrast, in GaAs NWs<br />

although Si doping leads sometimes to n-type, but most <strong>of</strong>ten p-type conductivity is observed<br />

in Si-doped GaAs NWs [1,2]. The problem <strong>of</strong> doping <strong>of</strong> NWs has become now a major<br />

challenge to the growers <strong>of</strong> one-dimensional III-V semiconductor structures.<br />

To explain the observed phenomena we study theoretically the properties <strong>of</strong> GaAs and<br />

InAs NWs, in which one cation/anion is substituted by a dopant ion. Since the III-V<br />

semiconductor NWs can grow in both, zinc-blende (ZB) and wurtzite (WZ) structures, we<br />

check whether the crystal structure <strong>of</strong> the wire has an impact on the doping level and the<br />

distribution <strong>of</strong> impurities in the NW and, most important, on the electronic properties <strong>of</strong> the<br />

doped NWs. The calculations show that the distribution <strong>of</strong> impurities in the wires as well as<br />

the conductivity depend crucially on the crystal structure.<br />

We consider ZB NWs oriented along (111) axis and WZ NWs along (0001) axis. These<br />

growth directions are chosen because they are the most energetically preferred for both GaAs<br />

and InAs NWs [3]. Using ab initio methods based on the density functional theory we have<br />

calculated the formation and segregation energies for GaAs and InAs NWs doped with<br />

different ions. In GaAs NWs, we consider Be atom, which substitutes a cation and should lead<br />

to p-type conductivity and Si atom which substitutes either anion (p-type) or cation (n-type).<br />

The formation energies for Si in GaAs NWs calculated for various sets <strong>of</strong> chemical potentials<br />

suggest that the energy needed for substituting the anion and the cation, and thus the type <strong>of</strong><br />

conductivity obtained by Si-doping, can be different in ZB and WZ wires. As-grown InAs<br />

NWs have usually high concentration <strong>of</strong> electrons in the conduction band. Thus, in InAs NWs<br />

we show, by comparing the formation energies, that among Be, Zn or Si impurities, which can<br />

be used to obtain p-type wires, Be is preferred.<br />

The segregation energy shows where the dopants prefer to stay – it is defined as the<br />

difference between the energy <strong>of</strong> a NW with the impurity ion in a given position in the wire<br />

and in its center. It has been obtained that in ZB structures the impurities prefer to stay at the<br />

side surfaces <strong>of</strong> the NW. In WZ NWs the positions inside the wire are preferred. For all kinds<br />

<strong>of</strong> impurities studied, the segregation energy in WZ NWs is much smaller than in ZB NWs,<br />

thus, the distribution <strong>of</strong> impurities in WZ wires should be much more homogenous.<br />

To study the electronic properties we have calculated the density <strong>of</strong> states <strong>of</strong> the doped<br />

NWs. In contrast to the formation and segregation energies, which have been calculated for<br />

wires with the surfaces not saturated by foreign ions, the density <strong>of</strong> states has been obtained<br />

for wires with side surfaces fully passivated by hydrogen atoms.<br />

[1] M. Hilse et al., Appl. Phys. Lett. 96, 193104 (2010)<br />

[2] J. Dufouleur et al., Nano Lett. 10, 1734 (2010)<br />

[3] M. Galicka, et al., J. Phys: Condens. Matter 20, 454226 (2008)<br />

Work supported by EC network SemiSpinNet (PITN-GA-2008-215368).<br />


MoO4 _______________________________________________________________<br />

40th "Jaszowiec" <strong>2011</strong> International School and Conference on the Physics <strong>of</strong> Semiconductors<br />

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∗ �Email: franz.schuster@cea.fr<br />


40th _______________________________________________________________<br />

"Jaszowiec" <strong>2011</strong> International School and Conference on the Physics <strong>of</strong> Semiconductors MoO5<br />

Growth <strong>of</strong> optically active CdTe quantum dots in ZnTe nanowires<br />

�<br />

P. Wojnar 1 , E. Janik 1 , S. Kret 1 , A. Petrouchik 1 , M. Goryca 2 ,<br />

T. Kazimierczuk 2 , P. Kossacki 2 , G. Karczewski 1 and T. Wojtowicz 1<br />

�<br />

1 Institute <strong>of</strong> Physics, Polish Academy <strong>of</strong> Sciences, Al Lotników 32/46, 02-668 Warsaw, Poland<br />

2 Institute <strong>of</strong> Experimental Physics, University Warsaw, ul Ho�a 69, 00-681 Warsaw, Poland<br />

Quantum dots consisting <strong>of</strong> nanometer sized insertions <strong>of</strong> low energy gap semiconductor in large<br />

energy gap nanowire have recently risen a great interest because <strong>of</strong> their emerging applications as<br />

single photon sources at room temperature [1], biological markers, nanobarcodes [2] and also in view<br />

<strong>of</strong> electronic coupling <strong>of</strong> several quantum dots.<br />

In this work the growth and optical properties <strong>of</strong> CdTe<br />

quantum dots in ZnTe nanowires are reported. The structure is<br />

grown by molecular beam epitaxy in the vapor-liquid-solid<br />

growth mechanism induced by gold catalysts. Nanometer sized<br />

droplets <strong>of</strong> gold/gallium eutectic are formed on GaAs substrate<br />

by thermal treatment <strong>of</strong> a 1 nm thick gold layer deposited in a<br />

separate growth process. Subsequently, ZnTe nanowires with<br />

the length <strong>of</strong> about 1.5 �m are grown at relatively high<br />

substrate temperature <strong>of</strong> 420°C. For the deposition <strong>of</strong> the CdTe<br />

insertion, the temperature is decreased to 380°C, which is still<br />

a too high temperature for the epitaxial growth and, therefore,<br />

ensures that CdTe growth occurs in the axial direction <strong>of</strong> the<br />

nanowire. Deposition time <strong>of</strong> CdTe is set to 60s. The further<br />

growth <strong>of</strong> 200 nm ZnTe nanowire segment takes place at<br />

380°C.<br />

Photoluminescence measurements performed at 5 K show<br />

clearly that the presence <strong>of</strong> CdTe insertions in the ZnTe<br />

nanowires results in the appearance <strong>of</strong> a strong emission in the<br />

2.0 eV – 2.2 eV energy region. When the excitation spot<br />

Figure Low temperature micro-<br />

photoluminescence <strong>of</strong> the ZnTe<br />

nanowires with CdTe insertions –<br />

(a) “as grown” sample - several objects<br />

excited simultaneously (b) nanowires<br />

dispersed on silicon – only one<br />

emission line present, 9 K, excitation:<br />

532 nm (2.33 eV), 100 �W<br />

diameter is reduced to the size <strong>of</strong> the order <strong>of</strong> 1 �m, this broad<br />

band splits into several sharp lines with the spectral width <strong>of</strong><br />

the order <strong>of</strong> 2 meV, figure a. Those lines are related to the<br />

emission from individual quantum dots in nanowires as<br />

confirmed by our further study. In order to better resolve the<br />

individual emission lines, the nanowires are removed from the<br />

GaAs substrate in a ultrasonic methanol bath and dispersed on<br />

a silicon substrate, figure b.<br />

The emission lines exhibit a relatively large degree <strong>of</strong> linear<br />

polarization ranging from 70% to 95% depending on the particular line. This effect is due to large<br />

contrast <strong>of</strong> the dielectric constant <strong>of</strong> the nanowire and the surrounding and, thus, confirms that the<br />

emissions come from an object placed inside a nanowire. On the other hand, photon correlation<br />

measurements prove the zero dimensional character <strong>of</strong> studied objects. A clear antibunching at the zero<br />

delay time between arriving <strong>of</strong> two subsequent photons from the same emission line is observed<br />

confirming that these individual objects cannot emit simultaneously two photons at the same energy.<br />

Moreover, the power dependence <strong>of</strong> the emission and the cross-correlation measurements allow us to<br />

indentify biexcitonic emissions with binding energies ranging from 7 - 12 meV<br />

The research has been supported by the European Union within European Regional Development Fund through Innovative<br />

Economy grant (POIG.01.01.02-00-008/08) and “support for the creation <strong>of</strong> joint research infrastructure <strong>of</strong> scientific<br />

institutions.” - POIG.02.02.00-00-003/08<br />

[1] A. Tribu et al., NanoLett., 8, 4326 (2008)<br />

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MoO6<br />

_______________________________________________________________<br />

40th "Jaszowiec" <strong>2011</strong> International School and Conference on the Physics <strong>of</strong> Semiconductors<br />

Electronic properties <strong>of</strong> graphite<br />

J. M. Schneider 1,2 , B. A. Piot 2 , I. Sheikin 2 , N. A. Goncharuk 3 , P. Vaˇsek 3 , P. Svoboda 3 , Z. V´yborn´y 3 , L. Smrčka 3 ,<br />

M. Orlita 2 , M. Potemski 2 , and D. K. Maude 2<br />

1 Instituto de Física, Universidade de São Paulo, PB 66318, 05315-970 São Paulo, SP, Brazil<br />

2 Grenoble High Magnetic Field Laboratory, Centre National de la Recherche Scientifique, 38042 Grenoble, France<br />

3 Institute <strong>of</strong> Physics, Academy <strong>of</strong> Science <strong>of</strong> the Czech Republic, 162 53 Prague 6, Czech Republic<br />

Despite more than 50 years <strong>of</strong> reserach, graphite is not yet fully understood. From the viewpoint <strong>of</strong> graphene, a<br />

single layer <strong>of</strong> carbon atoms set in a honeycomb lattice, graphite can be considered as a quasi-two-dimensional (2D)<br />

system consisting <strong>of</strong> a macroscopic stack <strong>of</strong> graphene layers. A fundamental question concerns the link between<br />

the coupling <strong>of</strong> the graphene layers and the dimensionality <strong>of</strong> the electronic system in graphite. According to the<br />

Slonczewski, Weiss, and McClure (SWM) band structure calculations <strong>of</strong> graphite the weak coupling leads to the form<br />

<strong>of</strong> the in-plane dispersion depending upon the momentum kz in the direction perpendicular to the layers. The carriers<br />

occupy a region along the H-K-H edge <strong>of</strong> the hexagonal Brillouin zone. The Fermi surface <strong>of</strong> graphite reveals two<br />

extremal majority charge carriers cross sections: Electrons at the K point (kz = 0) and holes at kz = 0.35. For both<br />

types <strong>of</strong> charge carriers the in-plane dispersion is parabolic (massive fermions). Only at the H point (kz = 0.5) the<br />

in-plane dispersion is linear, similar to that <strong>of</strong> charge carriers in graphene (massless Dirac fermions). However, the<br />

extremal cross section at the H point is negligible (minority charge carriers) and does not contribute significantly to<br />

the electronic properties <strong>of</strong> graphite.<br />

This work represents a review <strong>of</strong> recent studies <strong>of</strong> the electronic properties <strong>of</strong> graphite using the Shubnikov-de Haas<br />

(SdH) and the de Haas -van Alphen (dHvA) effect at millikelvin temperatures [1, 3–5]. Because <strong>of</strong> the low temperatures<br />

used, both the SdH and the dHvA effect are much richer than previously published data. Quantum oscillations<br />

are observed for both majority electrons and holes with an orbital quantum number up to almost N = 100. The<br />

high quality <strong>of</strong> the data allows a detailed investigation <strong>of</strong> the fine structure <strong>of</strong> the Fermi surface [1]. Using both the<br />

semiclassical model and the full magnetic field Hamiltonian we show that these oscillations are fully consistent with<br />

the three-dimensional (3D) SWM bandstructure calculations <strong>of</strong> graphite. At magnetic fields B > 2 T, a significant<br />

deviation from the 1/B periodicity occurs. A self-consistent calculation <strong>of</strong> the Fermi energy shows that the deviation<br />

from the 1/B periodicity is caused by a considerable movement <strong>of</strong> the Fermi energy when the quantum limit is approached.<br />

This seriously questions the validity <strong>of</strong> using the high-field data to extract the phase <strong>of</strong> the Shubnikov-de<br />

Haas oscillations and hence the nature <strong>of</strong> the charge carriers [2, 3]. The detailed information <strong>of</strong> the band structure<br />

<strong>of</strong> graphite and the movement <strong>of</strong> the Fermi energy in magnetic field allows to examine the spin splitting <strong>of</strong> quantum<br />

features. We show that an effective g-factor value <strong>of</strong> g ∗ = 2.5 is required to explain the data [4].<br />

We performed dHvA measurements to make a complete map <strong>of</strong> the Fermi surface <strong>of</strong> graphite. For tilt angles θ < 60 ◦ ,<br />

the fundamental frequencies <strong>of</strong> both the electrons and holes follow almost exactly a 1/cos(θ) dependence (quasi-2D<br />

behavior). For θ > 60, a clear deviation from the quasi-2D behavior is observed. The deviation is a signature <strong>of</strong><br />

the dispersion along kz and unequivocally demonstrates the 3D character <strong>of</strong> the Fermi surface which arises from the<br />

coupling between the graphene layers. Furthermore, the observation <strong>of</strong> quantum oscillations at θ = 90 ◦ is a direct<br />

pro<strong>of</strong> that the Fermi surface <strong>of</strong> graphite is closed. Additionally, the θ = 90 ◦ data reveals minority holes with a Dirac<br />

like energy spectrum much like as for charge carriers in graphene [5].<br />

References<br />

[1] J. M. Schneider, Phys. Rev. Lett. 102, 166403 (2009).<br />

[2] I. A. Luk’yanchuk and Y. Kopelevich, Phys. Rev. Lett. 97, 256801 (2006).<br />

[3] J. M. Schneider, Phys. Rev. Lett. 104, 119702 (2010).<br />

[4] J. M. Schneider et al, Phys. Rev. B 81, 195204 (2010).<br />

[5] J. M. Schneider, to be submitted to Phys. Rev. Lett.<br />


40th _______________________________________________________________<br />

"Jaszowiec" <strong>2011</strong> International School and Conference on the Physics <strong>of</strong> Semiconductors MoP1<br />

Nonlocal transport in PbTe/PbEuTe microstructures<br />

K. A. Kolwas 1 , G. Grabecki 1,2 , S. Trushkin 1 , Ł. Cywiński 1 , M. Aleszkiewicz 1 ,<br />

T. Dietl 1,3 , G. Springholz 4 , G. Bauer 4<br />

1 <strong>Instytut</strong> <strong>Fizyki</strong> <strong>PAN</strong>, al. Lotnikow 32/46, PL-02-668 Warszawa, Poland<br />

2 Wydział Matematyczno-Przyrodniczy, Szkoła Nauk Ścisłych, UKSW, ul. Wóycickiego 1/3,<br />

PL 01-938 Warszawa, Poland<br />

3 <strong>Instytut</strong> <strong>Fizyki</strong> Teoretycznej, UW, ul.Hoża 69 PL 00-681 Warszawa, Poland<br />

4 Institut für Halbleiter-und-Festkörperphysik, Johannes Kepler University,<br />

Altenbergerstr. 69, A-4040 Linz, Austria)<br />

Heterostructures <strong>of</strong> narrow-gap semiconductors have recently received a renewed attention, due to the discovery<br />

<strong>of</strong> Quantum Spin Hall phase in HgTe/HgCdTe quantum wells [1], the fingerprint <strong>of</strong> which is the presence <strong>of</strong><br />

large quantized nonlocal voltage signals due to the edge currents [2]. However, even in the topologically trivial<br />

phase, a large nonlocal voltage signal has been seen in Hall bar structures <strong>of</strong> HgTe [3] due to the presence <strong>of</strong><br />

Spin Hall Effect (SHE) in the ballistic regime [4].<br />

Heterostructures <strong>of</strong> IV-VI compounds (Pb,Eu,Sn)Te are another attractive candidates in which one can<br />

search for physics related to the crucial influence <strong>of</strong> the spin-orbit coupling on the electronic structure <strong>of</strong><br />

relevant bands. In contrast to HgTe case, there is no inversion asymmetry (because <strong>of</strong> rock-salt crystal structure)<br />

and the Dresselhaus splitting is absent. The huge dielectric constant <strong>of</strong> PbTe (e0 > 1000) makes it an ideal<br />

material in which quantum ballistic transport effects can be observed [5].<br />

We have fabricated, by means <strong>of</strong> the e-beam<br />

lithography techniques, multi-probe quantum wires (see<br />

Fig. 1 inset) based on an MBE-grown PbTe quantum<br />

well (QW), <strong>of</strong> the thickness <strong>of</strong> 12 nm, embedded<br />

between<br />

Pb1-xEuxTe barriers, deposited on BaF2 substrates. The<br />

wires were equipped with side gates for tuning the<br />

electron concentration. The wire widths W were as<br />

narrow as 1 mm, and their lengths L = 5 mm. The electron<br />

mobility in the initial QW was 5 m 2 /Vs, which<br />

corresponds to the electron mean free path le = 3 mm.<br />

Therefore, W < le < L , and our structures were in quasiballistic<br />

regime. The application <strong>of</strong> negative voltage to<br />

the gates has led to the total depletion <strong>of</strong> the wires. The<br />

measured resistance was then above 10 7 Ohms.<br />

The low-temperature transport measurements have<br />

shown characteristic negative magnetoresistance maxima<br />

which were almost temperature independent in the range<br />

Fig. 1: Local magnetoresistance <strong>of</strong> structure T < 20 K (Fig. 1). We assign it to the suppression <strong>of</strong> the<br />

shown in the inset.<br />

electron non-specular backscattering by the wire<br />

boundaries [6].<br />

Nonlocal resistance measurements have been performed in order to search for the signatures <strong>of</strong> the SHE and<br />

the inverse SHE [3, 4]. The obtained signal at B = 0 is at least one order <strong>of</strong> magnitude larger than the value<br />

expected from the classical van der Pauw formula. However, it is suppressed by the magnetic field with a<br />

dependence distinctly correlated with the local magnetoresistance. This suggests that the nonlocal resistance may<br />

be related to non-specular boundary scattering in our wires.<br />

Acknowledgments: This work was supported by Ministry <strong>of</strong> Science (Poland) under Grant<br />

No. 1247/B/H03/2008/35 and under Iuventus Plus grant No. IP2010 006070.<br />

References<br />

[1] Markus König et al., Science 318, 766 - 770 (2007).<br />

[2] Andreas Roth et al., Science 325, 294 - 297 (2009).<br />

[3] C. Brune et al., Nature Physics 6, 448 - 454 (2010).<br />

[4] E. M. Hankiewicz et al., Phys. Rev. B 70, 241301 (2004).<br />

[5] G. Grabecki et al., Phys. Rev. B 72, 125332 (2005).<br />

[6] C. W. J Beenakker and H. van Houten, Cond-Mat/0412664 (2004).<br />


MoP2 _______________________________________________________________<br />

40th "Jaszowiec" <strong>2011</strong> International School and Conference on the Physics <strong>of</strong> Semiconductors<br />

Optical transitions in PbTe/CdTe quantum wells grown by molecular beam<br />

epitaxy on GaAs (001) and BaF2 (111) substrates<br />

M. Szot,* L. Kowalczyk, E. Smajek, B. Taliashvili, P. Dziawa, W. Kn<strong>of</strong>f, A. Reszka,<br />

V. Domukhovski, E. Łusakowska, P. Dłu ewski, M. Wiater, T. Wojtowicz, T. Story,<br />

M. Bukała, R. Buczko, P. Kacman<br />

Institute <strong>of</strong> Physics, Polish Academy <strong>of</strong> Sciences, Al. Lotników 32/46, 02-668 Warsaw, Poland<br />

IV-VI narrow band gap semiconductors PbTe, PbSe, and PbS constitute materials system<br />

alternative to III-V semiconductors in the area <strong>of</strong> mid-infrared lasers and detectors. The<br />

attractiveness <strong>of</strong> PbTe/CdTe heterosystem results, inter alia, from a very strong confinement<br />

<strong>of</strong> electrons in PbTe caused by large difference in the energy gaps <strong>of</strong> these semiconductors<br />

( Eg=1.2 eV at 300 K) and from low Auger recombination rate in PbTe. Additionally, PbTe<br />

and CdTe exhibit excellent matching <strong>of</strong> their lattice parameters at room temperature, even<br />

tough they crystallize in different cubic lattices: zinc blende (a0=6.48 Å) for CdTe and rock<br />

salt (a0=6.46 Å) for PbTe. Such features are strongly desirable for efficient optoelectronic<br />

devices operating at room temperature. In this paper, we undertake experimental and<br />

theoretical studies <strong>of</strong> ground-state photoluminescence (PL) from PbTe/CdTe quantum wells<br />

grown along different crystal directions by molecular beam epitaxy (MBE).<br />

The PbTe/CdTe QWs were grown by the MBE on (001) oriented GaAs and (111) oriented<br />

BaF2 substrates with thick (1÷4 µm) high quality CdTe buffer layers. The structural quality <strong>of</strong><br />

the heterostructures was examined by x-ray diffraction (XRD) and cross-sectional TEM and<br />

SEM electron microscopy techniques. The PbTe/CdTe heterostructures with the best crystal<br />

quality were obtained for the growth at low temperatures (about 260 °C). The XRD rocking<br />

curve width parameter below 200 arcsec was found for both PbTe and CdTe. For multiple-<br />

QWs diffraction satellites up 7 th order were observed. The single- and multiple-QWs (3<br />

repetitions <strong>of</strong> a basic PbTe/CdTe bilyer) were grown with the thicknesses <strong>of</strong> PbTe quantum<br />

wells varying from 4 to 20 nm and 60 nm thick CdTe barriers.<br />

The optical properties <strong>of</strong> the heterostructures were examined by the PL measurements using<br />

1064 nm line <strong>of</strong> YAG:Nd pulsed laser for excitation. For both (001) and (111) PbTe QWs we<br />

observed relatively narrow and strong PL which we attribute to the ground-state transitions in<br />

the PbTe well. The PL energy at T=4 K shifts from 310 meV to 460 meV for (001) PbTe<br />

QWs with thickness varying from 20 nm to 4 nm, respectively. For a given quantum well<br />

thickness <strong>of</strong> 12 nm, the PL energy observed for (111) PbTe QW grown on BaF2 is about 70<br />

meV lower than for (001) PbTe well. The observed blue shift <strong>of</strong> the PL line is in good<br />

agreement with our theoretical analysis <strong>of</strong> the influence <strong>of</strong> quantum size effect on the groundstate<br />

electronic transitions in the PbTe wells. Both studied growth directions were considered<br />

theoretically. Our calculations were performed within two methods: tight-binding and<br />

effective mass approximation. The dependence on band <strong>of</strong>fsets in the conduction and valence<br />

band in the type-I band alignment at the PbTe/CdTe interface was studied. The strong<br />

effective mass anisotropy and strain induced change <strong>of</strong> the band gap in PbTe were taken into<br />

account. The latter effect results from the large difference between PbTe and CdTe<br />

temperature expansion coefficients.<br />

This work was partially supported by the European Union within the European Regional<br />

Development Fund, through the Innovative Economy grant (POIG.01.01.02-00-108/09).<br />


40th _______________________________________________________________<br />

"Jaszowiec" <strong>2011</strong> International School and Conference on the Physics <strong>of</strong> Semiconductors MoP3<br />

Properties <strong>of</strong> three-dimensional topological insulators<br />

studied by microwave resonance spectroscopy<br />

A. Wolos, 1,2 A. Drabinska, 1 M. Kaminska, 1 G. Strzelecka, 3 A. Hruban, 3 A. Materna, 3<br />

M. Piersa, 3 Z. Wilamowski 2<br />

1 Faculty <strong>of</strong> Physics, University <strong>of</strong> Warsaw, ul Hoza 69, 00-681 Warsaw, Poland.<br />

2 Institute <strong>of</strong> Physics, Polish Academy <strong>of</strong> Sciences, Al. Lotnikow 32/46, 02-668 Warsaw,<br />

Poland.<br />

3 Institute <strong>of</strong> Electronic Materials Technology, ul. Wolczynska 133, 01-919 Warsaw, Poland.<br />

Bismuth selenide and telluride, as well as other related materials have been recently<br />

proposed as three-dimensional topological insulators with a single Dirac cone on the surface<br />

[1]. This class <strong>of</strong> materials is predicted to show an insulating bulk gap together with gapless<br />

surface states. Occurrence <strong>of</strong> the topological surface states has been confirmed by angleresolved<br />

photoemission spectroscopy [2]. There are, however, only a few reports in literature<br />

concerning experimental observation <strong>of</strong> electric transport in the conducting surface, owing to<br />

large number <strong>of</strong> bulk carriers (either electrons or holes) originating from crystal defects. In<br />

order to override presence <strong>of</strong> highly conductive bulk states shading electric properties <strong>of</strong> the<br />

surface, we propose to apply a contactless method basing on a standard electron spin<br />

resonance (ESR) spectrometery.<br />

In this communication we discuss results <strong>of</strong> microwave resonance experiments<br />

performed on Bi2Te3, Bi2Se3, and Bi2Te2Se crystals grown by vertical Bridgman method in<br />

Institute <strong>of</strong> Electronic Materials Technology, Warsaw. The experiment was performed using<br />

Bruker ELEXSYS E580 spectrometer operating in X- and Q-bands. Semiconducting samples<br />

placed in a cavity <strong>of</strong> a standard ESR spectrometer show, in addition to spin resonance spectra,<br />

also spectral features related to sample conductivity. Using this technique, we have earlier<br />

investigated Shubnikov-de Haas oscillations, cyclotron resonance or coupled plasmoncyclotron<br />

resonance in two dimensional electron gas (2DEG) in GaN-based heterostructures<br />

[3] and in Si-based quantum wells.<br />

First results obtained for bismuth selenide and telluride samples show a broad<br />

resonance line dependent only on the component <strong>of</strong> the external magnetic field perpendicular<br />

to the sample plane. This feature is characteristic for the cyclotron-related resonances in two<br />

dimensional electron gas. The relation <strong>of</strong> the observed resonance line to the topological<br />

surface states, together with parameters <strong>of</strong> the 2DEG (concentration and mobility) derived<br />

from the resonance will be discussed.<br />

[1] H. Zhang et al. Nature Phys. 5, 438 (2009).<br />

[2] Y. Xia et al. Nature Phys. 5, 398 (2009).<br />

[3] A. Wolos et al. Phys. Rev. B 76, 045301 (2007).<br />


MoP4 _______________________________________________________________<br />

40th "Jaszowiec" <strong>2011</strong> International School and Conference on the Physics <strong>of</strong> Semiconductors<br />

Analysis <strong>of</strong> Optical Properties and Pressure Dependence <strong>of</strong> the Energy Gap <strong>of</strong><br />

ZnO Layers, Bulk and Nano-Powders<br />

A. Duzynska 1,* , A. Kaminska 1 , H.Teisseyre 1,2 , E. Przezdziecka 1 , D. Dobosz 1 ,<br />

Z.R. Zytkiewicz 1 , A. Kozanecki 1 , J. D. Fidelus 1,2 , A. Durygin 3 , V. Drozd 3 , R. Hrubiak 3 ,<br />

A. Suchocki 1,4<br />

1 Institute <strong>of</strong> Physics, Polish Academy <strong>of</strong> Sciences, Al. Lotników 32/46, 02-668 Warsaw, Poland<br />

2 Institute <strong>of</strong> High Pressure Physics, Polish Academy <strong>of</strong> Sciences, Sokołowska 29/37, 01-142 Warsaw, Poland<br />

3 CeSMEC, Florida International University, University Park, Miami, FL 33199, USA<br />

4 Institute <strong>of</strong> Physics, University <strong>of</strong> Bydgoszcz, Weyssenh<strong>of</strong>fa 11, 85-072, Bydgoszcz, Poland<br />

* Corresponding author: A. Duzynska, e-mail address duzyn@ifpan.edu.pl<br />

Nowadays zinc oxide (ZnO) is a promising wide-band-gap semiconductor for<br />

optoelectronic applications, especially in the blue and ultraviolet spectral ranges. An important<br />

factor affecting mechanical, electric and optical properties <strong>of</strong> ZnO are methods/conditions <strong>of</strong><br />

growth as well as its crystal form.<br />

In this project we have studied pressure dependence <strong>of</strong> the near band gap<br />

photoluminescence (using the diamond anvil cell technique) <strong>of</strong> several various samples:<br />

- two zinc oxide thin layers grown by plasma-assisted molecular beam epitaxy (PA-MBE) on<br />

sapphire substrates (thickness <strong>of</strong> the layers: 1µm and 200 nm),<br />

- bulk ZnO crystal grown by vapor-phase deposition during industrial process <strong>of</strong> production <strong>of</strong><br />

zinc white,<br />

- two samples <strong>of</strong> hexagonal ZnO nano-powders (size <strong>of</strong> crystallite: 1µm and 65 nm) grown by<br />

plasma technique.<br />

We also investigated the stability and the volume behaviour under hydrostatic pressure by<br />

X-ray powder-diffraction <strong>of</strong> bulk crystal using synchrotron radiation. The values the bulk moduli<br />

(B0), and their pressure derivative (B0’), before and after phase transition from wurtzite (B4) to<br />

rock salt (B1) structure were obtained from these measurements. We found that for wurtzite<br />

structure B0 is equal to (156 ± 13) GPa and for rock salt phase B0 = (180 ± 59) GPa. In zinc oxide<br />

the B4-B1 transition has been earlier reported at ~ 10 GPa. However, the change <strong>of</strong> nanoscale<br />

grain size has a huge influence on the characteristic <strong>of</strong> phase transition. We supposed that in the<br />

case <strong>of</strong> nano-powders a tetragonal intermediate (iT) appears at the B4/B1 interface and a<br />

hexagonal intermediate (iH) is also showed as an effect <strong>of</strong> axial compression [1]. These results<br />

are visible in our experimental data <strong>of</strong> pressure dependence <strong>of</strong> the energy gap EG (≈ EPL),<br />

especially for nano-powder with 65 nm crystallite size. Moreover experimental band gap<br />

pressure coefficients for bulk crystal and films grown on sapphire are about 20 meV/GPa, but for<br />

nano-powders are lower, about 15 meV/GPa. This effect will be also discussed.<br />

The photoluminescence is quenched at about 10 GPa for ZnO bulk materials as results <strong>of</strong><br />

transition from direct to indirect gap in the rock salt structure. Nonetheless for smaller size nanopowder<br />

the photoluminescence vanishes at 20 GPa. The origin <strong>of</strong> this phenomenon will be<br />

analysed in this work.<br />

References:<br />

1. S. E. Boulfelfel and S. Leoni, Phys. Rev. B. 78, 125204 (2008).<br />

Acknowledgement: This work was partly supported by the European Union within European Regional Development<br />

Fund, through the Innovative Economy grants (POIG.01.01.02-00-008/0 and POIG.01.01.02-00-108/2009/2) and<br />

the project no N N202 010134 <strong>of</strong> Polish Ministry <strong>of</strong> Science and Higher Education.<br />


40th _______________________________________________________________<br />

"Jaszowiec" <strong>2011</strong> International School and Conference on the Physics <strong>of</strong> Semiconductors MoP5<br />

Defect structure <strong>of</strong> two-dimensional Wigner crystals<br />

A. Radzvilavičius and E. Anisimovas<br />

Department <strong>of</strong> Theoretical Physics, Vilnius University, Saul˙etekio al. 9-III, LT-10222<br />

Vilnius, Lithuania<br />

As the density and temperature <strong>of</strong> the electron gas is lowered beyond a certain critical<br />

value, the potential Coulomb energy dominates the kinetic energy and particles tend to<br />

crystallize. The lowest energy structure <strong>of</strong> a two-dimensional system thus corresponds to<br />

the perfect hexagonal Wigner lattice [1]. Finite-size clusters, however, are usually formed<br />

due to the presence <strong>of</strong> an external confinement. In many experimental realizations, such<br />

structures are trapped by a symmetric parabolic potential; these include, but are not<br />

limited to, micro-metric charged spheres in low-temperature dusty plasmas and electrons<br />

in semiconductor quantum dots [2]. The structural properties <strong>of</strong> finite two-dimensional<br />

Wigner clusters strongly depend on the system size and energetic state.<br />

In the present contribution, we report the results <strong>of</strong><br />

the numerical Monte Carlo and minima hopping [3] experiments,<br />

and analyze the structural properties <strong>of</strong> twodimensional<br />

Wigner clusters, consisting <strong>of</strong> classical point<br />

charges, confined by an external parabolic potential trap.<br />

Systems <strong>of</strong> up to N = 1000 particles are considered.<br />

Thenumber<strong>of</strong>metastableconfigurations,corresponding<br />

to the local minima <strong>of</strong> the potential energy, grows<br />

very rapidly with the number <strong>of</strong> particles. Structurally<br />

different states are realized with different probabilities.<br />

By analyzing the number and probabilities <strong>of</strong> metastable<br />

configurations we conclude that the configurational uncertainty<br />

(entropy), grows faster than exponentially – as<br />

the logarithm <strong>of</strong> the factorial <strong>of</strong> the number <strong>of</strong> particles<br />

N [4].<br />

Small crystals exhibit near-circular symmetry – shell<br />

structure, induced by the external confinement. Ground<br />

Figure 1: The triangulated<br />

structure and defects <strong>of</strong> 600particle<br />

Wigner cluster.<br />

states <strong>of</strong> larger clusters correspond to a few circular shells at the exterior and an ordered<br />

hexagonal core. Due to the necessity to satisfy Euler’s theorem and to lower strain energy<br />

<strong>of</strong> the lattice, the presence <strong>of</strong> intrinsic topological defects is unavoidable; these are mainly<br />

located in the transition region between the circular surface and ordered interior, at the<br />

six corners <strong>of</strong> a near-perfect hexagon (figure 1). The number <strong>of</strong> defective vertices in<br />

six defect groups grows non-uniformly with the system size. A few distinct species <strong>of</strong><br />

defects are distinguished: disclinations, dislocations, lengthy grain boundaries, unique<br />

triangular vacancies and interstitial sites. Large pentagonal rosette defects (see figure)<br />

are also observed in large clusters. As the energy <strong>of</strong> the metastable state grows, the<br />

orientational symmetry <strong>of</strong> the crystal is lost. Long grain boundaries in the interior, large<br />

number<strong>of</strong>dislocationsandnear-surfacecomplexestendtoappearinthehighlymetastable<br />

configurations.<br />

[1] E. Wigner, Phys. Rev. 46, 1002 (1934),<br />

[2] A. V. Filinov, M. Bonitz and Yu. E. Lozovik, Phys. Rev. Lett. 86, 3851 (2001),<br />

[3] S. Goedecker, J. Chem. Phys. 120, 9911 (2004),<br />

[4] A. Radzvilavičius and E. Anisimovas, J. Phys.: Condens. Matter 23, 075302 (<strong>2011</strong>).<br />


MoP6 _______________________________________________________________<br />

40th "Jaszowiec" <strong>2011</strong> International School and Conference on the Physics <strong>of</strong> Semiconductors<br />

Gauge invariant computational scheme for heterostructures in magnetic field<br />

Anna Korbecka and Jacek A. Majewski<br />

Institute <strong>of</strong> Theoretical Physics, Faculty <strong>of</strong> Physics, University <strong>of</strong> Warsaw,<br />

ul. Hoża 69 PL-00-681, Poland<br />

Multiband k·p models together with the envelope function theory have become a standard<br />

approach for studies <strong>of</strong> the electronic structure <strong>of</strong> not only bulk semiconductors but also their<br />

heterostructures, other low-dimensional forms, and semiconductors in the translational symmetry<br />

breaking magnetic field. This continuum scheme proved to be very useful for modeling <strong>of</strong><br />

semiconductor nanostructures, albeit not free from flaws. In applications <strong>of</strong> this method for<br />

structures where the translational symmetry is broken, the corresponding component <strong>of</strong> the wave<br />

vector is substituted by differential operator. Together with the fact that parameters <strong>of</strong> the k·p<br />

Hamiltonian are position dependent, it leads to the situation where the operators representing<br />

individual components <strong>of</strong> the total matrix operator are not hermitian. Therefore, these operators are<br />

modified (so-called ‘symmetrization’ is performed) to acquire hermitian form. It solves some <strong>of</strong> the<br />

severe problems, however the ‘symmetrization’ procedure is not unique and very <strong>of</strong>ten leads to<br />

unphysical spurious solutions [1] and nonphysical outcomes for the heavy-hole subbands [2]. The<br />

problem has been disputed hotly for a long time in semiconductor physics without a final solution<br />

on the horizon. In the meantime, the new approach has emerged that is based on the so-called Burt’s<br />

exact envelope function theory [3] and formulates the whole operator matrix in a proper form<br />

instead <strong>of</strong> ‘symmetrizing’ its individual components [4], i.e., the whole matrix <strong>of</strong> operators is<br />

hermitian, however, its individual operator components are not. This procedure <strong>of</strong> B.A. Foreman is<br />

not plagued by spurious solutions problem and gives in most cases physically reasonable results,<br />

which can be, however, quite different from ones obtained within the standard approach. In<br />

addition, it has been recently proved [2] that the Foreman’s scheme is the only one that guarantees<br />

the gauge invariance <strong>of</strong> the k·p Hamiltonian for semiconductor low-dimensional structures in the<br />

constant magnetic field.<br />

We have implemented Foreman’s theory for low dimensional semiconducting systems in<br />

magnetic field into nextnano 3 simulation package [5] together with the magnetic p-d exchange<br />

interaction [6] and interface corrections for systems without mirror symmetry (e.g., no-common<br />

atom systems like InAs/GaSb). The Hamiltonian has been solved on discrete lattice. This allows us<br />

for reliable studies <strong>of</strong> the Landau level formation in the valence bands in non-magnetic<br />

heterostructures (GaAs/AlGaAs, InGaAs/GaAs, InAs/GaSb) and also heterostructures with<br />

modulated magnetization (such as (Ga,Mn)As/AlGaAs and (Ga,Mn)Sb/InAs). The scheme deals<br />

also with the doped structures, by including the electrostatic potential obtained from the Poisson’s<br />

equation self-consistently into Foreman’s k·p Hamiltonian.<br />

In particular, we have computed the electronic structure <strong>of</strong> GaAs/Al0.33Ga0.67As [001] and<br />

[311] heterostructures in the magnetic field. The obtained results help to interpret experiments on gfactor<br />

in 2D-hole gases [6] and the intrinsic photo-induced anomalous Hall effect [7].<br />

[1] E. P. Pokalitov, V. A. Fonoberov, V. M. Fomin, and J.T. Devreese, Phys. Rev. B. 64, 245328 (2001).<br />

[2] T. Andlauer, R. Morsch, P. Vogl, Phys. Rev. B. 78, 075317 (2008).<br />

[3] M. G. Burt, J. Phys. Condens. Matter 4, 6651 (1992).<br />

[4] B. A. Foreman, Phys. Rev. B. 48, 4964 (1993); ibid. 56, R12 748 (1997).<br />

[5] http://www.nextnano.de<br />

[6] D.A. Vasyukov, A.S.Plaut, M.Henini, Physica E 42, 964 (2010).<br />

[7] D.A. Vasyukov, A.S.Plaut, M.Henini, L.N.Pfeiffer, K.W.West, C.A.Nicoll, I.Farrer, and D.A. Ritchie,<br />

Physica E 42, 940 (2010).<br />


40th _______________________________________________________________<br />

"Jaszowiec" <strong>2011</strong> International School and Conference on the Physics <strong>of</strong> Semiconductors MoP7<br />

Modeling PbTe-based low dimensional structures<br />

M. Bukała 1 , P. Sankowski 2 , R. Buczko 1 and P. Kacman 1<br />

1 Institute <strong>of</strong> Physics, Polish Academy <strong>of</strong> Sciences, Warsaw 02-668, Poland<br />

2 Institute <strong>of</strong> Informatics, University <strong>of</strong> Warsaw, Warsaw 02-097, Poland<br />

We present a theoretical study <strong>of</strong> various PbTe low dimensional structures. First, we<br />

consider rock-salt PbTe inclusions in zinc-blende CdTe matrix as well as <strong>of</strong> zinc-blende CdTe<br />

nano-clusters in rock-salt PbTe matrix i.e. structures with possibly improved thermoelectric<br />

properties. Next, we model PbTe-based 2D heterostructures, in which quantum spin-Hall<br />

phase is expected. For these purposes a tight-binding parameterisation for PbTe has been<br />

obtained, which, in contrast to all available in literature parameterisations, leads not only to<br />

proper values <strong>of</strong> the energy gaps but also to adequate effective masses in the valence and<br />

conduction bands. Our tight-binding model includes the sp3 atomic orbitals with spin-orbit<br />

interactions and accounts for the nearest neighbour and next near neighbour interactions. We<br />

have checked on 2D PbTe/CdTe heterostructures that such tight-binding description and the<br />

effective mass approach lead to similar energy structures.<br />

The aim <strong>of</strong> the former research is to show how introducing nanostructures <strong>of</strong> different<br />

size and shape changes the electron density <strong>of</strong> states near the Fermi level and thus can be used<br />

to optimize the thermoelectric power Seeback coefficient. We have considered the Fermi level<br />

situated near the top <strong>of</strong> the valence band (in p-type) as well as at the bottom <strong>of</strong> the conduction<br />

band (in n-type). We have studied structural and electronic properties <strong>of</strong> PbTe quantum wires<br />

in the CdTe matrix, and CdTe anti-wires and CdTe anti-dots in the PbTe matrix. For structures<br />

with less than 500 atoms in the unit cell, all atomic positions in the model nanostructures, in<br />

particular at the interfaces, have been calculated using first principles methods with relaxation<br />

and re-bonding allowed. The obtained relaxed atomic positions have been further used in the<br />

calculations <strong>of</strong> electron density <strong>of</strong> states <strong>of</strong> the studied structures, which are performed within<br />

the tight-binding approximation. In larger structures the relaxation has not been taken into<br />

account.<br />

Our calculations show that all kinds <strong>of</strong> inclusions can lead to shape and size dependent<br />

changes <strong>of</strong> the density <strong>of</strong> states at the Fermi level. Thus, the electronic contribution to the<br />

Seebeck coefficient can be optimized by changing the shape, size, density as well as the<br />

spatial arrangement <strong>of</strong> the inclusions.<br />

The same tight-binding model, supported by effective mass approach, have been used to<br />

study the PbTe-based 2D heterostructures. In particular, it has been shown that states related<br />

to the interfaces can appear in PbSnTe/PbTe structure, in which the Sn content leads to band<br />

inversion. By changing the Sn content and the diameter <strong>of</strong> the heterostructure, we are looking<br />

for systems, in which such states may lead to the spin-Hall effect.<br />

The work was partially supported by the European Union within the European Regional<br />

Development Fund, through grant Innovative Economy (POIG.01.01.02-00-108/09)<br />


MoP8 _______________________________________________________________<br />

40th "Jaszowiec" <strong>2011</strong> International School and Conference on the Physics <strong>of</strong> Semiconductors<br />

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40th _______________________________________________________________<br />

"Jaszowiec" <strong>2011</strong> International School and Conference on the Physics <strong>of</strong> Semiconductors MoP9<br />

Atomistic calculation <strong>of</strong> screened Coulomb interactions in semiconductor<br />

nanostructures.<br />

M. Chwastyk, P. T. Różański, and M. Zieliński<br />

<strong>Instytut</strong> <strong>Fizyki</strong> UMK, ul. Grudziądzka 5, 87-100 Toruń, Poland<br />

Dielectric screening <strong>of</strong> Coulomb interactions affects optical and electronic properties<br />

<strong>of</strong> semiconductor nanostructures including multi-exciton generation rates in colloidal<br />

nanocrystals [1], excitonic spectra and self-energy corrections in nanopillar or graphene<br />

quantum dots [2,3]. Using tight-binding approach we compare several methods <strong>of</strong> accounting<br />

for dielectric screening in atomistic calculation including semi-classical approximation [4],<br />

Thomas-Fermi approach [5] and real-space dielectric matrix inversion [6]. We propose an<br />

efficient method in which dielectric screening can be decomposed into contribution from<br />

volume and surface-polarization terms, further screened by microscopic Thomas-Fermi like<br />

contribution. We illustrate our approach by calculating electronic and optical properties <strong>of</strong><br />

several different spherical semiconductor nanocrystals differing in size and composition. We<br />

compare results obtained with proposed approach and real-space dielectric matrix inversion<br />

and study role <strong>of</strong> different approximation used in calculation. We discuss possibility<br />

generalizing our model for a multi-million atom systems like self-assembled quantum dots.<br />

[1] Z. Lin, A. Franceschetti, and M.T. Lusk, arXiv:1009.0417<br />

[2] Y. M. Niquet et al., Phys. Rev. B 73, 165319 (2006).<br />

[3] D. Guclu, P. Potasz, and P. Hawrylak Phys. Rev. B 82, 155445 (2010).<br />

[4] A. Franceschetti and M. C. Troparevsky, Phys. Rev. B 72, 165311 (2005).<br />

[5] S. Lee et al., Phys. Rev. B 63, 195318 (2001).<br />

[6] F. Trani, D. Ninno, and G. Iadonisi, Phys. Rev. B 76, 085326 (2007).<br />


MoP10 _______________________________________________________________<br />

40th "Jaszowiec" <strong>2011</strong> International School and Conference on the Physics <strong>of</strong> Semiconductors<br />

Van der Waals Density Functionals in Materials Science<br />

Magda Birowska, Karolina Milowska, and Jacek A. Majewski<br />

Faculty <strong>of</strong> Physics, University <strong>of</strong> Warsaw, ul. Hoża 69, 00-681 Warszawa, Poland<br />

The density functional theory (DFT) has revolutionized materials science making it possible<br />

to quantitatively predict numerous properties <strong>of</strong> materials. However, exact in principle, the DFT<br />

relies on various approximations to the exchange and correlation functionals that are nowadays<br />

prerequisites for practical implementations <strong>of</strong> DFT. In spite <strong>of</strong> a long list <strong>of</strong> successes, the local<br />

density approximation (LDA) or generalized gradient approximation (GGA) that are commonly<br />

used in so-called ab-initio calculations have as well many flaws. One <strong>of</strong> its weaknesses is rather<br />

poor description <strong>of</strong> Van der Waals (VdW) type <strong>of</strong> bonding in solids. The best example is the<br />

geometry unit cell geometry <strong>of</strong> graphite. Whereas the atomic distances in the carbon planes with<br />

strong covalent bonds are excellently predicted by LDA or GGA, the distances between the<br />

planes, which are determined by Van der Waals type <strong>of</strong> interactions, are not. Therefore, recently<br />

new correlation functionals have emerged that take into account polarization effects and could<br />

improve the description <strong>of</strong> Van der Waals forces [1,2].<br />

In this communication, we present the results <strong>of</strong> theoretical studies <strong>of</strong> various systems with<br />

considerable role <strong>of</strong> VdW forces where the new functionals have been employed. The results <strong>of</strong><br />

test studies for graphite are very promising, providing the solution to the long standing problem<br />

and giving the distance between the carbon layers equal to 3.349 Å (with DRSLL functional [1])<br />

or 3.345 Å (with LMKLL one [2]) in excellent agreement with experimental value <strong>of</strong> 3.36 Å<br />

(LDA and GGA lead to distances equal to 3.117 Å and 3.425 Å, respectively). However, in the<br />

studies we focus on graphene single-, bi-, and multi-layers, either free standing or bound in an<br />

incommensurable way to silicon carbide substrate. We consider also functionalized graphene<br />

layers. The correct description <strong>of</strong> the structural and electronic properties <strong>of</strong> these systems is <strong>of</strong><br />

crucial importance for the quickly developing electronics based on graphene. In our studies <strong>of</strong><br />

materials we have used the numerical package SIESTA with implemented VdW functionals and<br />

the standard LDA and GGA ones. This facilitates the direct comparison <strong>of</strong> the material<br />

properties obtained within both approaches. We find differences in the electronic structures <strong>of</strong><br />

these systems and their energetics. In particular, we find that the potential barriers in these<br />

systems calculated with VdW functional and standard approaches in many cases differ<br />

considerably. It is <strong>of</strong> crucial importance, particularly if one uses atomistic ab initio methods as a<br />

starting point for multi-scale modeling <strong>of</strong> materials and determines effective potentials on the<br />

basis <strong>of</strong> DFT calculations.<br />

This paper has been supported by the project “SiCMAT” (POIG.01.03.01-14-155/09-00).<br />

[1] M. Dion, H. Rydberg,E. Schoder, D. C. Langreth, and B. I. Lundqvist, Phys. Rev. Lett. 92,<br />

246401 (2004)<br />

[2] K. Lee, E. Murray, L. Kong, B. I. Lundqvist and D. C. Langreth, arXiv:1003.5255v1 (2010).<br />


40th _______________________________________________________________<br />

"Jaszowiec" <strong>2011</strong> International School and Conference on the Physics <strong>of</strong> Semiconductors MoP11<br />

Hopkinson-Like Effect in Single-Crystalline CdCr2Se4 Semiconductor<br />

T. Groń 1 , E. Malicka 2 , A.W. Pacyna 3 , H. Duda 1 and J. Krok-Kowalski 1<br />

1 University <strong>of</strong> Silesia, Institute <strong>of</strong> Physics, ul. Uniwersytecka 4, 40-007 Katowice, Poland<br />

2 University <strong>of</strong> Silesia, Institute <strong>of</strong> Chemistry, ul. Szkolna 9, 40-006 Katowice, Poland<br />

3 The Henryk Niewodniczański Institute <strong>of</strong> Nuclear Physics, Polish Academy <strong>of</strong> Sciences,<br />

ul. Radzikowskiego 152, 31-342 Kraków, Poland<br />

The magnetization <strong>of</strong> many compounds shows a peak near the Curie temperature, TC,<br />

when heating the sample in a fixed (small) magnetic field [1]. This behavior is commonly<br />

called the Hopkinson effect [2]. The accepted explanation [3] <strong>of</strong> the Hopkinson effect is based<br />

only on domain wall motion. This mechanism is obviously inapplicable to the case <strong>of</strong> singledomain<br />

particles. However, a thermomagnetic effect which is quite similar to the Hopkinson<br />

effect has been experimentally observed in most <strong>of</strong> the amorphous magnetic materials as well<br />

as in some spin glasses where the existence <strong>of</strong> multi-domain particles is questionable or even<br />

practically impossible [2]. In Nd2Fe14B-type ribbons the existence <strong>of</strong> a maximum in the<br />

thermomagnetic curves <strong>of</strong> thermally demagnetized samples in low fields was connected with<br />

the processes <strong>of</strong> irreversible rotation <strong>of</strong> magnetic moments <strong>of</strong> non-interacting uniaxial single<br />

domain particles according to the Stoner-Wohlfarth model [2,4].<br />

Magnetization, ac and dc magnetic susceptibility <strong>of</strong> the p-type CdCr2Se4 semiconductor<br />

[5] were measured in the zero-field-cooled mode using a Lake Shore 7225 dc<br />

magnetometer/ac susceptometer at 4.3 K and in applied external magnetic fields up to 60 kOe,<br />

and a Faraday type Cahn RG automatic electrobalance in the temperature range 4.5-400 K and<br />

'<br />

"<br />

at 1 kOe, respectively. The in-phase c ( T)<br />

and out-<strong>of</strong>-phase c ( T)<br />

components <strong>of</strong> the ac<br />

1<br />

fundamental susceptibility were recorded in the temperature range 4.5-160 K using an<br />

oscillating field Hac = 1 Oe with frequency <strong>of</strong> 120 Hz for external magnetic fields Hdc = 0, 100<br />

Oe, 200 Oe, 450 Oe and 1 kOe. The signals <strong>of</strong> the second (c2) and third (c3) harmonics were<br />

detected at the same temperature range, for the same amplitude and frequency as the ac c1<br />

measurements without an external static magnetic field.<br />

The dc magnetic measurements showed ferromagnetic order with a Curie temperature<br />

TC = 130 K and a saturation magnetization <strong>of</strong> 5.91 µB/f.u. at 4.5 K and at 60 kOe. The ac<br />

'<br />

magnetic measurements revealed a spectacular peak at 450 Oe and 1 kOe in the c 1(<br />

T)<br />

curve<br />

near TC, similar to the Hopkinson peak. The increasing dc magnetic field suppresses the<br />

'<br />

magnetic susceptibility intensity <strong>of</strong> c ( T)<br />

and slightly shifts the Hopkinson peak to higher<br />

1<br />

"<br />

temperatures. A small and positive value <strong>of</strong> c1( T)<br />

below TC indicating a small energy loss<br />

correlates rather with strong spin fluctuations than with spin rearrangement processes. This is<br />

consistent with zero values <strong>of</strong> second and third harmonic ac susceptibility in the temperature<br />

range <strong>of</strong> 4.5-160 K, suggesting the lack <strong>of</strong> short-range magnetic interactions. One can<br />

conclude that the peaks in the thermomagnetic curves at 450 Oe and 1 kOe can be<br />

qualitatively explained with the aid <strong>of</strong> the Hopkinson-like effect.<br />

This work is partly founded from science Grant No. N N204 145938.<br />

[1] J. Hopkinson, Proc. R. Soc. London 48, 1 (1890).<br />

[2] O. Popov, and M. Mikhov, J. Magn. Magn. Mater. 75, 135 (1988).<br />

[3] M. Kersten, Z. Angew. Phys. 8, 313 (1956).<br />

[4] E.C. Stoner, and E.P. Wohlfarth, Phil. Trans. R. Soc. A 240, 599 (1948).<br />

[5] H.W. Lehmann, Phys. Rev. 163, 488 (1967).<br />

49<br />


MoP12 _______________________________________________________________<br />

40th "Jaszowiec" <strong>2011</strong> International School and Conference on the Physics <strong>of</strong> Semiconductors<br />

Optical and structural characterization <strong>of</strong> zinc oxide<br />

nanostructures obtained by Atomic Layer Deposition method<br />

Ł. Wachnicki 1 , B. S. Witkowski 1 , S. Gierałtowska 1 , E. Janik 1 , M. Godlewski 1,2 ,<br />

E. Guziewicz 1<br />

1 Polish Academy <strong>of</strong> Sciences, Institute <strong>of</strong> Physics, al. Lotników 32/46, Warszawa 02-668,<br />

Poland<br />

2 Cardinal Stefan Wyszynski University, College <strong>of</strong> Science, Department <strong>of</strong> Mathematics and<br />

Natural Sciences, Warszawa, Poland<br />

Zinc oxide has been extensively investigated in the past few years mainly<br />

because <strong>of</strong> a large variety <strong>of</strong> potential applications. For example, it can be used<br />

in light-emitting diodes and transistors, as gas sensors, and transparent<br />

conductive oxide in solar cells.<br />

Among many applications, ZnO is also a prospective material for sensor<br />

technology, where developed surface morphology is very important. In this<br />

work we present growth <strong>of</strong> ZnO nanostructures using ALD. As a substrate we<br />

used gallium arsenide with gold eutectic mixture prepared on the surface at<br />

600°C. To obtain eutectic solution we spread gold thin film on a GaAs substrate<br />

and then annealed the sample in a nitrogen gas flow. Au-Ga droplets where<br />

formed in this way. The so-prepared substrate was used for growth <strong>of</strong> ZnO<br />

nanostructures in the ALD system. We applied deionized water and zinc<br />

chloride as an oxygen and zinc precursors, respectively. The eutectic mixture<br />

plays a role <strong>of</strong> a catalyst for the ZnO nanostructures growth. Au-Ga droplets<br />

flow on the front <strong>of</strong> growth forcing the growth <strong>of</strong> ZnO nanostructures. Optical<br />

and structural characterization was investigated by photoluminescence, scanning<br />

electron microscope (SEM) and atomic force microscope (AFM). SEM images<br />

show ZnO nanorods in a form <strong>of</strong> crystallites <strong>of</strong> up to 1 m length and 100 nm<br />

diameters. It is the first demonstration <strong>of</strong> ZnO nanostructures growth by ALD<br />

using VLS (vapour-liquid-solid) approach.<br />

The research was supported by the European Union within European Regional<br />

Development Fund, through grant Innovative Economy (POIG.01.01.02-00-<br />

008/08).<br />


40th _______________________________________________________________<br />

"Jaszowiec" <strong>2011</strong> International School and Conference on the Physics <strong>of</strong> Semiconductors MoP13<br />

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MoP14 _______________________________________________________________<br />

40th "Jaszowiec" <strong>2011</strong> International School and Conference on the Physics <strong>of</strong> Semiconductors<br />

Photoluminescence and chromaticity properties <strong>of</strong> ZnO nanopowders<br />

obtained by a microwave solwothermal method<br />

E. Wolska 1 , D. Sibera 2 , B.S. Witkowski 1 , S.A. Yatsunenko 1 , I. Pełech 2 ,<br />

U. Narkiewicz 2 , M. Godlewski 1,3<br />

1 Institute <strong>of</strong> Physics, Polish Academy <strong>of</strong> Sciences, Al. Lotników 32/46,<br />

02-668 Warsaw, Poland<br />

2 Institute <strong>of</strong> Chemical and Environment Engineering, West Pomeranian University <strong>of</strong><br />

Technology, K. Puławskiego 10, 70-322 Szczecin, Poland.<br />

3 Dept. <strong>of</strong> Mathematics and Natural Sciences College <strong>of</strong> Science,<br />

Cardinal S. Wyszy ski University, Dewajtis 5, 01-815 Warsaw, Poland<br />

ZnO is one <strong>of</strong> the most intensively studied wide band gap semiconducting material.<br />

Different applications <strong>of</strong> ZnO in transparent electronics, optoelectronics, photovoltaics<br />

and spintronics were demonstrated recently, which explains the increasing interest in this<br />

material. Moreover, new applications in medicine and biology are predicted. It is claimed<br />

that ZnO nanopowders can be used as luminescence markers for such applications.<br />

Luminescence markers are promising materials to be used for early recognition <strong>of</strong> many<br />

<strong>of</strong> diseases, including cancer.<br />

In the present work we discuss growth and properties <strong>of</strong> ZnO nanopowders obtained by a<br />

microwave solvothermal method. We compare properties <strong>of</strong> nanopowders obtained by<br />

two methods. In the first process as a zinc precursor we used zinc nitrate hexahydrate<br />

(Zn(NO3)2 6H2O) dissolved in distilled water. In the second process we used zinc nitrate<br />

hexahydrate and ethanol. For each <strong>of</strong> these two processes we performed growth at five<br />

different values <strong>of</strong> pressure in the microwave reactor during the solvothermal synthesis.<br />

The so-obtained powders were studied as-grown or after annealing at 750 o C in the air<br />

atmosphere.<br />

Each type <strong>of</strong> the samples were characterized with photoluminescence,<br />

cathodoluminescence and their size was determined from SEM and XRD investigations.<br />

The growth processes were optimized to obtain the brightness light emission and, in some<br />

cases, the best chromaticity. The latter study was motivated by possibility <strong>of</strong> white color<br />

emission <strong>of</strong> ZnO upon UV excitation.<br />

We discuss relations between synthesis parameters in the solvothermal microwave<br />

processes and optical properties <strong>of</strong> the samples obtained.<br />

The research was partially supported by the European Union within European Regional<br />

Development Fund, through grant Innovative Economy (POIG.01.01.02-00-008/08).<br />


40th _______________________________________________________________<br />

"Jaszowiec" <strong>2011</strong> International School and Conference on the Physics <strong>of</strong> Semiconductors MoP15<br />

Technology and testing the mechanical properties <strong>of</strong> the<br />

InGaP/GaAs/InGaP microcantilevers<br />

Tomáš Ščepka, Dagmar Gregušová, Róbert Kúdela, Štefan Gaži and Vladimír<br />

Cambel<br />

Institute <strong>of</strong> Electrical Engineering, Slovak Academy <strong>of</strong> Sciences, Dúbravská cesta 9, SK-<br />

84104 Bratislava, Slovakia<br />

Microcantilevers are at the heart <strong>of</strong> atomic force microscopes and also perspective for<br />

future chemical and biological sensing applications. Most <strong>of</strong> the commercially produced<br />

cantilevers are made <strong>of</strong> silicon or silicon nitride. However, microcantilever probes based on<br />

III-V semiconductor materials and their heterostructures can be equipped with novel<br />

functionalities through the incorporation <strong>of</strong> electronic and opto-electronic devices designed<br />

directly on them.<br />

GaAs is the most used III-V material for cantilever probe development thanks to its<br />

optoelectronic properties, high mobility <strong>of</strong> electrons, sufficient mechanical properties and<br />

high piezoresistivity (values higher than those <strong>of</strong> silicon) [1]. Application <strong>of</strong> mature<br />

deposition methods, such as metal organic chemical vapor deposition, allows for the<br />

formation <strong>of</strong> sandwich layers whose thickness can be defined with extreme precision.<br />

This work is a first step towards sensors with improved sensitivity and integrability.<br />

We focus on the fabrication and initial mechanical testing <strong>of</strong> microcantilevers based on an<br />

InGaP/GaAs/InGaP heterostructure (Fig. 1.). The heterostructure consists <strong>of</strong> a GaAs layer,<br />

that determined the cantilever thickness. The layer is embedded between two InGaP etch-stop<br />

layers [2]. All layers are lattice matched to a thinned semi-insulating GaAs (100) substrate.<br />

The front-side patterning was accomplished using optical photolithography, Ar ion milling <strong>of</strong><br />

InGaP layer, and wet chemical etching <strong>of</strong> GaAs. The substrate was at first thinned to ~250 µm<br />

and then cantilevers were released via back-side bulk micromachining in two solutions. The<br />

1H2SO4:8H2O2:1H2O mixture was used as a fast etchant<br />

which was followed by a finer 1H3PO4:2H2O2:8H2O<br />

etchant. The experimental cantilevers presented here<br />

were designed as simple rectangles, nominally 100-300<br />

µm long and 35-65 µm wide, with a thickness <strong>of</strong> 0.1, 0.2,<br />

0.5, 1, and 2 µm. We have investigated the influence <strong>of</strong><br />

geometrical dimensions, especially thickness <strong>of</strong> GaAs<br />

layer, on the fabrication process and mechanical<br />

properties <strong>of</strong> the cantilevers. To determine the resonance<br />

frequencies, a detection system <strong>of</strong> an Atomic Force<br />

Microscope (NT-MDT NTEGRA) was used. The<br />

measured resonance frequencies <strong>of</strong> the cantilevers<br />

correspond with theoretical values.<br />

This work is the result <strong>of</strong> the ESF project implementation, CENTE - 2 nd stage, ITMS code<br />

26240120019, supported by the R&D Operational Program funded by the ERDF.<br />

[1] K. Hjort et al., J. Micromech. Microeng. 4, 1-13 (1994).<br />

[2] D. Gregušová et al., J. Micromech. Microeng. 20, 097001 (2010).<br />

53<br />

Figure 1. SEM micrograph <strong>of</strong><br />

fabricated microcantilever.

MoP16 _______________________________________________________________<br />

40th "Jaszowiec" <strong>2011</strong> International School and Conference on the Physics <strong>of</strong> Semiconductors<br />

ZnO thin films <strong>of</strong> different crystalline structures grown on Si (100)<br />

substrates by reactive DC sputter deposition<br />

Michał A. Borysiewicz 1 , El bieta Dynowska 1,2 , Valery Kolkovsky 2 , Jan Dyczewski 2 ,<br />

Eliana Kami ska 1 , Anna Piotrowska 1<br />

1 Institute <strong>of</strong> Electron Technology, Al. Lotników 32/46, 02-668 Warsaw, Poland<br />

2 Institute <strong>of</strong> Physics, Polish Acad. <strong>of</strong> Sci., Al. Lotników 32/46, 02-668 Warsaw, Poland<br />

ZnO is considered an advantageous material in the transparent electronics,<br />

optoelectronics and photovoltaics due to its wide band-gap <strong>of</strong> 3.37 eV and high exciton<br />

binding energy <strong>of</strong> about 60 meV. Most research on ZnO thin films is performed on layers<br />

deposited on sapphire substrates. The sapphire has the advantage <strong>of</strong> having a crystalline<br />

structure matching that <strong>of</strong> the ZnO, but also a disadvantage <strong>of</strong> a large lattice mismatch <strong>of</strong><br />

~18%. Therefore, to grow high-quality ZnO thin films on sapphire substrates, buffer layers<br />

have to be introduced [1-3]. On the other hand, in order to enable the integration <strong>of</strong> electronic<br />

and optoelectronic elements on one chip and take advantage <strong>of</strong> the high quality, large wafer<br />

sizes and low price, ZnO growth on silicon (100) substrates should be studied, regardless <strong>of</strong><br />

the lattice mismatch to ZnO.<br />

This work reports on the structure <strong>of</strong> thin ZnO films on Si (100) substrates deposited<br />

using room-temperature reactive DC sputtering from a Zn target in an Ar-O2 mixture. Films<br />

grown in several deposition processes are compared. The power fed to the Zn target was kept<br />

at 75 W and the total gas pressures and amounts <strong>of</strong> oxygen in the gas mixture were changed<br />

between the processes. The structure <strong>of</strong> the films was studied by means <strong>of</strong> X-ray diffraction in<br />

the -2 geometry and scanning electron microscopy <strong>of</strong> the surface as well as the crosssections.<br />

The stoichiometry was estimated using Rutherford backscattering spectrometry.<br />

Transport properties were determined through Hall effect measurements. The<br />

characterisations were carried out after deposition and after subsequent 15 minutes-long<br />

annealing processes in an RTP furnace in O2 flow at 400 o C, 600 o C and 800 o C. We found, that<br />

by varying the parameters <strong>of</strong> the growth accordingly, films with very different structures<br />

could be produced. In particular, growth in oxygen-poor conditions resulted in porous<br />

polycrystalline structures (Fig. 1) potentially suited for applications in chemical sensors or<br />

dye-sensitized solar cells and growth in oxygen-rich conditions led to the formation <strong>of</strong> dense<br />

columnar films (Fig. 2) which may be applied in LEDs.<br />

This study was partially supported by the European Union within European Regional<br />

Development Fund, through grant Innovative Economy (POIG.01.03.01-00-159/08,<br />

"InTechFun").<br />

Fig. 1. Fig. 2.<br />

[1] B. Pecz et al., J. Appl. Phys. 100, 103506 (2006)<br />

[2] M.W. Cho et al., Semicond. Sci. Technol. 20, S13–S21 (2005)<br />

[3] M.A. Borysiewicz et al., Acta Phys. Pol. A 119, 333 (<strong>2011</strong>)<br />


40th _______________________________________________________________<br />

"Jaszowiec" <strong>2011</strong> International School and Conference on the Physics <strong>of</strong> Semiconductors MoP17<br />

Comparison <strong>of</strong> the valence band <strong>of</strong> amorphous and crystalline<br />

GeTe and (Ge,Mn)Te layers<br />

W. Kn<strong>of</strong>f 1 , M.A. Pietrzyk 1 , B.A. Orłowski 1 , B. Taliashvili 1 , T. Story 1 , R.L. Johnson 2<br />

1 Institute <strong>of</strong> Physics, PAS, Al. Lotników 32/46, 02-668 Warsaw, Poland<br />

2 Institute <strong>of</strong> Experimental Physics, University <strong>of</strong> Hamburg, Luruper Chaussee 149,<br />

D-22761 Hamburg, Germany<br />

(Ge,Mn)Te belongs to the IV-VI family <strong>of</strong> diluted magnetic semiconductors and attracts<br />

considerable attention due to its ferroelectricity and carrier-induced ferromagnetism governed<br />

by the RKKY indirect exchange mechanism. GeTe is also known as a phase change material<br />

exhibiting at the nanosecond scale a transformation from an amorphous to a polycrystalline<br />

phase, accompanied by insulator to metal transition. In (Ge,Mn)Te layers this structural<br />

transformation results also in paramagnet to ferromagnet transition. The aim <strong>of</strong> this work is to<br />

compare electronic structure <strong>of</strong> amorphous (a-) and crystalline (c-) GeTe and Ge0.9Mn0.1Te<br />

semiconductors using resonant photoemission spectroscopy (RPES) method.<br />

GeTe and (Ge,Mn)Te layers were grown on BaF2 (111) substrates by molecular beam epitaxy<br />

technique using effusion cells with GeTe, Mn, and Te2. For the growth <strong>of</strong> amorphous and<br />

crystalline GeTe and (Ge,Mn)Te layers, the substrate temperature was kept at room<br />

temperature and at T=250 0 C, respectively. Mn content in the layers was determined by energy<br />

dispersive X-ray fluorescence analysis. No evidence <strong>of</strong> crystalline phases was found in<br />

a-GeTe and a-(Ge,Mn)Te layers by X-ray diffraction (XRD) measurements. GeTe and<br />

(Ge,Mn)Te layers deposited at higher temperature (T=250 0 C) grow epitaxially and are<br />

monocrystalline with [111] growth direction. For GeTe and (Ge,Mn)Te layers the<br />

photoemission spectra were measured in the photon energy range 45-60 eV. The three-peak<br />

structure related to 4p and 5p Te orbitals (about 2.6 eV), Ge 4s orbitals (7.6 eV), and Te 5s<br />

orbitals (12 eV) was experimentally observed in the valence band <strong>of</strong> both c-GeTe and a-GeTe<br />

[1-3]. The spectra obtained for c-(Ge,Mn)Te and a-(Ge,Mn)Te in the energy range<br />

corresponding to the transition from Mn 3p to Mn 3d states reveal the same structure <strong>of</strong> peaks<br />

and Mn 3d orbitals contribution at 4 eV.<br />

Intensity (arb.u.)<br />

a)<br />

c-GeTe<br />

a-GeTe<br />

hυ = 50 eV<br />

0 5 10 15<br />

Binding Energy (eV)<br />

0 5 10 15 20 25<br />

Binding Energy (eV)<br />

Fig.1 The valence band photoemission spectra <strong>of</strong> amorphous and crystalline GeTe (figure a) and (Ge,Mn)Te<br />

(figure b). The spectrum <strong>of</strong> (Ge,Mn)Te was measured at the energy corresponding to the Mn 3p-3d threshold.<br />

[1] N.J. Shevchik, J. Tejada, W.W. Langer, M. Cardona, Phys. Rev. Lett., 30 659 (1973).<br />

[2] B.J. Kowalski, M.A. Pietrzyk, W. Kn<strong>of</strong>f, et al., Physics Procedia 3 1357 (2010).<br />

[3] M.A. Pietrzyk, B.J. Kowalski, B.A. Orłowski, et al., Acta Phys. Pol. A 112, 275 (2007).<br />

c-(Ge,Mn)Te<br />

a-(Ge,Mn)Te<br />

hν=51 eV<br />

This work was partially supported by the EC Network SemiSpinNet (PITN-GA-2008-215368).<br />

55<br />

Intensity (arb.u.)<br />


MoP18 _______________________________________________________________<br />

40th "Jaszowiec" <strong>2011</strong> International School and Conference on the Physics <strong>of</strong> Semiconductors<br />

Local Structure Study <strong>of</strong> GaAs:Te Using Te K-edge X-ray Absorption Fine<br />

Structure.<br />

A. Pietnoczka 1 , R. Bacewicz 1 , T. Słupi ski 2 , S. H. Wei 3 , M. Jie 3 , J. Antonowicz 1 ,<br />

T. Drobiazg 1<br />

1 Faculty <strong>of</strong> Physics, Warsaw University <strong>of</strong> Technology, ul. Koszykowa 75, 00-662 Warsaw,<br />

Poland<br />

2 Faculty <strong>of</strong> Physics, University <strong>of</strong> Warsaw, Ho a 69, Warsaw 00-681, Poland<br />

3 NREL, 1617 Cole Blvd., Golden, CO 80401, USA<br />

The annealing <strong>of</strong> heavily doped GaAs:Te can significantly change the value <strong>of</strong> the free<br />

electron concentration in a reversible manner. These changes <strong>of</strong> electrical properties are<br />

accompanied by the structural changes <strong>of</strong> GaAs:Te solid solution.[1] In this contribution we<br />

present an attempt to determine local changes around Te atoms for different states <strong>of</strong> the<br />

GaAs:Te crystals caused by the annealing corresponding to different electron concentrations.<br />

Extended X-ray Absorption Fine Structure (EXAFS) and X-ray Absorption Near Edge<br />

Structure (XANES) techniques have been employed to investigate the local structure around<br />

tellurium atom and its valence. Tellurium K-edge absorption has been measured in a<br />

fluorescence method at the X1 beamline in the HASYLAB. The samples were kept at the<br />

temperature <strong>of</strong> 80 K in order to minimize the thermal disorder.<br />

Various models <strong>of</strong> Te defect complex have been used to fit the data for low<br />

concentration state: VGa- TeAs complexes [2], TeAs+ TeAs pairs [1], as well as several DX-like<br />

configurations <strong>of</strong> tellurium. DX configurations have been calculated using density functional<br />

theory (DFT). The EXAFS and XANES spectra for different Te location have been simulated<br />

and compared with the experimental data. Multi-parameter fitting <strong>of</strong> the EXAFS data<br />

provided information on the local structure around Te (the bond lengths, the coordination<br />

numbers and Debye-Waller factor). The charge state <strong>of</strong> Te has been determined from the<br />

XANES data. The EXAFS and XANES were useful for ruling out the possibility <strong>of</strong> existence<br />

GaTe and Ga2Te3 precipitations.<br />

The XAFS results have been compared with the studies <strong>of</strong> structural disorder using the<br />

X-ray diffuse scattering.<br />

References:<br />

[1] T.Słupi ski, E.Zieli ska-Rohozi ska, Thin Solid Films 367, 227 (2000).<br />

[2] D.T.J. Hurle, J. Appl. Phys. 85, 6957 (1999).<br />


40th _______________________________________________________________<br />

"Jaszowiec" <strong>2011</strong> International School and Conference on the Physics <strong>of</strong> Semiconductors MoP19<br />

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MoP20 _______________________________________________________________<br />

40th "Jaszowiec" <strong>2011</strong> International School and Conference on the Physics <strong>of</strong> Semiconductors<br />

Investigation <strong>of</strong> thermal properties <strong>of</strong> AlGaAs/GaAs Quantum<br />

Cascade Lasers by thermoreflectance spectroscopy<br />

Małgorzata Iwi ska 1,2 , Dorota Pier ci ska 1 , Kamil Pier ci ski 1 ,<br />

Anna Szerling 1 , Piotr Karbownik 1 , Maciej Bugajski 1,2<br />

1 Institute <strong>of</strong> Electron Technology, al. Lotników 32/46, 02-668 Warsaw, Poland<br />

2 Faculty <strong>of</strong> Physics, WUT, ul. Koszykowa 75, 00-662 Warsaw, Poland<br />

Abstract<br />

Quantum Cascade Lasers (QCLs) are semiconductor unipolar devices in<br />

which intersubband transitions <strong>of</strong> carriers lead to lasing. The QCL design’s<br />

characteristic feature is the active regions cascading scheme. The wavelength <strong>of</strong><br />

emitted radiation in QCLs depends mainly on the width and the depth <strong>of</strong> quantum<br />

wells and to the less extend on the material. QCLs emit wavelengths within the<br />

infrared region <strong>of</strong> the spectrum and which covers most <strong>of</strong> molecules’ absorption<br />

bands. This makes them ideal sources for trace gases detection, air pollution<br />

monitoring, human’s exhaled breath monitoring or biomedical imaging. Most <strong>of</strong><br />

QCL applications require devices operating at room temperature (RT), <strong>of</strong>ten in<br />

CW mode, which is not an easy task to achieve.<br />

For full exploitation <strong>of</strong> QCLs application potential understanding <strong>of</strong><br />

thermal management in the devices is necessary. For CW RT operation the active<br />

region temperature has to be kept under control. Its value depends on such<br />

parameters as the pulse frequency, the pulse width, device mounting technology<br />

and the heat sink temperature. The excessive temperature <strong>of</strong> the active region<br />

causes escape <strong>of</strong> the electrons from the upper laser level to continuum states and<br />

thermal backfilling <strong>of</strong> the lower laser level, both processes deteriorating device<br />

performance by lowering the laser gain.<br />

The AlGaAs/GaAs QCLs investigated in this work are designed and<br />

fabricated at the Institute <strong>of</strong> Electron Technology in Warsaw [1]. The aim <strong>of</strong> the<br />

study was to determine the main factors influencing temperature rise in QCLs and<br />

to optimise their thermal properties. The measurements included temperature<br />

dependence <strong>of</strong> light current (I-V) characteristics and measurements <strong>of</strong> temperature<br />

distributions within devices. The latter were obtained by spatially resolved<br />

thermoreflectance measurements [2,3], which is the technique allowing for high<br />

resolution temperature mapping on working devices.<br />

1. K. Kosiel, M. Bugajski, A. Szerling, J. Kubacka-Traczyk, P. Karbownik, E. Pruszy ska-<br />

Karbownik, J. Muszalski, A. Łaszcz, P. Romanowski, M. Wasiak, W. Nakwaski, I.<br />

Makarowa, P. Perlin , Photonics Letters <strong>of</strong> Poland, vol.1, 16-18, (2009)<br />

2. M. Bugajski, T. Piwoñski, D. Wawer, T. Ochalski, E. Deichsel, P. Unger, B. Corbett,<br />

Materials Science in Semiconductor Processing, vol.9, 188-197 (2006)<br />

D. Wawer, T. J. Ochalski, T. Piwo ski, A. Wójcik-Jedli ska, M. Bugajski, H. Page, phys.<br />

stat. sol. (a) 202, 1227–1232 (2005)<br />


40th _______________________________________________________________<br />

"Jaszowiec" <strong>2011</strong> International School and Conference on the Physics <strong>of</strong> Semiconductors MoP21<br />

Luminescence properties <strong>of</strong> Sm 3+ in different phases <strong>of</strong> TiO2<br />

P. Łach 1 , M. G. Brik 2 , I. Sildos 2 , A. Kamińska 1 , and A. Suchocki 1,3<br />

1<br />

- Institute <strong>of</strong> Physics, Polish Academy <strong>of</strong> Sciences, al. Lotników 32/46, 02-668, Warsaw,<br />

Poland<br />

2<br />

- Institute <strong>of</strong> Physics, University <strong>of</strong> Tartu, Riia 142, Tartu 51014, Estonia<br />

3<br />

- Institute <strong>of</strong> Physics, Kazimierz Wielki University, Weyssenh<strong>of</strong>fa 11, Bydgoszcz 85-072,<br />

Poland<br />

Titanium dioxide (TiO2) belongs to the wide-gap (3.0-3.2 eV) semiconductors and it<br />

has a very good performance as a photocatalyst and an oxygen sensor. Recently, it has been<br />

considered as a host suitable for doping with rare (RE) ions. In particular, samarium-doped<br />

titania reveals interesting phenomena like intense host-emission, high sensitivity <strong>of</strong> the<br />

emission on the ambient gas and enhanced photocatalytic activity [1].<br />

We have carried out studies <strong>of</strong> the photoluminescence <strong>of</strong> Sm 3+ -doped TiO2 within the<br />

temperature range 10 – 300 K in order to show the energy levels <strong>of</strong> the trivalent impurity ion<br />

in two different phases <strong>of</strong> titania. The anatase TiO2 crystallizes in I41/amd space group, in a<br />

tetragonal structure, the lattice parameters are equal to 3.7850 and 9.5140 Ǻ. The rutile TiO2<br />

crystallizes in the P42/mnm space group and lattice parameters are equal to 4.593 and 2.959<br />

Ǻ. In order to obtain crystalline titania powder in both <strong>of</strong> these two phases our samples were<br />

annealed: up to 700°C and at 1100 °C for anatase and rutile phases, respectively. Thanks to<br />

that we have obtained a maximum <strong>of</strong> Sm emission intensity [2], which depends on the<br />

ambient around the samples. The best luminescence signals were observed during the<br />

measurements in the atmospheric oxygen. We have used the over band-gap excitation in both<br />

cases. The PL intensity <strong>of</strong> rutile titania was much lower than in the corresponding anatase<br />

TiO2 at room temperature. In the two cases we have also observed the PL spectra at low<br />

temperatures. Unfortunately in argon atmosphere our samples have not given any signals.<br />

An example <strong>of</strong> PL spectrum <strong>of</strong> Sm 3+ in TiO2 at room temperature under the band-toband<br />

excitation <strong>of</strong> the anatase structure is presented below.<br />

PL intenity [au]<br />

0,007<br />

0,006<br />

0,005<br />

0,004<br />

0,003<br />

0,002<br />

0,001<br />

0,000<br />

6 H7/2<br />

6 H5/2<br />

6 H9/2<br />

6 H11/2<br />

500 550 600 650 700 750<br />

Wavelength [nm]<br />

Sm 3+ in anatase TiO 2<br />

λ exc = 325 nm<br />

T=300K<br />

In order to check the influence <strong>of</strong> high pressure on the photoluminescence properties<br />

<strong>of</strong> both phases titanium oxide doped with Sm we have used the intra-center excitation (488<br />

nm) in a diamond anvil-cell at low temperature. Pressure coefficients <strong>of</strong> the various<br />

luminescence lines were established. Pressure dependence <strong>of</strong> the crystals field parameters f<br />

Sm 3+ ions were calculated on this basis.<br />

Acknowledgements: This work was partly supported by the European Union within European Regional<br />

Development Fund, through the Innovative Economy grant POIG.01.01.02-00-108/2009/3<br />

References:<br />

[1] M.G. Brik et al., Physica B 405 (2010) 2450-2456<br />

[2] V. Kiisk et al., J. Phys. D: Appl. Phys. 42 (2009) 125107<br />


MoP22 _______________________________________________________________<br />

40th "Jaszowiec" <strong>2011</strong> International School and Conference on the Physics <strong>of</strong> Semiconductors<br />

Investigation <strong>of</strong> electro-optical properties <strong>of</strong> 2D macroporous silicon<br />

structures with surface nanocrystals<br />

Lyudmila A. Karachevtseva a , Stepan Ya. Kuchmii b , Oleg A. Lytvynenko a ,<br />

Fedor F. Sizov a , Olena J. Stronska a , Alexandr L. Stroyuk b<br />

a V. Lashkaryov Institute <strong>of</strong> Semiconductor Physics <strong>of</strong> NASU, Kyiv, Ukraine<br />

b L.V. Pisarzhevskii Institute <strong>of</strong> Physical Chemistry <strong>of</strong> NASU, Kyiv, Ukraine<br />

One <strong>of</strong> the perspective materials for the development <strong>of</strong> 2D photonic structures is<br />

macroporous silicon that can be obtained by the method <strong>of</strong> photoanodic etching. In view <strong>of</strong><br />

the potential barrier on a macropore surface or heterojunction on «macropore-nanocoating»<br />

boundary, one should take into account processes on the local surface centres at energies<br />

below that <strong>of</strong> the indirect interband transition.<br />

Optical absorption in 2D photonic macroporous silicon structures was investigated<br />

taking into account the linear electro-optical effect. The spectral dependence <strong>of</strong> optical<br />

absorption in the near-IR area (impurity absorption) has oscillating structure and varies under<br />

the “3/2” law at long wavelengths. This correlates with frequency dependence <strong>of</strong> the<br />

imaginary part <strong>of</strong> permittivity for optical transitions between impurity levels and the allowed<br />

bands <strong>of</strong> a crystal in an electric field (the impurity Franz-Keldysh effect) [1]. The<br />

experimental absorption spectra <strong>of</strong> macroporous silicon agree well with the corresponding<br />

spectral dependences <strong>of</strong> the electro-optical energy and the imaginary part <strong>of</strong> permittivity in<br />

the weak electric field approximation, thus confirming realization <strong>of</strong> the impurity Franz-<br />

Keldysh effect. The electric field <strong>of</strong> the reflected electromagnetic wave at the grazing angle <strong>of</strong><br />

light incidence onto macropore surface changes effectively a local surface electric field.<br />

The near-IR light absorption oscillations for 2D macroporous silicon structures with<br />

microporous silicon and as well as CdTe, ZnO surface nanocrystals were investigated taking<br />

into account electro-optical effect within the strong electric field approximation. Wellseparated<br />

oscillations with one order amplitude at spectral ranges <strong>of</strong> the surface bonds were<br />

observed. An analysis <strong>of</strong> the experimental absorption spectra is carried out within the model<br />

<strong>of</strong> the resonant electron scattering on impurity states in the strong electric field with<br />

difference between two resonant energies equaled to Wannier-Stark ladder [2]. The model <strong>of</strong><br />

the resonant electron scattering on impurity states in an electric field <strong>of</strong> heterojunction<br />

«silicon-nanocoating» on macropores surface and the realization <strong>of</strong> Wannier-Stark effect on<br />

the randomly distributed surface bonds were considered. The Wannier-Stark ladders are not<br />

destroyed by impurities due to the longer scattering lifetime relatively the oscillation period<br />

<strong>of</strong> an electron in an external electric field at all investigated spectral region for macroporous<br />

silicon structures with CdTe and ZnO surface nanocrystals.<br />

[1] L.A. Karachevtseva, V.I. Ivanov, O.O. Lytvynenko, K.A. Parshin, O.J. Stronska, Appl.<br />

Surf. Sci. 255(5), 3328 (2008).<br />

[2] L. Karachevtseva, S. Kuchmii, O. Lytvynenko, F. Sizov, O. Stronska, A. Stroyuk, Appl.<br />

Surf. Sci. 257, 3331 (<strong>2011</strong>).<br />


40th _______________________________________________________________<br />

"Jaszowiec" <strong>2011</strong> International School and Conference on the Physics <strong>of</strong> Semiconductors MoP23<br />

Investigation <strong>of</strong> multi-phonon excitations in<br />

ZnO textured crystalline films by Raman spectroscopy<br />

George V. Lashkarev 1 , Anatoliy M. Yaremko 2 , Vitaliy A. Karpyna 1<br />

1 I.N.Frantsevich Institute for Problems <strong>of</strong> Material Science, National Academy <strong>of</strong> Sciences <strong>of</strong><br />

Ukraine, 3 Krzizhanovsky st., Kyiv, Ukraine<br />

2 V.E.Lashkarev Institute for Semiconductor Physics, National Academy <strong>of</strong> Sciences <strong>of</strong><br />

Ukraine, 45 Prospect Nauky, Kyiv, Ukraine<br />

ZnO is a direct bandgap semiconductor (Eg = 3.37eV at RT) which has got much<br />

attention due to its ability to generate ultraviolet (UV) radiation. A large binding energy <strong>of</strong><br />

excitons (~60 meV) favors to excitonic transitions at room and higher temperatures. Besides<br />

ZnO has a strong enough constant <strong>of</strong> electron- (exciton) phonon interaction (EPI) what<br />

promtes 8-9 phonon replicas observed in Raman spectrum <strong>of</strong> this crystal. Investigation <strong>of</strong><br />

photoluminescence (PL) and Raman scattering (RS) spectra are essential for understanding<br />

the mechanism <strong>of</strong> light generation and absoorption. In the series <strong>of</strong> papers the phonon replicas<br />

in ZnO were revealed in PL and Raman investigations but without theoretical consideration.<br />

ZnO films were deposited by radio frequency reactive magnetron sputtering on sapphire<br />

(0001) substrate. To grow textured films <strong>of</strong> high structural perfectness we used layer-by-layer<br />

growth mode described in [1]. This growth mode was realized by several stage <strong>of</strong> deposition<br />

at which next ZnO layer grow on the previously grown one, in result, ZnO films <strong>of</strong> improved<br />

quality is formed. The resonant Raman scattering is a powerful method for the investigation<br />

<strong>of</strong> elementary excitations in crystals. Raman measurements were carried out in backscattering<br />

geometry (z(x,x)z� polarization at which z direction oriented parallel to the c-axis) at room<br />

temperature using Horiba Jobin Yvon T64000 system, equipped with an Olympus confocal<br />

optical microscope. The experiment was performed in the wavenumber range from 200 to<br />

5000 cm -1 with a spectral resolution < 2 cm -1 .<br />

Multi-phonon structure <strong>of</strong> RS spectra can be<br />

Intensity, arb.units<br />

50<br />

40<br />

30<br />

20<br />

10<br />

A LO<br />

1<br />

ZnO:Cu/sapphire<br />

2A LO<br />

1<br />

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5A<br />

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1<br />

6A LO<br />

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1<br />

0<br />

0<br />

0 1000 2000 3000 4000 5000<br />

Raman shift, cm -1<br />

50000<br />

40000<br />

30000<br />

20000<br />

10000<br />

Intensity, arb. units<br />

Fig.1. Comparison <strong>of</strong><br />

experimental RS spectra<br />

(curve 1) <strong>of</strong> ZnO:Cu film with<br />

calculated one (curve 2).<br />

1<br />

2<br />

explained in the frames <strong>of</strong> theoretical approach developed<br />

in [2]. To consider the RS we have to study the response<br />

<strong>of</strong> the system on electromagnetic field in the second order<br />

<strong>of</strong> perturbation theory. Fig.1 shows experimental RS<br />

spectrum and theoretical result. It is seen that positions <strong>of</strong><br />

maxima <strong>of</strong> experimental spectrum are well enough<br />

described by theoretical dependences. In particular, we<br />

can see that number <strong>of</strong> maxima is eight, which is<br />

practically the same as for good quality bulk crystal [3].<br />

The numerical calculations show also that character <strong>of</strong><br />

spectrum, i.e. the number <strong>of</strong> maxima and distribution <strong>of</strong><br />

intensity different line is very sensitive to constant <strong>of</strong> EPI.<br />

The theoretical consideration allows determining<br />

following main parameters, described LO phonons in RS:<br />

energy �LO=72 meV, constant <strong>of</strong> EPI �� 0,1 = 0.2 eV, damping constant � = 10 meV.<br />

[1] A.I. Ievtushenko, V.A. Karpyna, V.I. Lazorenko, et.al., Thin Solid Films 518 4529 (2010)<br />

[2] Yaremko A.M., V.M. Dzhagan, V.O. Yukhymchuk, et.al., J. Mol. Struct., 976 205 (2010)<br />

[3] J. F. Scott, Phys. Rev. B, 2 1209 (1970)<br />

The authors are grateful to V. Strelchuk for Raman measurements <strong>of</strong> ZnO films<br />


MoP24 _______________________________________________________________<br />

40th "Jaszowiec" <strong>2011</strong> International School and Conference on the Physics <strong>of</strong> Semiconductors<br />

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"Jaszowiec" <strong>2011</strong> International School and Conference on the Physics <strong>of</strong> Semiconductors MoP25<br />

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MoP26 _______________________________________________________________<br />

40th "Jaszowiec" <strong>2011</strong> International School and Conference on the Physics <strong>of</strong> Semiconductors<br />

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40th _______________________________________________________________<br />

"Jaszowiec" <strong>2011</strong> International School and Conference on the Physics <strong>of</strong> Semiconductors MoP27<br />

Properties <strong>of</strong> high-k oxides grown by Atomic Layer Deposition method for<br />

transparent electronics<br />

S. Gierałtowska 1 , Ł. Wachnicki 1 , B.S. Witkowski 1 , T.A. Krajewski 1 , M. Godlewski 1,2 ,<br />

E. Guziewicz 1<br />

1 Polish Academy <strong>of</strong> Sciences, Institute <strong>of</strong> Physics, al. Lotników 32/46, Warszawa 02-668,<br />

Poland<br />

2 Cardinal Stefan Wyszynski University, College <strong>of</strong> Science, Department <strong>of</strong> Mathematics and<br />

Natural Sciences, Warszawa, Poland<br />

Transparent electronics is nowadays an emerging technology for the next generation <strong>of</strong><br />

electronic structures, including IC (integrated circuit), recent devices with 45nm node and<br />

smaller cross-bar memories and transistors for e-paper applications. The essential materials<br />

used in semiconductor manufacturing processes are high-k oxides gate dielectrics. The<br />

introduction <strong>of</strong> high-k for gate dielectrics is one <strong>of</strong> several strategies allowing further<br />

miniaturization <strong>of</strong> microelectronic components, colloquially referred to as extending the<br />

Moore's Law.<br />

Our research was focused on the optimization <strong>of</strong> technological parameters for composite<br />

layers <strong>of</strong> high-k oxides that include hafnium oxide (HfO2) and aluminum oxide (Al2O3). We<br />

obtain high quality thin films using Atomic Layer Deposition at low temperature (60°C –<br />

150°C). Growth parameters are elaborated for the maximum smoothness and amorphous<br />

microstructure required for gating. To fabricate thin film transistors (TFTs) and active<br />

elements <strong>of</strong> cross-bar memories, we deposit ZnO (zinc oxide) as an active channel and high-k<br />

oxides as gate dielectric at low temperature, both grown by the ALD technique. I-V<br />

characteristics <strong>of</strong> our TFT structure will be shown. The combination <strong>of</strong> low temperature and<br />

transparency processing makes the ZnO-TFT with high-k dielectric very promising for the<br />

next generation <strong>of</strong> invisible and flexible electronics.<br />

The research was partially supported by the European Union within European Regional<br />

Development Fund, through grant Innovative Economy (POIG.01.01.02-00-008/08).<br />


MoP28 _______________________________________________________________<br />

40th "Jaszowiec" <strong>2011</strong> International School and Conference on the Physics <strong>of</strong> Semiconductors<br />

High Spin-Low Spin Transitions in Cu0.2Co0.76Cr1.83Se4 Semiconductor<br />

E. Maciążek 1 , T. Groń 2 , A.W. Pacyna 3 , T. Mydlarz 4 and J. Krok-Kowalski 2<br />

1 University <strong>of</strong> Silesia, Institute <strong>of</strong> Chemistry, ul. Szkolna 9, 40-006 Katowice, Poland<br />

2 University <strong>of</strong> Silesia, Institute <strong>of</strong> Physics, ul. Uniwersytecka 4, 40-007 Katowice, Poland<br />

3 The Henryk Niewodniczański Institute <strong>of</strong> Nuclear Physics, Polish Academy <strong>of</strong> Sciences,<br />

ul. Radzikowskiego 152, 31-342 Kraków, Poland<br />

4 International Laboratory <strong>of</strong> High Magnetic Fields and Low Temperatures,<br />

ul. Gajowicka 95, 53-529 Wrocław, Poland<br />

Electrical and magnetic studies carried out on single-crystalline CuxCoyCrzSe4 spinels<br />

showed ferromagnetic ordering and p-type metallic conductivity for y = 0.06, 0.1 and 0.11 as<br />

well as a ferrimagnetic behaviour and n-type electrical semiconductivity for y = 0.23 [1].<br />

Later, structural and electrical investigations carried out on polycrystalline Cu1-xCoxCr2Se4<br />

spinels in the compositional range 0.0 ≤ x ≤ 1.0 revealed a transformation from cubic to<br />

monoclinic structure above x = 0.4 as well as metallic-type conductivity with high values <strong>of</strong><br />

activation energy for x ≤ 0.5 and polaron semiconducting with low activation energy for x ³<br />

0.8 [2]. Recently, X-ray diffraction studies on the single phase Cu1-xCoxCr2Se4 samples (x = 0,<br />

0.2, 0.8, 1) proved the existence <strong>of</strong> the cubic spinel-type structure for x ≤ 0.2 and the<br />

monoclinic Cr3S4-type one for x ³ 0.8 [3]. Magnetization and magnetic susceptibility <strong>of</strong> the<br />

CuxCoyCrzSe4 polycrystals measured in the zero-field-cooled mode showed a transition from<br />

ferromagnetic order via ferrimagnetic one to antiferromagnetic-like behaviour with increasing<br />

Co content. This transition was accompanied with a lowering symmetry from cubic to<br />

monoclinic and for the latter the spin crossover phenomenon occurred [4].<br />

Magnetization, dc and ac magnetic susceptibility <strong>of</strong> Cu0.2Co0.76Cr1.83Se4 were measured<br />

in the zero-field-cooled mode using a vibrating sample magnetometer with a step motor at 4.2<br />

K and in applied external magnetic fields up to 150 kOe, a Faraday type Cahn RG automatic<br />

electrobalance in the temperature range 4.2-370 K and at 800 Oe, and a Lake Shore 7225 ac<br />

'<br />

"<br />

susceptometer, respectively. The in-phase c 1(<br />

T)<br />

and out-<strong>of</strong>-phase c1( T)<br />

components <strong>of</strong> the ac<br />

fundamental susceptibility were recorded in the temperature range 4.2-170 K simultaneously<br />

as a function <strong>of</strong> temperature in an oscillating field Hac = 1 Oe with frequency <strong>of</strong> 120 Hz. The<br />

signals <strong>of</strong> the second (c2) and third (c3) harmonics associated with nonlinear susceptibilities<br />

were detected as a function <strong>of</strong> temperature using an oscillating field <strong>of</strong> 5 Oe with frequency <strong>of</strong><br />

120 Hz without the applied external magnetic field.<br />

'<br />

Two spectacular peaks were observed on the c1( T)<br />

curve at 128 K and 147 K which<br />

corresponded to the strong fall <strong>of</strong> the dc magnetic susceptibility. At the same temperatures<br />

these peaks were observed on the curves <strong>of</strong> the second and third harmonics <strong>of</strong> magnetic<br />

susceptibility. Taking into account also a low value <strong>of</strong> the magnetization <strong>of</strong> 0.22 µB/f.u. at 4.2<br />

K and at 140 kOe as well as a hysteresis between 15 kOe and 60 kOe one can conclude that in<br />

the Cu0.2Co0.76Cr1.83Se4 semiconductor the high spin-low spin transitions may be populated<br />

and cross over in thermal equilibrium.<br />

[1] T. Groń, E. Maciążek, J. Heimann, J. Kusz, I. Okońska-Kozłowska, K. Bärner, and Ch.<br />

Kleeberg, Physica B 254, 84 (1998).<br />

[2] E. Maciążek, A. Molak, and T. Goryczka, J. Alloys Compd. 441, 222 (2007).<br />

[3] V. Svitlyk, and Y. Mozharivskyj, Inorg. Chem. 48, 5296 (2009).<br />

[4] E. Maciążek, T. Groń, A.W. Pacyna, T. Mydlarz, B. Zawisza, and J. Krok-Kowalski, Acta<br />

Phys. Pol. A 119, (<strong>2011</strong>).<br />


40th _______________________________________________________________<br />

"Jaszowiec" <strong>2011</strong> International School and Conference on the Physics <strong>of</strong> Semiconductors MoP29<br />

Photoluminescence Study <strong>of</strong> ZnSe Scintillating Crystals Doped with<br />

Isovalent Tellurium and Oxygen<br />

D. Shevchenko 1 , J. Mickevi ius 1 , G. Tamulaitis 1<br />

N. Starzhinskiy 2 , K. Katrunov 2 , V. Ryzhikov 2<br />

1 Institute <strong>of</strong> Applied Research and Semiconductor Physics Department,<br />

Vilnius University, Sauletekio Ave. 9-III, LT-10222 Vilnius, Lithuania<br />

2 Institute for Scintillation Materials, 60 Lenin Ave., 61001 Kharkov, Ukraine<br />

Recently, zinc selenide has attracted considerable attention as a promising scintillating<br />

material for ionizing radiation detection. Compared to conventional CsI(Tl) scintillators,<br />

ZnSe –based scintillators have these advantages: considerably lower hygroscopicity, lower<br />

afterglow level, high light yield, and higher radiation stability (by 3 to 4 orders <strong>of</strong> magnitude).<br />

Thus, scintillators based on ZnSe semiconducting compounds are an attractive choice for<br />

applications in medical and industrial tomography, X-ray introscopy and many others.<br />

Operating photoemission band in the ZnSe scintillators results from optical transition<br />

involving deep-defect states. Deep-level-related radiative recombination efficiency can be<br />

improved by doping with isovalent impurities and subsequent thermal annealing in Zn vapor.<br />

Introduction <strong>of</strong> the isovalent impurities promote formation <strong>of</strong> thermally stable complex<br />

defects, which act as radiative recombination centers. Subsequent annealing in Zn vapor<br />

enhances the concentration <strong>of</strong> the complex defects. Therefore it is important to understand<br />

radiative and non-radiative recombination mechanisms and their relation with the crystal<br />

growth conditions.<br />

The study <strong>of</strong> deep-level-related photoluminescence was carried out in the ZnSe crystals<br />

doped with isovalent tellurium and oxygen. Doping <strong>of</strong> ZnSe by Te was accomplished by<br />

mixing tellurium into the melt for crystal growth. Two methods <strong>of</strong> doping with oxygen were<br />

used: ZnSe(O) sample was doped with O by annealing finelly ground row material in oxygenrich<br />

atmosphere, while ZnSe(O, Al) sample was doped with oxygen by mixing Al2O3 into the<br />

melt for crystal growth. Undoped ZnSe samples were also studied for reference. After growth,<br />

all the samples under study were annealed in Zn vapor in identical conditions.<br />

Photoluminescence (PL) spectra were measured under steady-state conditions using CW<br />

He-Cd laser as an excitation source. The wavelength dependence <strong>of</strong> the absolute quantum<br />

yield was measured using an integrating Ulbricht sphere. Monochromated light <strong>of</strong> a halogen<br />

lamp was used for PL excitation.<br />

It was determined that photoluminescence quantum yield depends on excitation photon<br />

energy. The quantum yield rapidly decreases when the excitation photon energy is increased<br />

above the ZnSe band gap. The highest quantum yield (~ 22 %) is observed in the sample<br />

doped with tellurium and annealed in Zn vapor. The quantum yield <strong>of</strong> this sample was by a<br />

factor <strong>of</strong> 2 higher than that in other crystals. The observed dependence <strong>of</strong> the quantum yield<br />

on excitation wavelength is explained by nonradiative carrier recombination on crystal surface<br />

and direct excitation <strong>of</strong> donor-acceptor pair luminescence in ZnSe crystal volume.<br />

The study <strong>of</strong> PL spectra showed that the deep–level–related photoemission efficiency<br />

can be also enhanced by doping with oxygen. PL intensity <strong>of</strong> ZnSe(O, Al) scintillating crystal<br />

is by a factor <strong>of</strong> 16 higher than that <strong>of</strong> ZnSe(O) scintillator. The photoluminescence spectra<br />

consist <strong>of</strong> two strongly overlapped bands due to recombination via M- and SA-type complex<br />

defects. Annealing results in an increase <strong>of</strong> PL intensity by factor <strong>of</strong> ~3 and a red-shift <strong>of</strong> the<br />

PL band, compared with those in unannealed crystals.<br />


MoP30 _______________________________________________________________<br />

40th "Jaszowiec" <strong>2011</strong> International School and Conference on the Physics <strong>of</strong> Semiconductors<br />

Native Deep-Level Defects in MBE-Grown p-Type CdTe<br />

Karolina Olender, Tadeusz Wosiński, Andrzej Mąkosa, Piotr Dłużewski,<br />

Valery Kolkovsky and Grzegorz Karczewski<br />

Institute <strong>of</strong> Physics, Polish Academy <strong>of</strong> Sciences, 02-668 Warsaw, Poland<br />

We present results <strong>of</strong> investigation <strong>of</strong> deep-level defects in p-type CdTe layers grown by<br />

the molecular-beam epitaxy (MBE) technique. The CdTe layers <strong>of</strong> 5 µm thickness were doped<br />

with nitrogen and grown on (001)-oriented p+-type, Zn-doped GaAs substrate. Prior to<br />

deposition <strong>of</strong> the investigated CdTe:N film, the GaAs substrate was covered with thin, 3monolayer-thick,<br />

undoped ZnTe layer to reduce the strong lattice mismatch between CdTe and<br />

GaAs and to stabilize the growth in the [001] direction.<br />

Deep-level transient spectroscopy (DLTS) was used to identify a set <strong>of</strong> deep electronic<br />

states in the band gap <strong>of</strong> p-CdTe layer employing Schottky barriers, obtained by sputtering<br />

aluminum. Room-temperature free carrier concentration in the CdTe layers was<br />

2.4 × 10 16 cm -3 , as found from capacitance–voltage measurements recorded at a frequency <strong>of</strong><br />

1 MHz.<br />

DLTS measurements, carried out in the temperature range from 90 to 400 K, showed<br />

five hole traps, at low concentrations <strong>of</strong> the order <strong>of</strong> 10 13 cm -3 , called H1 to H5. Explicit<br />

saturation <strong>of</strong> the dependence <strong>of</strong> DLTS-signal amplitude on the filling-pulse time was observed<br />

for the H2 and H3 traps. They were characterized by the exponential kinetics <strong>of</strong> carrier<br />

capture, which means that the observed DLTS peaks originate from isolated point defects or<br />

impurities. The H2 trap, with the activation energy for hole emission <strong>of</strong> 0.46 eV, most likely is<br />

the same trap, which we observed as a minority-carrier trap in n-type CdTe:I layer [1]. We<br />

attributed this trap to the Cd vacancy, VCd. The H3 trap, with the activation energy <strong>of</strong> 0.75 eV,<br />

has been tentatively ascribed to the complex formed <strong>of</strong> VCd and the Te antisite defect, TeCd.<br />

In the case <strong>of</strong> H1 and H4 traps the dependence <strong>of</strong> DLTS-signal amplitude on the filling-<br />

pulse time, observed over the range <strong>of</strong> four orders <strong>of</strong> magnitude <strong>of</strong> the time, was logarithmic.<br />

It suggests that the observed traps are related to the hole states <strong>of</strong> dislocations. Thorough<br />

analysis <strong>of</strong> the dependence <strong>of</strong> DLTS-peak shape on the filling time allowed us to determine<br />

the type <strong>of</strong> electronic states associated with these dislocations. The H1 trap, with the<br />

activation energy <strong>of</strong> 0.22 eV, is characteristic <strong>of</strong> localized states. On the other hand, the H4<br />

trap, with the activation energy <strong>of</strong> 0.49 eV, was assigned to the bandlike states <strong>of</strong> dislocations.<br />

Probably, an electron emission from the same dislocation states was observed in our DLTS<br />

experiments for n-type CdTe:I epitaxial layers [1]. We attributed both the H1 and H4 traps to<br />

threading dislocations, generated at the mismatched interface with the substrate and<br />

propagated through the CdTe layer.<br />

Finally, the H5 trap, displaying the logarithmic capture kinetics, has been associated<br />

with acceptor states at the CdTe surface. We observed a strong influence <strong>of</strong> electric field on<br />

the DLTS-peak position <strong>of</strong> this trap and its apparent activation energy. Measurements <strong>of</strong> the<br />

electric-field dependences <strong>of</strong> hole emission rates, performed at various temperatures, allowed<br />

us to determine the mechanism responsible for the field-induced enhancement <strong>of</strong> hole<br />

ionization rate from the traps as the phonon-assisted tunnelling.<br />

This work has been partially supported by the Polish Ministry <strong>of</strong> Science and Higher<br />

Education under Grant No. N N515 0719 37.<br />

[1] K. Olender, T. Wosinski, A. Makosa, S. Kret, V. Kolkovsky and G. Karczewski, Semicond.<br />

Sci. Technol. 26, 045008 (<strong>2011</strong>).<br />


40th _______________________________________________________________<br />

"Jaszowiec" <strong>2011</strong> International School and Conference on the Physics <strong>of</strong> Semiconductors MoP31<br />

Residual Paramagnetism at High-Temperature Semiconductors<br />

CuEu2W2O10 and Cu3Eu2W4O18<br />

P. Urbanowicz 1 , E. Tomaszewicz 2 , T. Groń 1 , H. Duda 1 and Z. Kukuła 1<br />

1 University <strong>of</strong> Silesia, Institute <strong>of</strong> Physics, ul. Uniwersytecka 4, 40-007 Katowice, Poland<br />

2 West Pomeranian University <strong>of</strong> Technology, Department <strong>of</strong> Inorganic and Analytical<br />

Chemistry, Al. Piastów 42, 71-065 Szczecin, Poland<br />

CuEu2W2O10 and Cu3Eu2W4O18 were synthesized by the solid-state reaction method<br />

using CuWO4 and Eu2WO6 as the starting materials. The CuEu2W2O10 and Cu3Eu2W4O18<br />

compounds crystallize in the monoclinic and triclinic system, respectively [1]. Copper<br />

tungstate, CuWO4, belongs to the triclinic distorted wolframite type structure [2–4] in which<br />

every metal ion is surrounded by six oxygen ions. It has a potential technological significance<br />

(scintillation detectors, optical fibres). Thin film <strong>of</strong> CuWO4 is expected to be a promising<br />

positive electrode material for high-performance rechargeable lithium batteries [5,6]. On the<br />

other hand, doped and undoped rare-earth metal molybdates and tungstates (e.g., Eu2WO6) are<br />

host candidates for luminescent applications. They are used for a fabrication <strong>of</strong> white lightemitting<br />

diodes showing high stability, energy-saving and safety [7,8]. In the case <strong>of</strong> Eu2WO6<br />

narrower multiplet widths comparable to the thermal energy cause a weak temperature<br />

dependence <strong>of</strong> the magnetic susceptibility without the Curie-Weiss region as well as a low ntype<br />

conduction [9].<br />

The magnetic susceptibility <strong>of</strong> the powder compounds under study was measured in the<br />

temperature range 2-300 K in the applied external magnetic fields H = 5 kOe using a SQIUD<br />

Magnetometer <strong>of</strong> the Quantum Design MPMS – XL – 5 type. The electrical resistivity has been<br />

measured by a four-probe dc method using the apparatus with Keithley K181 digital<br />

multimeters. The thermoelectric power was measured by a differential method using the<br />

temperature difference <strong>of</strong> about 2 K. For electrical measurements the powder samples were<br />

compacted to disc form (10mm in diameter and 1–2 mm thick) using a pressure <strong>of</strong> 1.5GPa<br />

and they were next sintered through 2 h at 873 K.<br />

The electrical and magnetic measurements <strong>of</strong> CuEu2W2O10 and Cu3Eu2W4O18 showed a<br />

residual paramagnetism, i.e. a low value <strong>of</strong> the magnetic susceptibility and its weak<br />

temperature dependence as well as a semiconducting behaviour at high temperatures. We<br />

have also found that the effective magnetic moment increases gradually from 0.5 µB at 2 K to<br />

5 µB at 300 K, suggesting a transition from the low spin (LS) state <strong>of</strong> Eu 3+ ions to their LS and<br />

high spin (HS) mixed states, as the effective number <strong>of</strong> Bohr magnetons (peff = 9.80 for S = 3,<br />

HS) has not been reached. It means that only 50 % conversion <strong>of</strong> the molecules from LS to<br />

HS occurred. This work is partly founded from science Grant No. N N209 336937.<br />

[1] E. Tomaszewicz, J. Typek, and S.M. Kaczmarek, J. Therm. Anal. Calorim. 98, 409(2009).<br />

[2] L. Kihlborg, and L. Gebert, Acta Crystallogr. B 26, 1020 (1970).<br />

[3] S. Klein, and H. Weitzel, J. Appl. Crystallogr. 8, 54 (1975) (in German).<br />

[4] P.F. Sch<strong>of</strong>ield, K.S. Knight, S.A.T. Redfern, and G. Cressey, Acta Crystallogr. B 53, 102<br />

(1997).<br />

[5] L.G. Van Uitert, and S.T. Preziosi, J. Appl. Phys. 33, 2908 (1962).<br />

[6] R.H. Gillette, Rev. Sci. Instrum. 21, 294 (1950).<br />

[7] S. Neeraj, N. Kijima, and A.K. Cheetham, Chem. Phys. Lett. 387, 2 (2004).<br />

[8] T. Kim, and S. Kang, J. Lumin. 122–123, 964 (2007).<br />

[9] P. Urbanowicz, E. Tomaszewicz, T. Groń, H. Duda, A.W. Pacyna, and T. Mydlarz, Physica<br />

B 404, 2213 (2009).<br />


MoP32 _______________________________________________________________<br />

40th "Jaszowiec" <strong>2011</strong> International School and Conference on the Physics <strong>of</strong> Semiconductors<br />

Ferromagnetic nanocomposite Co-Al2O3 as a spintronic material with<br />

engineered magnetic properties<br />

M.V. Radchenko 1 , G.V. Lashkarev 1 , M.E. Bugaiova 1 , V.I. Sichkovskyi 1 ,<br />

V.I. Lazorenko 1 , L.A. Krushynskaya 2 ,Y.A. Stelmakh 2 , W. Kn<strong>of</strong>f 3 , T. Story 3<br />

1 I.M. Frantsevych Institute for Problems <strong>of</strong> Material Science, National Academy <strong>of</strong> Sciences<br />

<strong>of</strong> Ukraine, 3 Krzhizhanovskogo str., Kyiv, Ukraine<br />

2 E.O.Paton Electric Welding Institute, Academy <strong>of</strong> Sciences <strong>of</strong> Ukraine, 68 Antonowich str.,<br />

Kyiv, Ukraine<br />

3 Institute <strong>of</strong> Physics, Polish Academy <strong>of</strong> Sciences, Al. Lotników 32/46, Warsaw, Poland<br />

Ferromagnetic cobalt nanoparticles (NP) embedded in alumina matrix constitute a new class<br />

<strong>of</strong> two-phase spintronic materials - ferromagnetic nanocomposites (FMNC). FMNC attract<br />

attention as artificially developed materials permitting efficient engineering <strong>of</strong> their magnetic<br />

and magnetotransport properties These materials are closely related to inhomogeneous diluted<br />

magnetic semiconductors containing ferromagnetic clusters. In this work, we study magnetic<br />

and electrical characteristics <strong>of</strong> Cox(Al2O3)100-x (x

40th _______________________________________________________________<br />

"Jaszowiec" <strong>2011</strong> International School and Conference on the Physics <strong>of</strong> Semiconductors MoP33<br />

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MoP34 _______________________________________________________________<br />

40th "Jaszowiec" <strong>2011</strong> International School and Conference on the Physics <strong>of</strong> Semiconductors<br />

Van der Waals surface <strong>of</strong> InSe as a standard nanorelief in the metrology <strong>of</strong><br />

nanoobjects<br />

Dmitriev A. I.<br />

. I. M. Frantsevich Institute for Problems <strong>of</strong> Material Science, National Academy <strong>of</strong><br />

Sciences <strong>of</strong> Ukraine, Krzhizhanivskogo st. 3, 03680 Kyiv, Ukraine, FAX: +38424-2131.<br />

E-mail: dmitr. kiev@gmail.com<br />

Development <strong>of</strong> new high technologies and new materials leading to the fundamental<br />

improvment <strong>of</strong> the technical level <strong>of</strong> production largely dependents on the level <strong>of</strong> metrological<br />

support. Intensive development <strong>of</strong> nanotechnology requires high-precision measurements on<br />

atomic and molecular levels by creating the metrological system for measurements, at the first<br />

turn, the length in the nanometer scale. Such measurements require standard samples <strong>of</strong><br />

nanorelief surface.<br />

At present, the nanorelief surface <strong>of</strong> highly oriented pyrolytic graphite (HOPG) is used for<br />

this purpase. Simultaneously with the scanning <strong>of</strong> an unknown surface the scan <strong>of</strong> the well-<br />

known surface <strong>of</strong> graphite (the period <strong>of</strong> the hexagonal crystal lattice: a = 2,464 ± 0,002 Ǻ) is<br />

carried out. S<strong>of</strong>tware recognition <strong>of</strong> atoms image <strong>of</strong> the graphite allows to generate a control<br />

signal for correcting the microscope manipulator. When cleaving HOPG many stages with<br />

dimensions less than 2x2 mm are formed. They are used as a model <strong>of</strong> nanorelief They appear<br />

at fracture <strong>of</strong> graphite planes, oriented not toward cleavage. Graphite planes have structural<br />

defects associated with dislocations. Different types <strong>of</strong> dislocations manifest themselves in the<br />

topography <strong>of</strong> the surface in many ways. Over time, the accumulation <strong>of</strong> the adsorbate on the<br />

surface, results in poor quality images.<br />

Semiconductor InSe belongs to a group <strong>of</strong> layered compounds A3B6.Each layer <strong>of</strong> this<br />

crystal consists <strong>of</strong> atomic groups Se-In-In-Se with strong covalent bonds in the layer. Se atoms<br />

<strong>of</strong> neighboring layers are linked by weak Van der Waals (VdW) forces and form VdW surface.<br />

High chemical stability and the absence <strong>of</strong> dangling bonds (defects) prevents the appearance <strong>of</strong><br />

the adsorbat. Low surface roughness <strong>of</strong> cleaved surface at the areas up to 5x5 mm allows to<br />

use the VdW surface <strong>of</strong> InSe single crystals as the standard nanorelief. This is evidenced by<br />

the researches <strong>of</strong> morphology for VdW surface by scanning tunneling microscopy [1] in which<br />

images with atomic resolution were obtained. A single crystal InSe, grown by vertical<br />

Bridgman-Stockbarger method. The crystal has -g-polytype rhombohedral structure . with<br />

periods a = 4.003 Å, and c = 24.9553 Å (in hexagonal axes).<br />

Thus InSe is appropriate standard nanorelief for probe microscopy.<br />

1. Physics <strong>of</strong> the Solid State,53, 3, 622 (<strong>2011</strong>).<br />


40th _______________________________________________________________<br />

"Jaszowiec" <strong>2011</strong> International School and Conference on the Physics <strong>of</strong> Semiconductors MoP35<br />

Crystal field analysis <strong>of</strong> the Yb 3+ energy level scheme in III-V<br />

semiconductors<br />

A. Kami�ska 1 , C.-G. Ma 2 , M.G. Brik 2 , A. Kozanecki 1 , M. Bo�kowski 3 , E. Alves 4 ,<br />

A. Suchocki 1,5<br />

1 Institute <strong>of</strong> Physics, Polish Academy <strong>of</strong> Sciences, 02-668 Warsaw, Poland<br />

2 Institute <strong>of</strong> Physics, University <strong>of</strong> Tartu, Riia 142, Tartu 51014, Estonia<br />

3 Institute <strong>of</strong> High Pressures Physics ‘Unipress’, Polish Academy <strong>of</strong> Sciences, 01-142 Warsaw,<br />

Poland<br />

4 Instituto Tecnologico e Nuclear, Estrada Nacional 10, 2686-953 Sacavém, Portugal<br />

5 Institute <strong>of</strong> Physics, University <strong>of</strong> Bydgoszcz, Weyssenh<strong>of</strong>fa 11, 85-072 Bydgoszcz, Poland<br />

Rare earths (RE) doped semiconductors are nowadays attracting a lot <strong>of</strong> attention due<br />

to their unique optical and electrical properties and high potential <strong>of</strong> these materials in new<br />

optoelectronic device applications, for instance in electrically-pumped light-emitting diodes<br />

or lasers [1]. Among the RE doped III–V semiconductors InP:Yb is one <strong>of</strong> the most<br />

intensively studied material. Recently an important effort has been devoted also to study the<br />

optical properties <strong>of</strong> ytterbium doped GaN [2, 3]. These materials reveal strong Yb-related<br />

luminescence at about 1 �m. Yb 3+ ions (4f 13 electron configuration) have a simple energy<br />

level scheme, which consists <strong>of</strong> two states only: 2 F7/2 and 2 F5/2 separated by about<br />

10 000 cm -1 . Such a scheme excludes excited state absorption and all related energy losses.<br />

From this point <strong>of</strong> view, Yb 3+ ion is now the most promising for use as a non-Nd lasing center<br />

in the infrared spectral region.<br />

In the present work we report on combination <strong>of</strong> the high-pressure measurements with<br />

use <strong>of</strong> diamond anvil cell (DAC) <strong>of</strong> the luminescence spectra <strong>of</strong> InP:Yb 3+ and GaN:Yb 3+<br />

crystals with ab initio calculations <strong>of</strong> the electronic and optical properties <strong>of</strong> these systems.<br />

The CASTEP module [4] <strong>of</strong> the Materials Studio was used to calculate the electronic and<br />

optical properties at ambient and elevated pressure. In addition, crystal field analysis <strong>of</strong><br />

splitting <strong>of</strong> the 2 F7/2 and 2 F5/2 states has been performed, with an aim <strong>of</strong> assigning all features<br />

<strong>of</strong> the experimental luminescence spectra.<br />

A thorough analysis <strong>of</strong> the pressure behavior <strong>of</strong> the Yb 3+ luminescence lines in InP<br />

reveals interesting effects, namely: after 6 GPa blue shift <strong>of</strong> the luminescence tends to<br />

saturation, which was explained by pressure-induced shift <strong>of</strong> the InP valence band<br />

approaching the energies <strong>of</strong> the Yb 4f-4f transitions. This overlap behaves exactly as the<br />

luminescence lines with pressure. An analysis <strong>of</strong> the pressure dependence <strong>of</strong> the Yb 3+<br />

luminescence in GaN and fitting the trigonal crystal field Hamiltonian to the experimental<br />

data allowed to determine the ambient pressure values <strong>of</strong> crystal field parameters, spin orbit<br />

parameter and their pressure coefficients.<br />

Acknowledgements: This work was partly supported by the European Union within<br />

European Regional Development Fund, through the Innovative Economy grant<br />

(POIG.01.01.02-00-108/2009/2) and the project No. N N202 203838 <strong>of</strong> Polish Ministry <strong>of</strong><br />

Science and Higher Education.<br />

References:<br />

[1] J. H. Park et al., J. Appl. Phys. 98, 056108 (2005).<br />

[2] W.M. Jadwisienczak et al., Opt. Mater. 23, 175 (2003),<br />

[3] T. Koubaa et al., J. Alloys Compd. 496, 56 (2010).<br />

[4] M. D. Segall et al., J. Phys.: Condens. Matter 14, 2717 (2002).<br />


MoP36<br />

_______________________________________________________________<br />

40th "Jaszowiec" <strong>2011</strong> International School and Conference on the Physics <strong>of</strong> Semiconductors<br />

Studies <strong>of</strong> Electric Transport Properties <strong>of</strong> Single Layer Devices<br />

Based on the Polyazomethine Thin Films<br />

J. Weszka, M. Domański, J. Jurusik, B. Hajduk, M. Chwastek,<br />

and H. Bednarski<br />

Centre <strong>of</strong> Polymer and Carbon Materials, Polish Academy <strong>of</strong> Sciences, 34 M.<br />

Curie-Sk̷lodowska Str., 41-819 Zabrze, Poland<br />

Organic semiconductors (OS) have been known since a many years for their great potential<br />

applications in opto-electronic devices. Among the conjugated polymers, poly(pphenylenevinylene)<br />

(PPV) and its derivatives (like MEH-PPV) belong to the family<br />

<strong>of</strong> more extensively studied OSs and are known as revealing good properties for optoelectronic<br />

applications. Poly (p-phenyleneazomethine) (PPI), much less known and studied<br />

polymer than PPV, being its iso-elctronic counterpart and having nearly the same<br />

electronic structure, though containing nitrogen atoms in the backbone, has appeared to<br />

be very promising for optoelectronic applications, too [1,2].<br />

Single-layer organic devices have been produced based on chemical vapour deposited,<br />

in a homemade technological set-up [3]. Polyazomethine thin films were grown via polycondensation<br />

process <strong>of</strong> aromatic diamine and dialdehyde and sandwiched between metal<br />

electrodes with different work functions. The choice <strong>of</strong> the electrodes has appeared to<br />

influence greatly the device properties. For these reasons various pairs <strong>of</strong> electrode materials<br />

were used from ITO, Al and Au in order to check practically conditions <strong>of</strong> Schottky<br />

barrier formation and their applicability for photovoltaic devices. Metallic electrodes were<br />

thermally evaporated under vacuum <strong>of</strong> 10 −6 Tr, otherwise glass substrates covered with<br />

ITO layer were used.<br />

To verify quality <strong>of</strong> the as-prepared devices current-voltage characteristics have been<br />

taken as well as measurements <strong>of</strong> the current dependence on time were performed. Results<br />

have been analysed in terms <strong>of</strong> the unified device model for single layer organic light<br />

emitting diodes or photovoltaic cells with charge injection, transport and space charge<br />

effects [4]. The model describes both injection limited and space charge limited current<br />

flow and the transition between them. Determined by us relatively high value <strong>of</strong> the<br />

Schottky barrier height (higher then 0.5 eV) suggests that the current flow in the asprepared<br />

devices is injection charge limited, which is further supported by the net injected<br />

charge being relatively small. Therefore, for small bias voltage in the injection limited<br />

regime the thermionic emission is the dominant injection mechanism.<br />

Relatively low currents density, in comparison to those reported for PPV, recorded<br />

for our studied PPI based devices is attributed to the presence <strong>of</strong> surface states on the<br />

side <strong>of</strong> high work function electrode rather than to steric differences in the backbones <strong>of</strong><br />

the two polymers. This is consistent with the results we obtained on MEH-PPV devices<br />

prepared for comparison reasons.<br />

[1] A. Iwan and D. Sek, Prog. Polym. Sci. 33 (2008) 289-345.<br />

[2] J. Weszka, H. Bednarski and M.Domanski, J. Chem. Phys. 131 (2009) 024901.<br />

[3] J. Weszka, M. Domanski, B. Jarzabek, J. Jurusik, J. Cisowski, A. Burian, Thin<br />

Solid Films 516 (2008) 3098.<br />

[4] P.S. Davids, I.H. Campbell, and D.L. Smith, J. Appl. Phys. 82 (1997) 6319.<br />


40th _______________________________________________________________<br />

"Jaszowiec" <strong>2011</strong> International School and Conference on the Physics <strong>of</strong> Semiconductors MoP37<br />

Optical properties <strong>of</strong> black silicon with precipitated silver and gold<br />

nanoparticles<br />

R. Jarimavi i t -Žvalionien 1 , I. Prosy evas 1 , Ž. Kaminskien 1 , S. Lapinskas 2<br />

1 Institute <strong>of</strong> Materials Science, Kaunas University <strong>of</strong> Technology, Savanoriu pr. 271, Kaunas, Lithuania<br />

2 Institute <strong>of</strong> Applied Research, Vilnius University, Sauletekio str. 10, Kaunas, Lithuania<br />

Black silicon (BS) is a new generation material which may be useful for many<br />

applications, such as lasers, gas sensors, solar cells and etc. Black silicon (BS) is a type <strong>of</strong> porous<br />

silicon with low light reflectance and high light absorbance values. BS layers are especially<br />

useful and important in solar energy conversion. Forming BS layers on silicon increase the<br />

effective area and light absorbance as well as solar conversion efficiency. Another new way to<br />

increase solar energy conversion is use surface plasmons, i.e., collective surface oscillations <strong>of</strong><br />

conducting electrons in metal nanostructures that tend to trap optical waves near their surface.<br />

Plasmonics is a rapidly emerging subfield <strong>of</strong> nanotechnologies, which gives a possibility to<br />

control and guide light interaction with matter more effectively. Noble metals like silver and<br />

gold are ideal for this purpose as they do not have many interband transitions and do not absorb<br />

much light as a result. By combining black silicon with precipitated plasmonic nanoparticles in<br />

active layer has the potential to increase light interaction with matter more effectively and can<br />

revolutionary change the market <strong>of</strong> solar cells.<br />

Etching <strong>of</strong> silicon and formation <strong>of</strong> porous surfaces can be performed by many different<br />

methods. But the most simple in use is chemical metal assisted etching. In this work the porous<br />

silicon plates were prepared by wet chemical metal assisted method in a 1-minute single step<br />

liquid etch based upon catalysis by silver nanoparticles formed in a solution containing HF, V2O2<br />

and AgNO3. Processing time was reduced to 10 seconds by heating the samples at 80 o C. For<br />

synthesis <strong>of</strong> plasmonic nanoparticles silver nitrate (AgNO3) and chloroauric acid (HAuCl4) were<br />

used.<br />

Scanning electron microscope (SEM) analysis was done in order to evaluate the form and<br />

size <strong>of</strong> formed pores before precipitation <strong>of</strong> plasmonic nanoparticles and morphology <strong>of</strong> surface<br />

<strong>of</strong> porous silicon after precipitation. Optical properties <strong>of</strong> black silicon samples were investigated<br />

with UV-VIS and FTIR spectrometers and reflectance spectra were recorded during porous<br />

silicon formation at different times.<br />

SEM analysis <strong>of</strong> PS exhibited oval-shaped pores with Ø 150 – 200 nm and Ø 50 – 100<br />

nm in samples with longer and shorter processing times respectively. SEM analysis <strong>of</strong> BS with<br />

precipitated nanoparticles has revealed non-uniform distribution <strong>of</strong> gold and silver nanoparticles<br />

(Ø 70 - 150 nm) on BS surface and inside <strong>of</strong> pores. Reflectance analysis <strong>of</strong> BS has revealed that<br />

reflectance decreases with increase <strong>of</strong> processing time, herewith with pores depth, and increases<br />

after precipitation <strong>of</strong> nanoparticles in both cases. This phenomenon is still under investigation,<br />

but in general we can conclude, that formation <strong>of</strong> BS was successful and precipitation <strong>of</strong><br />

nanoparticles on its surface gives a possibility to control reflectivity <strong>of</strong> BS surfaces and increase<br />

photo efficiency through enhanced plasmonic properties <strong>of</strong> gold and silver nanoparticles.<br />


MoP38 _______________________________________________________________<br />

40th "Jaszowiec" <strong>2011</strong> International School and Conference on the Physics <strong>of</strong> Semiconductors<br />

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40th _______________________________________________________________<br />

"Jaszowiec" <strong>2011</strong> International School and Conference on the Physics <strong>of</strong> Semiconductors MoP39<br />

Self-Compensation Mechanism in Semi-Insulating CdMnTe:Sn Crystals<br />

Intended for X/γ-Ray Detectors<br />

N.S. Yurtsenyuk, L.A. Kosyachenko, V.M. Sklyarchuk,<br />

O.L. Maslyanchuk, E.V. Grushko<br />

Chernivtsi National University, Optoelectronics Department, 58012 Chernivtsi, Ukraine<br />

Cd1-xMnxTe is a typical diluted magnetic semiconductor with the zinc-blende structure<br />

within the 0 < x < 0.77 range. Owing to its unique magnetic and magneto-optic properties,<br />

Cd1-xMnxTe crystals can be used in many device applications, such as Faraday rotators,<br />

optical isolators, solar cells, magnetic field sensors and substrates for the epitaxial growth <strong>of</strong><br />

Hg1-xCdxTe and Hg1-xZnxTe [1]. More recently, Cd1-xMnxTe <strong>of</strong>fers several potential<br />

advantages over Cd1-xZnxTe as a material for room-temperature X/γ-ray detectors [2,3] that<br />

are widely used in science, medicine, industry, environment monitoring and other fields.<br />

Cd1-xMnxTe has a wide range <strong>of</strong> tunability <strong>of</strong> the band gap due to the large compositional<br />

influence <strong>of</strong> manganese. The band gap range 1.7 eV to 2.2 eV, required for substantiating an<br />

“ideal” radiation detector performance, can be realized with a relatively low (< 50%) amount<br />

<strong>of</strong> Mn compared with that <strong>of</strong> Zn in Cd1-xZnxTe (up to 70%) [4]. In fact, the energy band gap<br />

<strong>of</strong> Cd1-xMnxTe increases about 13 meV per atomic percent <strong>of</strong> Mn compared with 6.7 meV <strong>of</strong><br />

Zn in Cd1-xZnxTe. Another important advantage is that the segregation coefficient <strong>of</strong> Mn in<br />

CdTe is approximately 1 compared with 1.35 for Zn, that is, Cd1-xMnxTe crystals can have a<br />

more homogenous distribution <strong>of</strong> Mn throughout the ingot. These notable characteristics <strong>of</strong><br />

Cd1-xMnxTe increase the possibility <strong>of</strong> growing uniform large-volume stoichiometric crystals<br />

that can potentially be fabricated into different high-quality devices.<br />

In this work we study the conductivity mechanism <strong>of</strong> Cd1-xMnxTe single crystals doped<br />

by Sn. It is known that the introduction <strong>of</strong> Sn in the CdTe, as well as in other II-VI<br />

semiconductors, accompanied by the formation <strong>of</strong> complexes and self-compensation <strong>of</strong><br />

conductivity and leads to a semi-insulating state <strong>of</strong> the semiconductor, which is very<br />

important from the standpoint <strong>of</strong> its use in X/γ-ray detectors. We set a goal to find the<br />

ionization energy <strong>of</strong> the level responsible on electrical conductivity Ei <strong>of</strong> the material and the<br />

degree <strong>of</strong> its compensation ξ = Na/Nd. The band gap and its temperature dependence<br />

(necessary for calculations) were found from the optical transmission curves in the range <strong>of</strong><br />

the absorption edge <strong>of</strong> semiconductor. Based on the experimental dependences <strong>of</strong> the<br />

electrical characteristics <strong>of</strong> the material and statistics <strong>of</strong> electrons and holes, we have<br />

succeeded in determining Ei and ξ. Such a degree <strong>of</strong> compensation reveals to be close to<br />

100%. This is a direct confirmation <strong>of</strong> self-compensation, which provides semi-insulating<br />

state <strong>of</strong> the semiconductor. It seems that the obtained results lead to the conclusions which are<br />

important for technology.<br />

[1] J.K. Furdyna, J. Appl. Phys. 64(4), R29 (1988).<br />

[2] A. Burger, K. Chattopadhyay, H. Chen, J.-O. Ndap, X. Ma, S. Trivedi, S.-W. Kutcher, R. Chen,<br />

and R.-D. Rosemeier. J. Cryst. Growth 198/199, 872 (1999).<br />

[3] A. Mycielski, A. Burger,M. Sowinska, M. Groza, A. Szadkowski, P. Wojnar, B.Witkowska,<br />

W.Kaliszek, and P. Siffert, Phys. Stat. Sol. (c) 2, 1578 (2005).<br />

[4] J.E. Toney, T.E. Schlesinger, and R.B. James, Nucl. Instr. & Meth., A428, 14 (1999).<br />


MoP40 _______________________________________________________________<br />

40th "Jaszowiec" <strong>2011</strong> International School and Conference on the Physics <strong>of</strong> Semiconductors<br />

Influence <strong>of</strong> Cu, Ga and Au Dopants and Technology Conditions on the<br />

Magnetic Interactions in HgCr2Se4 Single Crystals<br />

J. Krok-Kowalski 1 , G. Władarz 1 , T. Groń 1 , H. Duda 1 , A.W. Pacyna 2 , K. Nikiforov 3 and<br />

P. Rduch 1<br />

1 University <strong>of</strong> Silesia, Institute <strong>of</strong> Physics, ul. Uniwersytecka 4, 40-007, Katowice, Poland<br />

2 The Henryk Niewodniczański Institute <strong>of</strong> Nuclear Physics, Polish Academy <strong>of</strong> Sciences,<br />

ul. Radzikowskiego 152, 31-342 Kraków, Poland<br />

3 Physico-Mathematical Faculty, K. Tsiolkovski State Pedagogical University,<br />

St. Razin str. 26, Kaluga 248023, Russia<br />

It is well known, that in the ferromagnetic spinel compound HgCr2Se4 (where TC and<br />

CW are <strong>of</strong> order 100 K and 150 K respectively) the divalent Hg 2+ ions occupy tetrahedral<br />

positions only, whereas trivalent Cr 3+ ions are located only in octahedral positions.<br />

Ferromagnetic coupling <strong>of</strong> the magnetic moments in this compound is realized via<br />

superexchange magnetic interactions [1]. It is also well known, that the full substitution in the<br />

tetrahedral positions <strong>of</strong> the divalent Hg 2+ ions by the monovalent, e.g. Cu + ions, leads to both<br />

the mixed valence <strong>of</strong> the chromium ions (it means, Cr 3+ and Cr 4+ in the ratio 1:1) and to the<br />

appearance and domination <strong>of</strong> the another mechanism <strong>of</strong> the magnetic coupling giving very<br />

strong ferromagnetic coupling, namely double exchange interaction. In this case both TC and<br />

CW become <strong>of</strong> order <strong>of</strong> 400 K [2,3]. However, the substitution <strong>of</strong> the Hg 2+ ions by the<br />

trivalent ions, e.g. Ga 3+ , leads to the collinear antiferromagnetism being the result <strong>of</strong> the<br />

superexchange interaction only, with the concentration <strong>of</strong> Ga 3+ ions up to 50%. The one aim<br />

<strong>of</strong> this work is to investigate how influences the substitution <strong>of</strong> Hg 2+ ions by Cu + , Au 2+ and<br />

Ga 3+ ions on the magnetic ordering in the sample, when the concentration <strong>of</strong> the substituting<br />

ions is lower than 10 %. The another one concerns <strong>of</strong> the influence <strong>of</strong> technology conditions<br />

on magnetic ordering in the sample.<br />

Using the chemical transport method the single crystals <strong>of</strong> HgCr2Se4 doped with copper<br />

(7%), gallium (8%) and gold (2%) were obtained. Additionally, the two non doped single<br />

crystals <strong>of</strong> HgCr2Se4 were obtained, in different synthesis conditions. One <strong>of</strong> them was<br />

synthesized with the deficiency <strong>of</strong> Hg (this procedure the produces vacancies), another one<br />

with Hg excess (this procedure brings to elimination <strong>of</strong> vacancies). For all crystals under<br />

study both the dc and ac magnetic susceptibility was measured in the temperature range <strong>of</strong><br />

4.2 – 250 K. The dc measurements were carried out with the use <strong>of</strong> the Cahn magnetic<br />

balance and SQUID susceptometer, whereas the ac measurements were carried out with the<br />

use <strong>of</strong> the Lake Shore 7225 magnetometer.<br />

As it follows from the obtained results, substitution monovalent Cu + ions and divalent<br />

Au 2+ ions in place <strong>of</strong> Hg 2+ ions causes the increase <strong>of</strong> the Curie temperature in comparison<br />

with the Curie temperature <strong>of</strong> undoped HgCr2Se4. Substitution <strong>of</strong> Hg 2+ ions by trivalent Ga 3+<br />

ions causes the lowering <strong>of</strong> TC. On the other hand, ZFC and FC measurements <strong>of</strong> the<br />

magnetic susceptibility shows that in the samples containing Cu + and Ga 3+ some trace<br />

frustration <strong>of</strong> magnetic interactions in spite <strong>of</strong> their ferromagnetic ordering occurs. Similar<br />

effect have been observed for single crystal synthesized with the deficiency <strong>of</strong> Hg. The latter<br />

effect is caused by the occurrence <strong>of</strong> vacancies generated in the sample by the technological<br />

process.<br />

[1] P.K. Baltzer, P.J. Wojtowicz, M. Robbins, and E. Lopatin, Phys. Rev. 151, 367 (1966).<br />

[2] F.K. Lotgering, Proc. Int. Conf. on Magnetism, Nottingham England, (1964) 533.<br />

[3] J. Krok, J. Spałek, J. Juszczyk, and J. Warczewski, Phys. Rev. B 28, 6499 (1983).<br />


40th _______________________________________________________________<br />

"Jaszowiec" <strong>2011</strong> International School and Conference on the Physics <strong>of</strong> Semiconductors MoP41<br />

The Photodetectors with Vertical Integration Based on ZnO Films<br />

A.I. Ievtushenko 1 , G.V. Lashkarev 1 , V.I. Lazorenko 1 , Zs.J. Horvath 2,3 , M.G. Dusheyko 4<br />

1 Frantsevich Institute for Problems <strong>of</strong> Materials Science, NASU, Kyiv, Ukraine<br />

2 Research Institute for Technical Physics and Materials Science, HAS, Budapest, Hungary<br />

3 Institute <strong>of</strong> Microelectronics and Technology, Budapest, Hungary<br />

4 National Technical University <strong>of</strong> Ukraine “KPI”, Kyiv, Ukraine<br />

ZnO is a wide-band (Eg 3.3 eV) direct-gap material which attracts a great attention to<br />

the development <strong>of</strong> the photodetectors for near-ultraviolet range. It is an electronic analog <strong>of</strong><br />

GaN, but ZnO have economic and ecological benefits in comparison with GaN. There are<br />

many papers devoted to the formation detectors <strong>of</strong> a planar configuration [1]. One <strong>of</strong> the<br />

promising directions <strong>of</strong> microelectronics is the development and study <strong>of</strong> devices with a<br />

vertical structure. The reason is that in this vertical configuration <strong>of</strong> electrodes the<br />

interelectrode distance can be easily reduced without using expensive lithography processes.<br />

Therefore, the purpose <strong>of</strong> this report is devoted to the study <strong>of</strong> technological aspects for the<br />

deposition <strong>of</strong> ZnO films as well as the investigation <strong>of</strong> the photoelectrical properties <strong>of</strong><br />

structures with vertical configuration <strong>of</strong> the electrodes.<br />

Early we reported the positive role <strong>of</strong> nitrogen in ZnO for UV detectors [2]. Therefore,<br />

the undoped and nitrogen doped ZnO films with different concentrations <strong>of</strong> the nitrogen were<br />

deposited by magnetron sputtering on n-Si and ITO/glass substrates. Structural and optical<br />

properties <strong>of</strong> the films were examined by XRD, EDX and optical measurements. Aluminum<br />

as ohmic contact to Si and Ni as Schottky contact to ZnO were deposited by electron beam<br />

evaporation with thickness control. Ni/ZnO/n-Si/Al and Ni/ZnO/ITO layered structures <strong>of</strong><br />

photodetectors were obtained. Current-voltage curves <strong>of</strong> photodiodes were studied in dark and<br />

under visible and UV irradiations using Keithley-236 Source Measure Unit. Features <strong>of</strong> the<br />

current-voltage characteristics and leakage current mechanisms are discussed. It was found an<br />

increase <strong>of</strong> photosensitivity with increasing nitrogen concentration in nitrogen doped ZnO<br />

films for all types <strong>of</strong> vertical structures. The maximum enhancement <strong>of</strong> reverse current under<br />

UV irradiation was two orders <strong>of</strong> magnitude. The influence <strong>of</strong> deposition technology <strong>of</strong> ZnO<br />

films on the photoelectrical properties <strong>of</strong> detectors as well as the effect <strong>of</strong> nitrogen<br />

incorporation will be presented. Prospects for the application <strong>of</strong> ZnO photodetectors with<br />

vertical integration and requirements for their design will be discussed.<br />

[1] K. Liu, M. Sakurai, M. Aono Sensors, 10, pp.8604-8634 (2010).<br />

[2] A.I. Ievtushenko, G.V. Lashkarev, V.I. Lazorenko, et al. Phys. Stat. Sol. ( ), 207, 7, pp.<br />

1746–1750 (2010).<br />


MoP42 _______________________________________________________________<br />

40th "Jaszowiec" <strong>2011</strong> International School and Conference on the Physics <strong>of</strong> Semiconductors<br />

Features <strong>of</strong> the Properties for Nitrogen Doping and Al-N Codoping <strong>of</strong> ZnO<br />

Films<br />

A.I. Ievtushenko 1 , G.V. Lashkarev 1 , V.I. Lazorenko 1 , O.Y. Khyzhun 1 , L.O. Klochkov 1 ,<br />

O.I. Bykov 1 , V.M. Tkach 2 , V.A. Baturin 3 , A.Y. Karpenko 3<br />

1 Frantsevich Institute for Problems <strong>of</strong> Materials Science, NASU, Kyiv, Ukraine<br />

2 Bakul Institute for Superhard Materials, Kyiv, Ukraine<br />

3 Institute <strong>of</strong> Applied Physics, Sumy, Ukraine<br />

Now zinc oxide is almost the most investigated material in the world because <strong>of</strong> its<br />

physical, ecological and economic benefits which make it promising one for photonic,<br />

optoelectronic and acoustooptic devices, etc. However the properties <strong>of</strong> ZnO films strongly<br />

depend on the concentration <strong>of</strong> various defects appearing at the stages <strong>of</strong> film growth and<br />

further processes in as-grown ZnO films.<br />

The nitrogen in ZnO is an acceptor impurity and widely used in attempts to obtain<br />

stable p-type conductivity in ZnO. However, based on our previous studies, we think that<br />

nitrogen doping leads to the passivation <strong>of</strong> oxygen vacancies <strong>of</strong> ZnO and therefore increase<br />

the photosensitivity, response speed and stability <strong>of</strong> photodetectors on their basis. Little<br />

known fact is the effect <strong>of</strong> increasing the concentration <strong>of</strong> nitrogen in ZnO films on their<br />

physical properties by using the increase <strong>of</strong> nitrogen pressure in the chamber at magnetron<br />

deposition. It is promising to investigate the way <strong>of</strong> increasing <strong>of</strong> nitrogen solubility in ZnO<br />

by using Al-N codoping.<br />

ZnO:N films with different nitrogen concentration were deposited on Si, SiNx and<br />

quartz substrates by magnetron sputtering using Zn, Zn:0.7%Al and Zn:1.4%Al targets. The<br />

investigations <strong>of</strong> XRD, AFM, PL, X-ray photoelectronic and EDX spectroscopy were carried<br />

out. In this report the influence <strong>of</strong> different types <strong>of</strong> nitrogen doping on the transformation <strong>of</strong><br />

the structure, optical properties and morphology <strong>of</strong> ZnO:N films will be presented.<br />


40th _______________________________________________________________<br />

"Jaszowiec" <strong>2011</strong> International School and Conference on the Physics <strong>of</strong> Semiconductors MoP43<br />

Influence <strong>of</strong> Oxygen Pressure on the Properties <strong>of</strong> AZO Films<br />

A.I. Ievtushenko 1 , G.V. Lashkarev 1 , V.I. Lazorenko 1 , L.O. Klochkov 1 , O.I. Bykov 1 ,<br />

O.M. Kutsay 2 , S.P. Starik 2 , V.A. Baturin 3 , A.Y. Karpenko 3<br />

1 Frantsevich Institute for Problems <strong>of</strong> Materials Science, NASU, Kyiv, Ukraine<br />

2 Bakul Institute for Superhard Materials, Kyiv, Ukraine<br />

3 Institute <strong>of</strong> Applied Physics, Sumy, Ukraine<br />

New oxide materials are very attractive for replacement <strong>of</strong> existing traditional ones in<br />

various fields <strong>of</strong> electronics and optoelectronics. Particularly the problems <strong>of</strong> creation the<br />

transparent conductive electrodes based on them attract more attention.<br />

The major drawback <strong>of</strong> widely-used ITO is a limited world reserve <strong>of</strong> indium.<br />

Opposite to ITO, zinc oxide has ecological and economical benefits and its components are<br />

widely distributed in Earth. Therefore, doped with aluminum (AZO), gallium (GZO) or<br />

indium (IZO) ZnO films are the best suited candidates to replace ITO. The development <strong>of</strong><br />

technology for deposition <strong>of</strong> transparent conductive electrodes based on AZO films will have<br />

the commercial future.<br />

Therefore the high quality aluminum doped ZnO films were deposited on glass<br />

substrates by layer by layer deposition method at a magnetron sputtering <strong>of</strong> Zn:1.4%Al target.<br />

The report presents the results <strong>of</strong> oxygen pressure influence on structural, morphological,<br />

electrical and optical properties <strong>of</strong> AZO films. It is demonstrated that optimal oxygen pressure<br />

<strong>of</strong> 0.03 Pa in the deposition chamber leads to the following AZO films characteristics: the<br />

resistivity is about 10 -3 Ohm cm, transparency - 95% and roughness - 6 nm. Our results on<br />

investigation <strong>of</strong> AZO films magnetron deposition technology will be discussed as well as the<br />

prospects <strong>of</strong> their applications.<br />


MoP44 _______________________________________________________________<br />

40th "Jaszowiec" <strong>2011</strong> International School and Conference on the Physics <strong>of</strong> Semiconductors<br />

Thermoelectric Power <strong>of</strong> CuCrxVySe4 p-Type Spinel Semiconductors<br />

H. Duda 1 , E. Malicka 2 , T. Groń 1 , A. Gągor 3 , R. Sitko 2 , J. Krok-Kowalski 1 and P. Rduch 1<br />

1 University <strong>of</strong> Silesia, Institute <strong>of</strong> Physics, ul. Uniwersytecka 4, 40-007 Katowice, Poland<br />

2 University <strong>of</strong> Silesia, Institute <strong>of</strong> Chemistry, ul. Szkolna 9, 40-006 Katowice, Poland<br />

3 Institute <strong>of</strong> Low Temperature and Structure Research, Polish Academy <strong>of</strong> Sciences,<br />

ul. Okólna 2, 50-950 Wrocław, Poland<br />

Detailed studies <strong>of</strong> the thermoelectric power, electrical conductivity, structural and<br />

magnetic properties carried out on polycrystalline CuCrxVySe4 spinels with x = 1.79, 1.64 and<br />

1.49 and y = 0.08, 0.22 and 0.45, respectively, are presented. Powder samples <strong>of</strong> the<br />

CuCrxVySe4 spinel series were prepared by annealing stoichiometric mixtures <strong>of</strong> high purity<br />

( 99.99 %) elements: Cu, Cr, V and Se. A precise atomic content determination <strong>of</strong> each<br />

sample with the aid <strong>of</strong> the energy-dispersive X-ray spectrometry (EDXRF) showed that the<br />

polycrystals <strong>of</strong> the CuCrxVySe4 spinel system revealed the cation deficiencies in the<br />

octahedral sites because x + y<br />

y<br />

2. A Rietveld refinement <strong>of</strong> the spinels under study was done<br />

using an X'Pert PRO X-ray diffractometer and a Jana-2000 program package. The electrical<br />

conductivity has been measured with the four-probe dc method using the apparatus with<br />

Keithley K181 digital multimeters. The thermoelectric power was measured with a<br />

differential method using the temperature gradient T <strong>of</strong> about 2 K. The electrical and thermal<br />

contacts between the sample and the copper rods were maintained with a silver lacquer<br />

mixture (Degussa Leitsilber 200). Magnetization and magnetic susceptibility were performed<br />

using a Faraday type Cahn RG automatic electrobalance at magnetic field up to 7 kOe. The<br />

electrical and magnetic measurements were done in the temperature ranges 77-432 K and 77-<br />

550 K, respectively.<br />

All the compounds under study crystallize in regular system <strong>of</strong> a normal spinel type<br />

MgAl2O4 structure with the space group symmetry Fd3m. With increasing V content a lattice<br />

parameter slightly increases from 1033.66(4) pm for y = 0.08 to 1033.71 (5) for y = 0.45<br />

while the positional anion parameter decreases from 0.2574(3) for y = 0.08 to 0.2568(3) for y<br />

= 0.45. The CuCrxVySe4 spinels are ferromagnets for that a Curie temperature TC and a<br />

paramagnetic Curie-Weiss temperature decrease as the V content increases, i.e. from TC =<br />

406 K and = 383 K for y = 0.08, via TC = 351 K and = 380 K for y = 0.22 via to TC = 282<br />

K and = 277 K for y = 0.45. The electrical conductivity measurements revealed the change<br />

<strong>of</strong> the hole conductivity character from the semiconductive into the metallic one close to TC.<br />

The thermoelectric power analysis was done with the aid <strong>of</strong> the modified Matoba, Anzai and<br />

Fujimori semi-empirical formula [1] including magnetic contribution [2]. The results <strong>of</strong><br />

thermopower analysis <strong>of</strong> the spinels under study show that the intensity <strong>of</strong> the magnon drag<br />

component is smaller in comparison with the single crystal sample <strong>of</strong> comparable V-content<br />

[2]. The magnon excitations, usually driven by the double exchange mechanism, are observed<br />

only at low temperatures. At higher temperatures the contribution to total thermopower<br />

originates mainly from the phonon component. At high temperatures, the diffusion component<br />

dominates and, according to the Mott formula, it is proportional to temperature. The non-zero<br />

value <strong>of</strong> the so-called impurity component <strong>of</strong> the thermopower described by the T 1/2 law<br />

suggests the presence <strong>of</strong> cation deficiencies in the octahedral sites.<br />

This work is partly founded from science Grant No. N N204 145938.<br />

[1] M. Matoba, S. Anzai, and A. Fujimori, J. Phys. Soc. Jpn. 63, 1429 (1994).<br />

[2] E. Malicka, T. Groń, D. Skrzypek, A.W. Pacyna, D. Badurski, A. Waśkowska, S. Mazur,<br />

and R. Sitko, Philos. Mag. 90, 1525 (2010).<br />


40th _______________________________________________________________<br />

"Jaszowiec" <strong>2011</strong> International School and Conference on the Physics <strong>of</strong> Semiconductors MoP45<br />

Critical Behaviour <strong>of</strong> the 3D-Heisenberg Ferromagnetic<br />

Semiconductors CdxCeyCr2Se4<br />

H. Duda 1 , P. Rduch 1 , E. Malicka 2 , T. Groń 1 and A. Gągor 3<br />

1 University <strong>of</strong> Silesia, Institute <strong>of</strong> Physics, ul. Uniwersytecka 4, 40-007 Katowice, Poland<br />

2 University <strong>of</strong> Silesia, Institute <strong>of</strong> Chemistry, ul. Szkolna 9, 40-006 Katowice, Poland<br />

3 Institute <strong>of</strong> Low Temperature and Structure Research, Polish Academy <strong>of</strong> Sciences,<br />

ul. Okólna 2, 50-950 Wrocław, Poland<br />

The pure CdCr2Se4 end member <strong>of</strong> the CdxCeyCr2Se4 spinel system combines the p-type<br />

semiconducting and ferromagnetic properties with a Curie temperature TC = 142 K and a<br />

Curie-Weiss temperature qCW = 190 K [1]. Magnetization <strong>of</strong> CdCr2Se4 reaches the full<br />

saturation <strong>of</strong> 5.98 µB per molecule [2] and the ferromagnetic properties are the result <strong>of</strong><br />

dominance <strong>of</strong> large and positive the nearest-neighbour Cr-Cr interactions [3]. Various<br />

theoretical models <strong>of</strong> the critical phenomena, the mean-field, the three-dimensional (3D)<br />

Heisenberg, the 3D-Ising and the tricritical mean-field model used to explain the critical<br />

properties showed that the CdCr2Se4 was in best accordance with the 3D-Heisenberg model<br />

for which the critical exponents: β = 0.36, γ = 1.38 and δ = 4.8 [4,5].<br />

The CdxCeyCr2Se4 spinel single crystals under study have a normal cation distribution,<br />

the Cd and Ce ions being located at the tetrahedral sites, while the Cr ions at the octahedral<br />

ones. Electrical and magnetic measurements revealed the semiconducting properties and a<br />

ferromagnetic order with TC = 132 K which insensibly decreases as the Ce content increases<br />

as well as a strong reduction <strong>of</strong> magnetic moment from 5.8 µB for y = 0.03 to 1.93 µB for y =<br />

0.13. The critical exponents <strong>of</strong> CdxCeyCr2Se4 single crystals were studied by measuring<br />

isothermal dc-magnetization around the paramagnetic-ferromagnetic (PM-FM) phase<br />

transition. Isothermal magnetization was measure with the aid <strong>of</strong> a Quantum Design System<br />

(MPMS XL) in the magnetic field up to 20 kOe and in the temperature range 125-140 K.<br />

The critical exponents (TC, β, γ and δ) were determined by analyzing the magnetization<br />

data in terms <strong>of</strong> various research techniques including modified Arrott plot [6], Kouvel-Fisher<br />

method [7], and critical isotherm analysis. Here β and γ are the critical exponents for the<br />

temperature dependence <strong>of</strong> the spontaneous magnetization just below TC and inverse initial<br />

susceptibility just above TC, respectively. In addition, the critical exponent δ is determined<br />

separately from the isothermal magnetization at TC. The critical exponent values are found to<br />

be consistent with the Widom scaling relation δ = 1 + γ/β, implying the critical exponents are<br />

reliable. The critical exponents characterizing the PM-FM transition are: β = 0.367 ±0.013, γ<br />

= 1.389 ±0.025 and δ = 4.763 ±0.043 at TC = 131 K for the sample Cd0.96Ce0.03Cr2Se4 and β =<br />

0.350 ±0.003, γ = 1.202 ±0.026 and δ = 4.487 ±0.095 at TC = 134 K for the sample<br />

Cd0.84Ce0.13Cr2Se4. These values <strong>of</strong> critical exponents concerning CdxCeyCr2Se4 system are<br />

close to the theoretical ones predicted by the 3D-Heisenberg model for CdCr2Se4.<br />

This work is partly founded from science Grant No. N N204 145938.<br />

[1] P.K. Baltzer, H.W. Lehmann, and M. Robbins, Phys. Rev. Lett. 15, 493 (1965).<br />

[2] R.C. LeCraw, H. von Philipsborn, and M.D. Sturge, J. Appl. Phys. 38, 965 (1967).<br />

[3] P.K. Baltzer, M. Robbins, and P.J. Wojtowicz, J. Appl. Phys. 38, 953 (1967).<br />

[4] L. Zhang, J. Fan, L. Li, R. Li, L. Ling, Z. Qu, W. Tong, S. Tan, and Y. Zhang, A Letters<br />

Journal Exploring the Frontiers <strong>of</strong> Physics (EPL) 91, 57001 (2010).<br />

[5] W. Zarek, Acta. Phys. Pol. A 52, 657 (1977).<br />

[6] A. Arrott, and J.E. Noakes, Phys. Rev. Lett. 19, 786 (1967).<br />

[7] J.S. Kouvel and M.E. Fisher, Phys. Rev. Lett. 136, A1626 (1964).<br />


MoP46 _______________________________________________________________<br />

40th "Jaszowiec" <strong>2011</strong> International School and Conference on the Physics <strong>of</strong> Semiconductors<br />

Optical properties <strong>of</strong> ZnO/ZnMgO single quantum wells grown by<br />

molecular beam epitaxy<br />

J.M. Sajkowski, M.A. Pietrzyk, D. Dobosz, M. Stachowicz, A. Droba, E.Przezdziecka,<br />

A.Wierzbicka, A. Kozanecki,<br />

Institute <strong>of</strong> Physics Polish Academy <strong>of</strong> Sciences, Al. Lotników 32/46 02-668, Warsaw, Poland<br />

In this work the high quality ZnMgO layers and ZnO/ZnMgO quantum wells (QWs) showing<br />

no phase separation were grown on (0001) sapphire substrates. ZnMgO layers and<br />

ZnO/ZnMgO heterostructures were grown at temperatures 445 - 460°C. No specific buffer<br />

layers were deposited on sapphire. Prior to growth the substrates were annealed in the growth<br />

chamber in oxygen (p=1,4*10 -5 Torr) at 600° C, and then a ~100 nm ZnMgO (Mg contents 10<br />

- 20%) barrier layer was grown. Then the growth was interrupted for couple <strong>of</strong> minutes and<br />

after this a ZnO wells with different thicknesses were grown. Finally, ZnMgO capping layers<br />

<strong>of</strong> the same composition as the ZnMgO barrier were deposited. After growth the structures<br />

were characterized using X-ray diffraction for the analysis <strong>of</strong> their crystalline quality and<br />

AFM was used for surface morphology studies. AFM images revealed very good flatness <strong>of</strong><br />

the surface – rms value was 1-2 nm.<br />

We present results <strong>of</strong> optical spectroscopy for a series <strong>of</strong> ZnO/ZnMgO single quantum wells<br />

<strong>of</strong> different widths using photoluminescence. Temperature dependent PL spectra were<br />

measured within a temperature range <strong>of</strong> 4-300K. We observed a blue shift <strong>of</strong> excitonic<br />

emission in comparison with bulk ZnO values which demonstrates quantum confinement in<br />

the ZnO wells with thicknesses smaller than 4 nm. We also demonstrate that excitonic<br />

emission from QWs is dominated by excitons bound to donors. The characteristic energy <strong>of</strong><br />

PL decay <strong>of</strong> the dominant excitonic lines agrees well with the values <strong>of</strong> localization energies<br />

<strong>of</strong> excitons to donor centers. For wider ZnO wells the quantum confined Stark effect shifts the<br />

PL spectra below the band gap <strong>of</strong> ZnO. This is a direct consequence <strong>of</strong> the very large built-in<br />

electric field developed in ZnO/ZnMgO QWs grown along the polar c-direction.<br />

Acknowledgement: Work supported in part within European Regional Development Fund,<br />

through grant Innovative Economy (POIG.01.01.02-00-008/08).<br />


40th _______________________________________________________________<br />

"Jaszowiec" <strong>2011</strong> International School and Conference on the Physics <strong>of</strong> Semiconductors MoP47<br />

Electronic structure <strong>of</strong> CdTe/PbEuTe/CdTe<br />

B.A. Orlowski 1 , S. P. Dziawa 1 , A. Reszka 1 , K. Gas 1 , S. Mickievicius 2 , S. Thiess 3 ,<br />

W. Drube 3 .<br />

! " # $<br />

$ % &<br />

' ( $) $ $ $ $ ) ( $ ' * + , - $ ! $ $<br />

,- * . $ "/ ,! ' ( $) % $<br />

Top part <strong>of</strong> the semiconductor nanostructure CdTe/Pb0.95Eu 0.05Te/CdTe with Pb0.95Eu 0.05Te<br />

buried under thin layer <strong>of</strong> CdTe (the layer transparent for part <strong>of</strong> photoemitted electrons) was<br />

studied with application <strong>of</strong> High Energy X-ray Photoemission Spectroscopy. The Energy<br />

Distribution Curves (EDC) corresponding to the valence band and core levels electrons <strong>of</strong> the<br />

buried layer and top cover CdTe layer were measured with application <strong>of</strong> synchrotron<br />

radiation <strong>of</strong> high energy h = 3510eV to obtain big escape depth <strong>of</strong> electrons photoemitted<br />

from buried layer. The measured peaks corresponding to the buried layer atoms were<br />

observed in the valence band region and in the high binding energy region for core levels<br />

Pb4f, Pb3d and Eu 3d. For top cover layer the contribution <strong>of</strong> valence band, Cd4d and<br />

Cd3d core levels were obtained. Measured Te4d, Te 3d and Te4d spectra possesses<br />

contribution as well from buried as from the top cover layer.<br />

The nanostructure was grown by MBE deposition method in the Institute <strong>of</strong> Physics, Polish<br />

Academy <strong>of</strong> Sciences in Warsaw [1]. The CdTe buffer layer was evaporated on GaAs wafer<br />

and Pb0.95Eu0.05Te layer <strong>of</strong> 6nm thick, evaporated on it. The layer <strong>of</strong> Pb0.95Eu 0.05Te was<br />

covered by CdTe layer <strong>of</strong> thickness 22 nm. The structure was exposed to the air and<br />

photoemission spectra were performed ex situ using the Tunable High Energy X-ray<br />

Photoemission Spectrometer at HASYLAB, Hamburg [2]. After sputtering <strong>of</strong> CdTe top layer<br />

the photoemission signal <strong>of</strong> buried layer elements remarkably increases (see Fig.1, Pb5d).<br />

Main parameters <strong>of</strong> the electronic structure <strong>of</strong> CdTe/Pb0.95Eu 0.05Te/CdTe were determined.<br />

Intensity [a. u.]<br />

1 . 4<br />

1 . 2<br />

1 . 0<br />

0 . 8<br />

0 . 6<br />

0 . 4<br />

0 . 2<br />

0 . 0<br />

C d T e / P b E u T e / C d T e<br />

H I G H E N E R G Y X P S<br />

h ν = 3 5 1 0 e V<br />

S p u t t e r i n g<br />

2 n d<br />

1 s t<br />

n o<br />

v b<br />

C d 4 d<br />

P b 5 d<br />

T e 4 d<br />

- 1 0 0 1 0 2 0 3 0 4 0 5 0<br />

B i n d i n g E n e r g y [ e V ]<br />

Fig.2. Set <strong>of</strong> EDC’s <strong>of</strong> the valence band, Cd4d, Pb5d and Te4d electrons region and measured after following<br />

steps <strong>of</strong> cleaning <strong>of</strong> the sample by sputtering CdTe 22nm top layer.<br />

The authors acknowledge support by MSHE <strong>of</strong> Poland research Projects DESY/68/2007 and by the European<br />

Community FP6 Programme "Integrating Activity on Synchrotron and Free Electron Laser Science" at DESY.<br />

1. M. Szot, L. Kowalczyk, E. Smajek, V. Domukhovski, J. Domagała, E. Łusakowska, B. Taliashvili,<br />

P. Dziawa, W. Kn<strong>of</strong>f, W. Wiater, T. Wojtowicz, T. Story, Acta Phys. Pol. 114, 1397 (2009).<br />

2. Th. Eickh<strong>of</strong>f, V. Medicherla, W. Drube, Jour.Electr. Spectr.Rel. Phen. 137-140, 85-88 (2004)<br />

orbro@ifpan.edu.pl<br />

85<br />

1 7<br />

0 4<br />

0 2

MoP48 _______________________________________________________________<br />

40th "Jaszowiec" <strong>2011</strong> International School and Conference on the Physics <strong>of</strong> Semiconductors<br />

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40th _______________________________________________________________<br />

"Jaszowiec" <strong>2011</strong> International School and Conference on the Physics <strong>of</strong> Semiconductors MoP49<br />

Multi-interface layered P-doped silicon structures for third generation<br />

photovoltaics<br />

Mikael Hosatte 1 , Marek Basta 1 , Bartłomiej S. Witkowski 2 , Zbigniew T. Kuznicki<br />

1 and Marek Godlewski 2<br />

1 Photonic Systems Laboratory, Pole Api, Blvd. Sebastien Brandt, 67414 Illkirch-<br />

Graffenstaden, France<br />

2 Institute <strong>of</strong> Physics, Polish Academy <strong>of</strong> Sciences, Al. Lotnikow 32/46, 02-668 Warszawa,<br />

Poland<br />

Nanoscale Si-layered systems represent an attractive way to enlarge optical and electrical<br />

functions in Si optoelectronic, photonic and PV technology. Physical interactions transform<br />

the initial Si material to a new Si-based metamaterial. The device architecture also plays a<br />

role in specific nonlinear features. The seemingly paradoxical behavior requires a better<br />

insight into understanding the mechanisms determining the macroscopic performance. We<br />

report here some specific optical and electrical properties resulting from the complexity <strong>of</strong><br />

different test structures. Optoelectronic properties <strong>of</strong> nanostructured materials can differ<br />

greatly comparing to those <strong>of</strong> bulk, or even thin film. The complexity <strong>of</strong> low-dimensional<br />

interactions renders impossible to directly use the description <strong>of</strong> the phenomena developed for<br />

previous device generations [1]. This is particularly true for Multi-interface Novel Devices<br />

(MINDs) which results mainly from the superposition <strong>of</strong> a PN junction [2], a nanoscale<br />

system [3] and a multi-interface architecture. In MINDs, whose collection efficiencies have<br />

been improved since the first generation, the buried substructure is presented as a<br />

metamaterial layer because it exhibits physical properties different than bulk silicon. In<br />

particular, it allows low energy-carrier multiplication.<br />

a) b)<br />

Figure 1. Architecture <strong>of</strong> MIND test structure (a) and SEM measurement showing buried nanoscale<br />

substructure as well as metamaterial layer (b)<br />

[1]P. Havu, V. Havu, M.J. Puska, M.H. Hakala, A. Foster, and R.M. Nieminen, “Finiteelement<br />

implementation for electron transport innanostructures”,J.Chem. Phys.124, 054707<br />

(2006)<br />

[2]C.T Sah, R.N. Noyce, W. Shockley, Proc. IRE 45, 1228 (1957). (2001).<br />

[3]Z. T. Kuznicki, Multi-interface novel devices, model with a continuous substructure, “3 rd<br />

Generation Photovoltaics for High Efficiency through Full Spectrum Utilization”, ed. A.<br />

Luque& A. Marti (Institute <strong>of</strong> Physics Publishing, Bristol, UK, 2003) pp. 165-195.<br />


MoP50 _______________________________________________________________<br />

40th "Jaszowiec" <strong>2011</strong> International School and Conference on the Physics <strong>of</strong> Semiconductors<br />

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40th _______________________________________________________________<br />

"Jaszowiec" <strong>2011</strong> International School and Conference on the Physics <strong>of</strong> Semiconductors MoP51<br />

ZnO-based Nanotubes Obtained by the Oxidation <strong>of</strong> ZnTe<br />

and ZnTe/Zn Nanowires<br />

K. Gas 1* , E. Dynowska 1 , E. Janik 1 , A. Kami ska 2 , S. Kret 1 , J.F. Morhange 3 ,<br />

I. Pasternak 4 , M. Wiater 1 , W. Zaleszczyk 1 , R. Hołyst 2 , E. Kami ska 4 , T. Wojtowicz 1 ,<br />

W. Szuszkiewicz 1<br />

1 Institute <strong>of</strong> Physics PAS, Al. Lotników 32/46, Warsaw, Poland<br />

2 Institute <strong>of</strong> Physical Chemistry, PAS, ul. Kasprzaka 44/52, Warsaw, Poland<br />

3 Institut des Nanosciences de Paris, UMR 7588, UMPC, 4 Place Jussieu, Paris, France<br />

4 Institute <strong>of</strong> Electron Technology, Al. Lotników 32/46, Warsaw, Poland<br />

One-dimensional (1D) semiconductor nanostructures like, e.g., nanowires (NWs) or<br />

nanobelts recently attracted a great deal <strong>of</strong> attention due to their potential application in<br />

electronic and optoelectronic nanodevices. Among many materials ZnO is particularly<br />

intensively studied due to its wide, direct energy gap (3.37 eV at 295 K) and large exciton<br />

binding energy <strong>of</strong> 60 meV. The non-intentionally doped ZnO usually exhibits n-type<br />

conductivity and it is difficult to convert it to p-type. The lack <strong>of</strong> p-type ZnO is the major<br />

obstacle in possible device applications <strong>of</strong> this material. Contrary to ZnO, ZnTe can be<br />

relatively easy highly p-type doped. Moreover, as it was already shown thermal oxidation <strong>of</strong><br />

nitrogen-doped ZnTe allows to obtain p-type ZnO [1], therefore one possible way to produce<br />

p-type, ZnO-based NW structure would be through the oxidation <strong>of</strong> p-type ZnTe NWs. In this<br />

paper we report a successful fabrication <strong>of</strong> the ZnO-containing nanotubes obtained by thermal<br />

oxidation. For this purpose MBE-grown ZnTe and ZnTe/Zn core/shell NWs were oxidized by<br />

annealing in flowing O2 gas using both a rapid thermal annealing apparatus and a furnace.<br />

Some samples were annealed in flowing Ar gas for a comparison.<br />

Scanning and transmission electron microscopy (SEM and TEM, respectively)<br />

demonstrated tubular character and some changes in the NWs’ morphology after the oxidation<br />

process. The TEM analyses also revealed that the nanotubes are built <strong>of</strong> small ZnO and TeO2<br />

crystallites with random crystallographic orientations. The average inner diameter <strong>of</strong> the<br />

nanotubes is about 40 nm whereas their outer diameter can reach up to 100 nm (measured in<br />

the central part <strong>of</strong> the NW). We determined the chemical composition <strong>of</strong> the samples using<br />

energy dispersive X-ray spectroscopy. Complementary information on the structure properties<br />

<strong>of</strong> the NWs was obtained by X-ray diffraction, the optical properties <strong>of</strong> the NWs were also<br />

investigated by Raman scattering. The influence <strong>of</strong> the technological conditions on the<br />

evolution <strong>of</strong> the nanostructures is shown and discussed. The results <strong>of</strong> our studies suggest that<br />

the most probable mechanism <strong>of</strong> formation <strong>of</strong> such tubular nanostructures consists <strong>of</strong> two<br />

major steps. Firstly, the ZnTe is partially decomposed by the reaction with oxygen both at the<br />

surface <strong>of</strong> the NW and along its axis at the center <strong>of</strong> the NWs. Due to very fast oxygen<br />

diffusion along the [111] direction a creation <strong>of</strong> a cavity starts in the central part <strong>of</strong> the NW.<br />

Next, the Zn from the surface produce the ZnO shell and the remaining Te aggregate into<br />

small nanocrystals, which decorate the hollow inner part <strong>of</strong> the ZnO NW [2] or (after an<br />

oxidation) form TeO2.<br />

The studies were partially supported by the European Union within European<br />

Regional Development Fund, through grant Innovative Economy (POIG.01.01.02-00-008/08).<br />

* Corresponding author, e-mail address: kgas@ifpan.edu.pl<br />

[1] E. Prze dziecka et al., Semicond. Sci. Technol. 22, 10 (2007).<br />

[2] P. Lu, D.J. Smith, phys. stat. sol. (a) 107, 681 (1988).<br />


MoP52 _______________________________________________________________<br />

40th "Jaszowiec" <strong>2011</strong> International School and Conference on the Physics <strong>of</strong> Semiconductors<br />

Composition-dependentthermalresistance<strong>of</strong>multilayeredstructurestakingintoaccountphononreflection,scatteringandtunneling.<br />

DariuszŻak 1,2 ,WłodzimierzNakwaski 2<br />

1 Institute<strong>of</strong>ElectronTechnology,<br />

al.Lotników32/46,02-668Warsaw,Poland.<br />

2 PhotonicsGroup,Institute<strong>of</strong>Physics,TechnicalUniversity<strong>of</strong>Lodz,<br />

ul.Wolczanska219,90-924Lodz,Poland.<br />

Abstract<br />

Developmentinthesynthesisandprocessing<strong>of</strong>nanoscaledevicescreatesthe<br />

demandtopredicttheirthermalproperties.Nowadays,superlatticestructures<br />

arebasecomponents<strong>of</strong>lightemittingandlightdetectingdevicesinmiddle<strong>of</strong><br />

infra-redspectrumrange.Thermalpropertiestakeimportantplaceindesigning<br />

process.Heatspreadinginmultilayersstructuresismuchdifferentthanthatin<br />

bulkmaterials.Mainpart<strong>of</strong>heattransportinthosestructures,isgovernedby<br />

thephonontransport.Thepresence<strong>of</strong>multipleinterfacesinmultilayerstructuresimpairssignificantlythephonontransport<strong>of</strong>heat,whichresultsinthe<br />

reducedthermalconductivities<strong>of</strong>thelayersandamountstoincreasedthermal<br />

resistivity<strong>of</strong>thedevices.Thebehaviour<strong>of</strong>thephonon/heattransportinthe<br />

presence<strong>of</strong>interfacesisachallengeinanalyticaldescription.Inthisarticlea<br />

semi-analyticalexpressionbasedontheacousticmismatchmodelandthediffuse<br />

mismatchmodelisproposed,thatpredictsthethermalconductivity<strong>of</strong>superlatticestructuresatroomtemperaturesaccurately.Themodelwasusedtopredictthethermalconductivities<strong>of</strong>Si/Ge,Si/Si0.3Ge0.7.Si/Gesuperlatticestructuresconsist<strong>of</strong>60,80,100,200,300periodsandhavedifferentlayerssizeratio<br />

foreachsample.AllSi/Si0.3Ge0.7superlatticestructureswithperiodwidths<br />

300˚A,150˚A,75˚A,45˚Aare3µmthinkandhave2:1layerratio.Theobtained<br />

calculations<strong>of</strong>thermalconductivityinrange<strong>of</strong>temperaturefrom200Kto300K<br />

areingoodagreementwiththeexperimentaldata.Thebehaviour<strong>of</strong>thermal<br />

conductivityinthepresents<strong>of</strong>changingnumbers<strong>of</strong>interfacespredictedbythis<br />

modelisconfirmedbymeasurements.<br />

1<br />


40th _______________________________________________________________<br />

"Jaszowiec" <strong>2011</strong> International School and Conference on the Physics <strong>of</strong> Semiconductors MoP53<br />

Pb1-xGexTe surface with Sm 2+ and Sm 3 + doping<br />

A. Reszka 1 , B.A. Orlowski 1 , M.A. Pietrzyk 1 , A. Szczerbakow 1 , S. Mickievicius 2 ,<br />

S. Balakauskas 2 , R.L. Johnson 3<br />

1 Institute <strong>of</strong> Physics, Polish Academy <strong>of</strong> Sciences, Al. Lotników 32/46, 02-668 Warsaw,<br />

Poland<br />

2 Semiconductor Physics Institute, A. Gostauto 11, 2600 Vilnius, Lithuania<br />

3 Institut für Experimentalphysik, Universität Hamburg, Luruper Chaussee 149, 22761<br />

Hamburg, Germany<br />

In the present work we report the experimental results <strong>of</strong> resonant photoemission study<br />

<strong>of</strong> single crystal Pb1-xGexTe surface doped with Sm. Doping <strong>of</strong> IV-VI semiconductors with<br />

rare earth atoms turns these materials into diluted magnetic semiconductors, exhibiting<br />

a number <strong>of</strong> specific properties [1].<br />

The main goals <strong>of</strong> the presented studies were:<br />

– Determination <strong>of</strong> Sm 4f electrons contribution to the valence band <strong>of</strong> Sm/Pb1-xGexTe.<br />

– Verification <strong>of</strong> Sm ions charge states (Sm 2+ and Sm 3+ ) to determine their structure<br />

and binding energies.<br />

– Investigation <strong>of</strong> the changes <strong>of</strong> Sm 2+ /Sm 3+ ratio during the process <strong>of</strong> Sm incorporation into<br />

the host lattice.<br />

The Pb1-xGexTe (x=0.03) samples were grown by the Bridgman method in the Institute<br />

<strong>of</strong> Physics Polish Academy <strong>of</strong> Sciences in Warsaw. The photoemission experiments were<br />

performed at the FLIPPER II beam line in HASYLAB in Hamburg, Germany.<br />

Typical feature <strong>of</strong> rare-earth photoemission spectroscopy (PES) is a characteristic<br />

behaviour <strong>of</strong> its intensity related to strongly localized 4f-shell electrons. Therefore, the PES<br />

investigations <strong>of</strong> Sm/Pb1-xGexTe were carried out for the varying photon energy through the<br />

region corresponding to Sm 4d 4f Fano resonance (h = 130-160 eV) [2]. In order to get<br />

a clean surface for experiment the sample was sputtered by Ar + ions to remove possible oxide<br />

contaminations and annealed at the temperature 300 ºC under UHV conditions. Samarium<br />

was deposited on the top <strong>of</strong> Pb1-xGexTe sequentially to obtain 1, 3 and 7 ML capping layers.<br />

Afterwards sample was annealed at 250 ºC for 0.5, 1.5 and 6.5 hours to incorporate Sm atoms<br />

into Pb1-xGexTe crystal matrix. After each deposition and annealing processes the set <strong>of</strong><br />

energy distribution curves (EDC) spectra was measured. Deposited samarium ions occur as<br />

Sm 2+ and Sm 3+ at the surface. Annealing led to the decrease <strong>of</strong> Sm 2+ /Sm 3+ ratio indicating<br />

change in the interaction <strong>of</strong> Sm ions with their neighbourhood.<br />

The authors acknowledge support by MSHE <strong>of</strong> Poland research Projects DESY/68/2007 and by the<br />

European Community via the Research Infrastructure Action under the FP6 Structuring the European<br />

Research Area" Programme (through the Integrated Infrastructure Initiative "Integrating Activity on<br />

Synchrotron and Free Electron Laser Science") at DESY. Partially supported by Innovative Economy<br />

(POIG.01.01.02-00-108/09) and (POIG.01.01.02-00-008/08)<br />

[1] T. Story. Semimagnetic Semiconductors Based on Lead Chalcogenides.<br />

Lead Chalcogenides: Physics and Applications. Taylor & Francis, New York 2002.<br />

[2] U. Fano, Phys. Rev. B 23 (1961) 1866.<br />


MoP54<br />

_______________________________________________________________<br />

40th "Jaszowiec" <strong>2011</strong> International School and Conference on the Physics <strong>of</strong> Semiconductors<br />

Studies <strong>of</strong> Optical Properties <strong>of</strong> Protonated Poliazomethine<br />

Thin Films<br />

H. Bednarski 1 , J. Weszka 1 , M. Domański 1 and V. Cozan 2<br />

1 Centre <strong>of</strong> Polymer and Carbon Materials, Polish Academy <strong>of</strong> Sciences, 34 M.<br />

Curie-Sk̷lodowska Str., 41-819 Zabrze, Poland<br />

2 Petru Poni Institute <strong>of</strong> Macromolecular Chemistry, Aleea Gr. Ghica Voda 41 A ,<br />

700487 Iasi, Romania<br />

The poly(p-phenyleneazomethine) (PPI) belongs to alternately conjugated polymers<br />

revealing semiconducting properties, thus belonging to the family <strong>of</strong> organic semiconductors.<br />

PPI is isoelectronic counterpart <strong>of</strong> poly(p-phenylene-vinilene) (PPV), which is well<br />

known for its wide applications in organic optoelectronics. Having nearly the same π electron<br />

system as PPV [1], the PPI differ structurally in the presence <strong>of</strong> nitrogen atoms. This<br />

structural feature opens an additional way <strong>of</strong> chemical doping, by nitrogen atoms lone<br />

pair protonation. Doping <strong>of</strong> conjugated polymers is a well established way <strong>of</strong> influencing<br />

on important electronic, opto-electronic and mechanical properties <strong>of</strong> semiconducting<br />

polymers.<br />

Doping <strong>of</strong> poliazomethines is realized mainly based on acid chemistry. Such a technique<br />

makes the process complex and difficult to control. Apart from intentional protonation<br />

always exist possibility <strong>of</strong> occurrence <strong>of</strong> unwanted side reactions like hydrolysis, decomposition<br />

etc. [2]. Moreover, the routine experimental methods such as IR, UV-vis, provide<br />

only averaged quantities, thus microscopic details are not fully resolved. Generally, the<br />

microscopic mechanism <strong>of</strong> protonation <strong>of</strong> polyazomethines is not well understood and<br />

many reported results are unclear.<br />

The purpose <strong>of</strong> this work is tw<strong>of</strong>old, namely experimental studies <strong>of</strong> optical properties<br />

<strong>of</strong> protonated poliazomethine thin films, as well as theoretical studies <strong>of</strong> protonation<br />

mechanism <strong>of</strong> this important group <strong>of</strong> materials. Performed theoretical calculations are<br />

aimed to rationalize observed experimental finding such as a red shift <strong>of</strong> the maximum <strong>of</strong><br />

the absorption band due to protonation.<br />

[1] J. Weszka, H. Bednarski and M.Domanski, J. Chem. Phys. 131 (2009) 024901.<br />

[2] A. Iwan and D. Sek, Prog. Polym. Sci. 33 (2008) 289-345.<br />


40th _______________________________________________________________<br />

"Jaszowiec" <strong>2011</strong> International School and Conference on the Physics <strong>of</strong> Semiconductors MoP55<br />

The reasons <strong>of</strong> destruction <strong>of</strong> nanoporous GaAs matrix<br />

fabricated by electrochemical etching<br />

A. V. Atrashchenko 1 , V. P. Ulin 1 , V. P. Evtikhiev 1<br />

1 I<strong>of</strong>fe Physical-Technical Institute <strong>of</strong> the Russian Academy <strong>of</strong> Sciences,<br />

Politekhnicheskaya 26, 194021, St Petersburg, Russia<br />

The goal <strong>of</strong> the present study is to find out the reasons <strong>of</strong> the destruction <strong>of</strong> porous<br />

matrices based on n-type GaAs oriented in (100) created by using anodic electrochemical<br />

etchinganddefineparameters<strong>of</strong>theporouslayers(poresdiameter, wallsthickness, depth,<br />

period <strong>of</strong> branching) for further application <strong>of</strong> such matrices in nanocomposite materials<br />

(matrixes with filled with metals or alloys (Ga, In, Ga-Au, In-Au)) [1,2].<br />

Aperspectivetechnique<strong>of</strong>creatingsuchmatricesisthemethod<strong>of</strong>anodicelectrochemical<br />

etching which allows one to obtain nanoporous matrices on large areas. Experiments<br />

in [3,4] show that a creation <strong>of</strong> nanoporous matrices by electrochemical etching method<br />

meets with a problem <strong>of</strong> destruction <strong>of</strong> porous layer with the increasing <strong>of</strong> the etching<br />

depth.<br />

Based on previously obtained data [4] and carried out experiments in this paper there<br />

were suggested three hypotheses explaining the destruction <strong>of</strong> the porous layer with the<br />

increasing <strong>of</strong> depth. 1) During the etching occurs a catastrophic heating which leads to a<br />

change <strong>of</strong> the electrical parameters <strong>of</strong> the reaction. The temperature measurements made<br />

while executing every experiment has refuted this hypothesis. 2) There is a different speed<br />

<strong>of</strong> movement <strong>of</strong> the electrochemical reaction (relative to the front <strong>of</strong> etching) in different<br />

halides. To check this hypothesis we changed the electrolyte composition by leaving only<br />

one halide. This led to predictable changes in the pores parameters [5], but didn’t remove<br />

the problem <strong>of</strong> porous layer destruction. Occasionally we found that with the increasing<br />

<strong>of</strong> the pore size the undamaged layer depth increases. 3) Mechanical destruction <strong>of</strong> the<br />

porous layer is due to high pressure <strong>of</strong> products <strong>of</strong> the etching. In etching form the<br />

products whose volume is larger than etched volume. The products go outside through<br />

nanochannels. The increase <strong>of</strong> the nanochannel length leads to drastic amplification <strong>of</strong><br />

resistance to the products outlet. At some moment a pressure <strong>of</strong> the etching products on<br />

a pore walls exceeds the value <strong>of</strong> the tensile strength so that the pressure at the front <strong>of</strong><br />

pore formation is higher than one in the middle <strong>of</strong> the porous layer. This explains why<br />

at stopping <strong>of</strong> the etching, when the destruction starts, we can see the destruction only<br />

at the front <strong>of</strong> etching.<br />

We have defined the reason <strong>of</strong> the porous matrices destruction while increasing <strong>of</strong> the<br />

porous layer depth by anodic electrochemical etching which is a mechanical destruction<br />

<strong>of</strong> the pores walls due to amplification <strong>of</strong> the etching products pressure on the walls.<br />

We have defined the limits <strong>of</strong> the parameters variation for subsequent usage them as a<br />

medium for creation <strong>of</strong> nanocomposite metal-semiconductor A3B5 materials: the pores<br />

diameter is from 20 nm to 70 nm (up to 200 nm in fluoride electrolytes with an abrupt<br />

decrease <strong>of</strong> the structural order), the walls thickness lies within 10-20 nm, the depth <strong>of</strong><br />

branching ranges from 150 nm to 1200 nm. We have outlined the methods allowing one<br />

to increase the porous layers depth by varying pulse current while etching.<br />

[1] J. B.Pendry, Phys. Rev. Lett. 85, 3966 (2000);<br />

[2] M. G. Silveirinha, C. A. Fernandes, Phys. Rev. B 78, 033108 (2008);<br />

[3] M. Christophersen et al., Phys. Stat. Sol. (a) 197, 197, (2003);<br />

[4] V. P. Ulin, S. G. Konnikov,Semiconductors 41, 832 (2007);<br />


MoP56 _______________________________________________________________<br />

40th "Jaszowiec" <strong>2011</strong> International School and Conference on the Physics <strong>of</strong> Semiconductors<br />

Effect <strong>of</strong> Substitution <strong>of</strong> Ti for Cd in CdCr2Se4 p-Type Semiconductor<br />

E. Malicka 1 , T. Groń 2 , A.W. Pacyna 3 , H. Duda 2 and J. Krok-Kowalski 2<br />

1 University <strong>of</strong> Silesia, Institute <strong>of</strong> Chemistry, ul. Szkolna 9, 40-006 Katowice, Poland<br />

2 University <strong>of</strong> Silesia, Institute <strong>of</strong> Physics, ul. Uniwersytecka 4, 40-007 Katowice, Poland<br />

3 The Henryk Niewodniczański Institute <strong>of</strong> Nuclear Physics, Polish Academy <strong>of</strong> Sciences,<br />

ul. Radzikowskiego 152, 31-342 Kraków, Poland<br />

The CdCr2Se4 spinel combines the p-type semiconducting and ferromagnetic properties<br />

with Curie temperature TC = 142 K and Curie-Weiss temperature qCW = 190 K [1,2].<br />

Magnetization <strong>of</strong> CdCr2Se4 reaches the full saturation <strong>of</strong> 5.98 µB per molecule [3]. The<br />

ferromagnetic properties <strong>of</strong> CdCr2Se4 are a result <strong>of</strong> dominating interactions between the<br />

nearest-neighbour chromium ions and <strong>of</strong> weaker superexchange couplings between the more<br />

distant chromium ones [4]. The Cr 2p XPS spectra <strong>of</strong> CdCr2Se4 showed the spin-orbit<br />

splitting between the final Cr 2p3/2 and Cr 2p1/2 states <strong>of</strong> 9.5 eV. The Cr 2p3/2 states are split<br />

into two peaks at 574.2 and 575.2 eV. The peak separation with the binding energy difference<br />

ΔE about 1 eV is typical <strong>of</strong> the 3d 3 elements with localized magnetic moment <strong>of</strong> 3 µB [5].<br />

CdCr2Se4 crystallizes in the cubic structure (Fd3m). The X-ray refinements showed that the<br />

(Cd) ions have a preference to be located in the tetrahedral sites and the [Cr] ions prefer to be<br />

located in the octahedral sites <strong>of</strong> the spinel structure [6].<br />

Chemical composition and structure refinement gave the sample composition and cation<br />

distribution according to the spinel formula: (Cd0.88Ti0.08)[Cr2]Se4. Magnetization, ac and dc<br />

magnetic susceptibility <strong>of</strong> (Cd0.88Ti0.08)[Cr2]Se4 were measured in the zero-field-cooled mode<br />

using a Lake Shore 7225 dc magnetometer/ac susceptometer at 4.3 K and in applied external<br />

magnetic fields up to 60 kOe, and a Faraday type Cahn RG automatic electrobalance in the<br />

temperature range 4.3-300 K and at 1 kOe and 2 kOe, respectively.<br />

The dc magnetic measurements <strong>of</strong> (Cd0.88Ti0.08)[Cr2]Se4 showed ferromagnetic order<br />

with Curie temperature TC = 129 K and Curie-Weiss temperature qCW = 138 K. Saturation<br />

magnetization <strong>of</strong> 4.73 µB/f.u. at 4.3 K and at 60 kOe seems to be strongly reduced in<br />

comparison with the CdCr2Se4 matrix. Such considerable lowering <strong>of</strong> magnetic moment<br />

induced by titanium substitution could be explained with only a transition <strong>of</strong> the certain<br />

number <strong>of</strong> Cr ions from the high-spin states to the low-spin states. Additionally, the ac<br />

magnetic measurements revealed a spectacular peak at 450 Oe and 1 kOe in the real part <strong>of</strong><br />

the first harmonic susceptibility curve near TC, similar to the Hopkinson peak. Zero-value <strong>of</strong><br />

its imaginary part below TC indicates a lack <strong>of</strong> the energy loss connected with the spin<br />

rearrangement processes. This is consistent with zero values <strong>of</strong> second and third harmonic ac<br />

susceptibility in the temperature range <strong>of</strong> 4.5-160 K, suggesting the lack <strong>of</strong> short-range<br />

magnetic interactions.<br />

This work is partly founded from science Grant No. N N204 145938.<br />

[1] H.W. Lehmann, Phys. Rev. 163, 488 (1967).<br />

[2] P.K. Baltzer, H.W. Lehmann, and M. Robbins, Phys. Rev. Lett. 15, 493 (1965).<br />

[3] R.C. LeCraw, H. von Philipsborn, and M.D. Sturge, J. Appl. Phys. 38, 965 (1967).<br />

[4] P.K. Baltzer, M. Robbins, and P.J. Wojtowicz, J. Appl. Phys. 38, 953 (1967).<br />

[5] V. Tsurkan, St. Plogmann, M. Demeter, D. Hartmann, and M. Neumann, Eur. Phys. J. B<br />

15, 401 (2000).<br />

[6] T. Groń, A. Krajewski, J. Kusz, E. Malicka, I. Okońska-Kozłowska, A. Waśkowska, Phys.<br />

Rev. B 71, 035208 (2005).<br />


40th _______________________________________________________________<br />

"Jaszowiec" <strong>2011</strong> International School and Conference on the Physics <strong>of</strong> Semiconductors MoP57<br />

Thermoelectric power in epitaxial Bi2Se3/Si(111) layers<br />

K. Dybko , M. Szot, T. Story and G. Karczewski,<br />

Institute <strong>of</strong> Physics, Polish Academy <strong>of</strong> Sciences,<br />

Al. Lotników 32/46, 02-668 Warszawa, Poland<br />

S. Schreyeck, C. Schumacher, K. Brunner and L.W. Molenkamp<br />

! "<br />

Bi2Se3 belongs to the class <strong>of</strong> three-dimensional topological insulators characterized<br />

by a bulk insulating state and conducting two-dimensional surface states with linear Diraclike<br />

dispersion. As grown Bi2Se3 samples are usually n-type due to inevitably present charged<br />

Se vacancies. Consequently, the conductivity <strong>of</strong> the system consists <strong>of</strong> parallel bulk and<br />

surface states conductivities. The observation <strong>of</strong> the surface states electrical properties<br />

requires minimalization <strong>of</strong> the bulk states contribution. It has been demonstrated in high field<br />

magnetotransport experiment [1] that beyond magnetic field corresponding to quantum limit<br />

for 3D carriers the clear Shubnikov de Haas oscillations <strong>of</strong> 2D character appear. This high<br />

field signature <strong>of</strong> surface states unambiguously proved the coexistence <strong>of</strong> both conducting<br />

channels in bulk samples.<br />

Here we study how the surface states affect thermoelectric properties <strong>of</strong> the whole<br />

electronic system in Bi2Se3. The samples were grown by molecular beam epitaxy on<br />

reconstructed (7x7)–(111) surface <strong>of</strong> Si semi-insulating substrate. Typically, samples are 100<br />

nm thick and have electron concentration <strong>of</strong> the order <strong>of</strong> 10 19 cm -3 . The Seebeck effect was<br />

measured over the temperature range <strong>of</strong> 10-300 K in ab-plane <strong>of</strong> the samples. The magnitude<br />

<strong>of</strong> the thermoelectric power decreases smoothly with lowering temperature until around 80 K,<br />

where it develops into a broad peak with maximum at 45-50 K. The absolute height <strong>of</strong> this<br />

peak is within 10 percent equal to the room temperature value (-60 V). The observed<br />

anomaly could be attributed to the theoretically predicted enhancement <strong>of</strong> the thermopower<br />

for surface states <strong>of</strong> topological insulators [2,3]. Here we measure the sum <strong>of</strong> individual<br />

contributions to the thermopower originating from bulk and surface states weighted by their<br />

electrical conductivities. Therefore, we also consider an alternative explanation that the<br />

effect is due to phonon drag, which may lead to the enhancement <strong>of</strong> the low temperature<br />

thermopower in semiconductors.<br />

[1] J.G. Analytis et al. , Nature Phys. 6, 960 (2010).<br />

[2] R. Takahashi and S. Murakami, Phys. Rev. B 81, 161302 (2010).<br />

[3] P. Ghaemi, R.S.K. Mong and J.E. Moore, Phys. Rev. Lett. 105, 166603 (2010).<br />


MoP58<br />

_______________________________________________________________<br />

40th "Jaszowiec" <strong>2011</strong> International School and Conference on the Physics <strong>of</strong> Semiconductors<br />

Diamagnetism <strong>of</strong> the pre-paired electronic systems:<br />

the flow equation study<br />

M. Zapalska and T. Domański<br />

Institute <strong>of</strong> Physics, M. Curie Sk̷lodowska University, 20-031 Lublin, Poland<br />

Magnetic ordering <strong>of</strong> the electronic systems can originate from a variety <strong>of</strong> mechanisms.<br />

It can be explained for instance by the energetic reasons (Pauli paramagnetism), assigned<br />

to the orbital effects (diamagnetism) or to the mutual electron-electron interactions which<br />

through the exchange coupling induce either the ferromagnetic or antiferromagnetic order.<br />

Among these phenomena the particular version <strong>of</strong> ordering (perfect diamagnetism)<br />

appears whenever the Fermi liquid becomes unstable towards the electron pair formation<br />

and the system becomes superconducting. It is well known that superconductivity goes<br />

hand in hand together with the perfect diamagnetism (Meissner effect). In his seminal<br />

paper P.W. Anderson explained this in terms <strong>of</strong> the Anderson-Higgs mechanism when the<br />

phasal Goldstone mode is swallen producing the massive term A 2 .<br />

Recent studies <strong>of</strong> the Princeton group [L. Li et al, Phys. Rev. B 81, 054510 (2010) by<br />

means <strong>of</strong> the high-presission torque magnetometry reveiled that in cuprate materials the<br />

remarkable diamagnetic response survies to temperatures much higher than critical Tc. It<br />

is expected that apparantly the preformed electron pairs (which above Tc are not capable<br />

to establish the long-range coherence) are responsible for such residual diamagnetism. We<br />

shall analyze this problem using the nonperturbative flow equation technique designed<br />

within the numerical renormalization group scheme.<br />


40th _______________________________________________________________<br />

"Jaszowiec" <strong>2011</strong> International School and Conference on the Physics <strong>of</strong> Semiconductors MoP59<br />

Quantum Monte Carlo vs. Density Functional Methods for the prediction <strong>of</strong><br />

relative energies <strong>of</strong> small Si-C clusters<br />

Nevill Gonzalez Szwacki and Jacek A. Majewski<br />

Faculty <strong>of</strong> Physics, University <strong>of</strong> Warsaw, ul Hoża 69, 00-681 Warszawa, Poland<br />

In many systems, prediction <strong>of</strong> the structural properties requires correct description <strong>of</strong><br />

the electron correlation, clearly exceeding the accuracy <strong>of</strong> the available exchange correlation<br />

functionals in the density functional theory (DFT). This is also the case in small systems with<br />

very non-homogenous distribution <strong>of</strong> electronic charge, like for example Si–C clusters, which<br />

play an important role are in technological processes, such as chemical vapor deposition used<br />

for fabrication <strong>of</strong> SiC thin films. The simulation <strong>of</strong> the growth processes is out <strong>of</strong> scope <strong>of</strong> abinito<br />

methods and has to be done using effective approaches after ‘coarse graining’ procedure.<br />

The quality <strong>of</strong> the effective potentials is crucial for reliability <strong>of</strong> the predictions. Therefore,<br />

the effective potentials are usually fitted to the results <strong>of</strong> the first-principles calculations for<br />

building blocks <strong>of</strong> a material.<br />

Numerous experimental and theoretical studies have been performed to investigate<br />

equilibrium geometries, vibrational frequencies, and relative energies <strong>of</strong> small SinCm cluster<br />

isomers [1,2]. Most <strong>of</strong> the calculations employed standard approximations to the DFT.<br />

In order to examine the accuracy <strong>of</strong> various DFT exchange-correlation (XC)<br />

functionals in predicting the relative energies <strong>of</strong> cluster isomers and to provide new<br />

benchmark data, we have performed diffusion quantum Monte Carlo (DMC) calculations for<br />

small SinCm clusters. We compare our DMC results with DFT ones obtained using the local<br />

density approximation (LDA), Perdew-Burke-Ernzerh<strong>of</strong> generalized gradient approximation<br />

(GGA), and the popular B3LYP hybrid Hartree-Fock/DFT functional. On the basis <strong>of</strong> our<br />

DMC results for Si-C clusters, we conclude that LDA and GGA XC functionals are not<br />

reliable for predicting the relative stability <strong>of</strong> various isomers <strong>of</strong> covalent SiC clusters,<br />

B3LYP does a little bit better, and generally the errors <strong>of</strong> various methods are completely<br />

unsystematic. This is illustrated in Fig. 1, where we plotted the energy (relative to the<br />

average) <strong>of</strong> four SinCm isomers. In particular, it is easily seen that the LDA approximation<br />

underestimates the relative energy <strong>of</strong> the planar (1) and (2) clusters, whereas the GGA<br />

approximation is unable to capture correctly the relative energy <strong>of</strong> the 3D clusters (2) and (3).<br />

The present studies clearly demonstrate that<br />

the determination <strong>of</strong> the effective potentials<br />

on the basis <strong>of</strong> DFT calculations with<br />

approximated exchange-correlation<br />

functionals can be misleading and<br />

computational schemes going beyond this<br />

approximation are very desirable.<br />

This paper has been supported by the project<br />

“SiCMAT” (Innovative Economy,<br />

POIG.01.03.01-14-155/09-00)<br />

[1] M. Bertolus, F. Finocchi, and P. Millie, J. Chem. Phys. 120, 4333 (2004).<br />

[2] C. R. Hsing et al., Phys. Rev. B 79, 245401 (2009).<br />


MoP60 _______________________________________________________________<br />

40th "Jaszowiec" <strong>2011</strong> International School and Conference on the Physics <strong>of</strong> Semiconductors<br />

Analysis <strong>of</strong> a Spatial Hole Burning Effect in a Square and<br />

Triangular Lattice Photonic Crystal Laser<br />

Marcin Koba 1,2 , and Pawe̷l Szczepański 1,2<br />

1 Warsaw University <strong>of</strong> Technology, Koszykowa 75, 00-662 Warsaw, Poland<br />

2 National Institute <strong>of</strong> Telecommunications, Szachowa 1, 04-894 Warsaw, Poland<br />

In this paper we present an approximate analysis <strong>of</strong> the nonlinear operation <strong>of</strong> the<br />

two-dimensional (2D) photonic crystal (PC) lasers taking into account nonlinear effects<br />

(gain saturation, spatial hole burning).<br />

Our analysis is conducted for the laser with an active medium that is confined in<br />

the square region with circular holes arranged in square or triangular Bravais’ lattice.<br />

The analysis is based on two-dimensional coupled wave equations [1]-[4] for TE and TM<br />

modes, modified by introduction <strong>of</strong> nonlinear gain and energy theorem developed earlier<br />

forvarioustypes<strong>of</strong>laser[5], includingDFBstructures[6]-[8]. Similarly, aswehavedonein<br />

e.g. [6]-[8] the field distributions appearing in the energy theorem (including nonlinearity)<br />

were approximated by those existing in the linear structure (i.e. at threshold).<br />

Following our derivation presented in [4],[9],[10] we introduce an approximate equations<br />

describing the small signal gain coefficient α0 as a function <strong>of</strong> system parameters,<br />

where we have introduced power saturation and spatial hole burning effects. We use these<br />

equations to calculate laser characteristics illustrating hole burning effect impact on cw<br />

operation <strong>of</strong> square and triangular lattice PC laser structure above the threshold. With<br />

thehelp<strong>of</strong>thepresentedtechniqueitispossibletocalculatetheoptimalcouplingstrength<br />

providing maximal power efficiency <strong>of</strong> the given 2D photonic laser structure.<br />

[1] K. Sakai, E. Miyai, and S. Noda, Opt. Express 15, 3981 (2007).<br />

[2] K. Sakai, E. Miyai, and S. Noda, IEEE J. Quantum Electron. 46, 788 (2010).<br />

[3] K. Sakai, J. Yue, and S. Noda, Opt. Express 16, 6033 (2008).<br />

[4] M. Koba, P. Szczepanski, and T. Kossek, IEEE J. Quantum Electron. 47, (13) <strong>2011</strong>.<br />

[5] L. Wosińska, P. Szczepański, and W. Woliński, Opto-Electron. Rev. 1, 26 (1992).<br />

[6] R. Paszkiewicz, A. Tyszka-Zawadzka, and P. Szczepański, Opt. Commun. 270, 314<br />

(2007).<br />

[7] P. Szczepański, D. Sikorski, and W. Woliński, IEEE J. Quantum Electron. 25, 871<br />

(1989).<br />

[8] P. Szczepanski, Appl. Opt. 24, 3574 (1985).<br />

[9] M. Koba, and P. Szczepanski, IEEE J. Quantum Electron. 46, 1003 (2010).<br />

[10] M. Koba, T. Osuch, and P. Szczepanski, J. Mod. Opt. submited for publication.<br />


40th _______________________________________________________________<br />

"Jaszowiec" <strong>2011</strong> International School and Conference on the Physics <strong>of</strong> Semiconductors TuI1<br />

Optical control <strong>of</strong> one and two spins in interacting quantum dots<br />

Alex Greilich, Samuel G. Carter, Danny Kim, Allan S. Bracker and Daniel Gammon<br />

Naval Research Laboratory, Washington DC 20375, USA<br />

The interaction between two quantum bits enables entanglement, the two-particle<br />

correlations that are at the heart <strong>of</strong> quantum information science. In semiconductor quantum<br />

dots much work has been focused on demonstrating single spin qubit control using optical<br />

techniques. However, optical control <strong>of</strong> two spin qubits remains a major challenge for scaling<br />

to a full-fledged quantum information platform. Here we combine advances in verticallystacked<br />

quantum dots with ultrafast laser techniques to achieve optical control <strong>of</strong> the<br />

entangled state <strong>of</strong> two spins.<br />

We demonstrate ultrafast optical control <strong>of</strong> two interacting electron or hole spins in<br />

two separate QDs using optical initialization, single-qubit gates with short pulses, and twoqubit<br />

gates with longer pulses or through precession in the exchange field. Local<br />

entanglement between the two spins is inferred from the coherent evolution <strong>of</strong> superposition<br />

states as measured in Ramsey fringes.<br />

The two-qubit gate speeds achieved here are over an order <strong>of</strong> magnitude faster than in<br />

other systems. These results demonstrate the viability and advantages <strong>of</strong> optically controlled<br />

quantum dot spins for multi-qubit systems.<br />

[1] Danny Kim, Samuel G. Carter, Alex Greilich, Allan S. Bracker and Daniel Gammon,<br />

Nature Physics 7, 223 (<strong>2011</strong>).��<br />


TuI2 _______________________________________________________________<br />

40th "Jaszowiec" <strong>2011</strong> International School and Conference on the Physics <strong>of</strong> Semiconductors<br />

Graphene Transport Properties�<br />

Shaffique Adam<br />

Center for Center for Nanoscale Science and Technology, National Institute <strong>of</strong> Standards and<br />

Technology<br />

Arguably, one <strong>of</strong> the most intriguing properties <strong>of</strong> graphene transport is the nonvanishing<br />

``minimum conductivity” at the Dirac point. The carrier density in these single<br />

monatomic sheets <strong>of</strong> carbon can be continuously tuned from electron-like carriers for large<br />

positive gate bias to hole-like carriers for negative bias. The physics close to zero carrier<br />

density (also called the intrinsic or Dirac region), is now understood to be dominated by<br />

fluctuations in local disorder potential, breaking the landscape into puddles <strong>of</strong> electrons and<br />

holes [1]. I will discuss the competing effects <strong>of</strong> disorder, electron-electron interactions, and<br />

quantum interference on graphene’s transport properties and demonstrate that, remarkably, a<br />

semi-classical effective medium theory captures the physics in the experimentally relevant<br />

regime [2]. Figure 1 shows a comparison <strong>of</strong> our theoretical predictions [3] with subsequent<br />

experimental data from Columbia, Manchester, Maryland and Exeter universities showing<br />

good agreement. To further test the range <strong>of</strong> validity <strong>of</strong> our model, I compare our results to a<br />

fully quantum-mechanical numerical calculation [4] <strong>of</strong> the conductivity and find that while<br />

the two theories are incompatible at weak<br />

disorder, they are compatible for strong<br />

disorder. Armed with this success, I will<br />

discuss how future graphene experiments<br />

could shed light on some long-standing<br />

open questions in condensed matter<br />

physics.<br />

Figure 1: Comparison <strong>of</strong> the self-consistent<br />

theory [3] with experimental data from<br />

various groups around the world. (See Ref.<br />

[2] for details). Solid blue line shows the<br />

Dirac point conductivity for graphene in<br />

the presence <strong>of</strong> charged impurities <strong>of</strong><br />

concentration nimp. The dashed line shows the quantum mechanical ballistic result 4e 2 /�h.<br />

Early results claimed that the conductivity was universal with a value <strong>of</strong> 4e 2 /h (dotted line).<br />

References:<br />

[1] S. Das Sarma, S. Adam, E. H. Hwang, E. Rossi, “Electronic transport in two dimensional<br />

graphene”, arXiv:1003.4731, Rev. Mod. Phys. (in press).<br />

[2] S. Adam, “Graphene carrier transport theory”, invited book chapter for Graphene<br />

Nanoelectronics: metrology, synthesis, properties and applications (Springer book series on<br />

nanoscience and technology).<br />

[3] S. Adam, E. H. Hwang, V. Galitski, and S. Das Sarma “A self-consistent theory for<br />

graphene transport” Proc. Nat. Acad. Sci. USA 104, 18392 (2007).<br />

[4] S. Adam, P. W. Brouwer, and S. Das Sarma “Crossover from quantum to Boltzmann<br />

transport in graphene” Phys. Rev. B Rapid Communications 79, 201404 (2009).<br />


40th _______________________________________________________________<br />

"Jaszowiec" <strong>2011</strong> International School and Conference on the Physics <strong>of</strong> Semiconductors TuI3<br />

Theoretical search for non-Abelian statistics in fractional quantum Hall states<br />

Arkadiusz Wójs<br />

Institute <strong>of</strong> Physics, Wroc�aw University <strong>of</strong> Technology, Wroc�aw, Poland<br />

Convincing demonstration <strong>of</strong> non-Abelian anyon quantum statistics (corresponding to<br />

the multi-dimensional realization <strong>of</strong> the braid group, allowed in two spatial dimensions) is a<br />

major challenge in today’s condensed matter physics. Promising physical systems for this<br />

quest are sought among correlated many-electron states a high magnetic field, responsible for<br />

the fractional quantum Hall effect at such Landau level fillings as �=5/2 or 12/5. In particular,<br />

the 5/2 state is believed to realize a superfluid condensate <strong>of</strong> p-paired “composite fermions”<br />

(electrons binding vortices <strong>of</strong> the many-body wave function) in an effectively compensated<br />

magnetic field. This hypothetical state, described by the “Pfaffian” wave function <strong>of</strong> Moore<br />

and Read, supports elementary charge excitations in form <strong>of</strong> fractional (e/4) quasiparticles<br />

which obey non-Abelian anyon quantum statistics. Anticipation <strong>of</strong> the Pfaffian ground state<br />

relies mostly on numerics, and it is confronted with rather ambiguous experimental evidence,<br />

including recent reports <strong>of</strong> the (unexpected) depolarization <strong>of</strong> spin at �=5/2. Therefore, the<br />

focus <strong>of</strong> this talk will be on available evidence concerning Pfaffian dynamics (more generally,<br />

non-Abelian quantum statistics) in realistic conditions <strong>of</strong> the fractional quantum Hall effect.<br />

We will begin by revisiting the question <strong>of</strong> spin polarization at �=5/2. The combination<br />

<strong>of</strong> exact diagonalization and quantum Monte Carlo in spherical geometry confirms quantum<br />

ferromagnetism in clean systems, as expected for the Pfaffian phase, but also reveals stability<br />

<strong>of</strong> Skyrmions (topological spin excitations) in sufficiently thick systems [1]. As pairing nature<br />

<strong>of</strong> the ferromagnetic (Pfaffian) ground state implies that Skyrmions at �=5/2 carry twice the<br />

charge (e/2) <strong>of</strong> spinless quasiparticles, the (realistic) lateral disorder aids Skyrmion formation,<br />

in turn leading to local depolarization – in agreement with its photoluminescence signatures.<br />

Having established the role <strong>of</strong> spin we will confine further analysis to the spin-polarized<br />

channel and turn to the question <strong>of</strong> adiabatic connection (i.e., essential equivalence) between<br />

the exact Pfaffian state and the realistic Coulomb ground state. The problem is subtler than at<br />

�=1/3 (where the famous Laughlin wave function is firmly established), because <strong>of</strong> a longer<br />

correlation length, a more fragile gap, and a competition between two- and three-body effects<br />

(the Pfaffian state being a unique zero-energy ground state <strong>of</strong> the contact triplet repulsion).<br />

Since the analysis <strong>of</strong> overlaps or pair correlation functions computed in the largest tractable<br />

systems is hardly conclusive, we studied features linked directly to the non-Abelian statistics.<br />

Specifically, in addition to the neutral boson pair-pair collective mode with the characteristic<br />

roton dispersion, the Pfaffian ground state should have a neutral fermion excitation associated<br />

with an unpaired particle (analogous to the Bogoliubov-deGennes excitation <strong>of</strong> a conventional<br />

superconductor). In the relevant weakly paired phase, the neutral fermion dispersion should<br />

have a minimum near the Fermi wave vector. More importantly, the neutral fermion mode is<br />

predicted to become gapless in the presence <strong>of</strong> distant quasiparticles – as a consequence <strong>of</strong><br />

their non-Abelian statistics within the Ising model with degenerate fusion channels (1 and �).<br />

Our calculations confirm [2] these qualitative features, in support <strong>of</strong> exotic statistics at �=5/2.<br />

Other issues will include Pfaffian/anti-Pfaffian duality, effects <strong>of</strong> Landau level mixing<br />

(e.g., on particle-hole symmetry) [3], and the pairing transition <strong>of</strong> composite fermions [4].<br />

[1] A. Wójs, G. Möller, S. H. Simon, and N. R. Cooper, Phys. Rev. Lett. 104, 086801 (2010).<br />

[2] G. Möller, A. Wójs, and N. R. Cooper, http://arxiv.org/abs/1009.4956<br />

[3] A. Wójs, C. T�ke, and J. K. Jain, Phys. Rev. Lett. 105, 096802 (2010).<br />

[4] A. Wójs, C. T�ke, and J. K. Jain, Phys. Rev. Lett. 105, 196801 (2010).<br />


TuI4 _______________________________________________________________<br />

40th "Jaszowiec" <strong>2011</strong> International School and Conference on the Physics <strong>of</strong> Semiconductors<br />

Polariton condensation in dynamic acoustic lattices<br />

D. Krizhanovskii 1 , E. Cerda-Méndez 2 , M.Wouters 3 , K. Biermann 3 , K.Guda 1 , R.Bradley 1 ,<br />

D.Sarker 1 , P. V. Santos 3 , R. Hey 3 and M. S. Skolnick 1<br />

1 Department <strong>of</strong> Physics and Astronomy, University <strong>of</strong> Sheffield, Sheffield, United Kingdom<br />

2 Paul-Drude-Institut für Festkörperelektronik, Berlin, Germany<br />

3 ITP, Ecole Polytechnique Fédérale de Lausanne, 1015 Lausanne, Switzerland<br />

Microcavity polariton condensates<br />

arising either from Bose-Einstein condensation<br />

(BEC) or from Optical Parametric Oscillation<br />

(OPO) exhibit collective superfluid-like<br />

behavior [1](OPO), spatial and temporal<br />

coherence (OPO, BEC) and vortices (OPO,<br />

BEC). Here, we study spatial coherence <strong>of</strong><br />

OPO polariton condensates formed by<br />

polariton-polariton scattering from the pump in<br />

external periodic potentials created by Surface<br />

Acoustic Waves (SAW)[2]. The effect <strong>of</strong><br />

strong polariton-phonon coupling results in<br />

modulation <strong>of</strong> the polariton dispersion with<br />

marked formation <strong>of</strong> Brillouin zones <strong>of</strong> period<br />

k=π/λSAW and energy stop band widths up to ~0.5<br />

meV (Fig.1a).<br />

Without SAW the OPO “signal” condensate<br />

is formed at k=0 above threshold with a coherence<br />

length <strong>of</strong> about ~20-25 µm. As the height <strong>of</strong> the<br />

SAW potential is increased we observe dramatic<br />

reduction <strong>of</strong> the spatial coherence length (Ly) along<br />

the SAW direction by a factor <strong>of</strong> 3-4 (Fig.1c), a result <strong>of</strong> suppressed tunnelling between the<br />

adjacent SAW minima. A smaller decrease <strong>of</strong> the coherence length Lx by a factor <strong>of</strong> 2<br />

perpendicular to the SAW direction (Fig.1d) is attributed to the increased phase fluctuations,<br />

predicted theoretically for 1D system <strong>of</strong> the non-equilibrium OPO. The observed effect<br />

corresponds to transition from condensate to insulator state. Nevertheless, this is not a Mott<br />

insulator: the number <strong>of</strong> particles per potential well is still large compare to the case <strong>of</strong><br />

superfluid-to Mott-insulator transition in cold atom system. Interestingly, at intermediate SAW<br />

powers the emission peaks at the edges <strong>of</strong> the 1 st Fig.1 a) Dispersion <strong>of</strong> polaritons subject to SAW<br />

potential. b) Image <strong>of</strong> condensate emission c)-d)<br />

Spatial coherence length <strong>of</strong> OPO signal parallel<br />

(LX) and perpendicular (LY) SAW at different<br />

pump powers.<br />

Brillouin zone (Fig.1b), indicating a phase<br />

difference <strong>of</strong> π between adjacent condensates. A likely reason for this is an additional scattering<br />

mechanism between the main pump beam and its diffraction replica.<br />

By contrast, inter-particle interactions at high densities screen the acoustic potential,<br />

partially reversing its effect on spatial coherence. Figs.1 c)-d) shows that with increasing<br />

excitation power the reduction <strong>of</strong> spatial coherence lengths occurs at higher SAW powers, thus<br />

indicating efficient screening <strong>of</strong> SAW potential at low acoustic powers as the polariton density<br />

is increased.<br />

Our work on polariton condensation in acoustic lattices provides new technology to<br />

investigate fundamental effects in hybrid light-matter system, such as coherent transport <strong>of</strong><br />

condensates, creation <strong>of</strong> entangled states and Josephson tunneling.<br />

1. A. Amo, et al, Nature 457, 291 (2009)<br />

Ly (P =140 mW)<br />

laser<br />

40<br />

Ly (P =200 mW)<br />

laser<br />

Ly (P =240 mW)<br />

laser<br />

30<br />

Ly (P =300 mW)<br />

laser<br />

-5 0 5 10 15<br />

0 5 10 15<br />

2. E. A. Cerda-Méndez, D. N. Krizhanovskii et al, Phys. Rev. Lett. 105, 116402 (2010)<br />

102<br />

Energy (eV)<br />

1.5365<br />

1.5360<br />

1.5355<br />

1.5350<br />

1.5345<br />

1.5340<br />

1.5365<br />

1.5360<br />

1.5355<br />

1.5350<br />

1.5345<br />

1.5340<br />

a)<br />

b)<br />

-2 -1 0 1 2<br />

k-vector ky<br />

c)<br />

d)<br />

Ly (P laser =68 mW)<br />

SAW power (dBm)<br />

50<br />

20<br />

10<br />

0<br />

40<br />

30<br />

20<br />

10<br />

0<br />

Coherence length L y (µm)<br />

Coherence length Lx (µm)

40th _______________________________________________________________<br />

"Jaszowiec" <strong>2011</strong> International School and Conference on the Physics <strong>of</strong> Semiconductors TuO1<br />

Resonant Terahertz Absorption by Magnetoplasmons<br />

in Grating-Gate GaN/AlGaN-based Field-Effect Transistors<br />

K. Nogajewski 1 , K. Karpierz 1 , M. Grynberg 1 , W. Knap 2 , R. Gaska 3 ,<br />

J. Yang 3 , M. S. Shur 4 , and J. ̷Lusakowski 1<br />

1 Faculty <strong>of</strong> Physics, University <strong>of</strong> Warsaw, ul. Ho˙za 69, 00-681 Warsaw, Poland<br />

2 CNRS - Université Montpellier2, Pl. E. Bataillon, 34095 Montpellier, France<br />

3 Sensor Electronic Technology, Inc., Columbia, South Carolina 29209, USA<br />

4 Rensselaer Polytechnic Institute, Troy, New York 12180, USA<br />

An interest to fabricate voltage-tunable detectors <strong>of</strong> THz radiation based on plasma<br />

excitations in field-effect transistors (FETs) is marked by an effort to test new types <strong>of</strong><br />

structures and new materials. High electron mobility FETs based on GaN/AlGaN heterostructures<br />

<strong>of</strong>fer a possibility <strong>of</strong> high-voltage/high-current THz applications in agressive<br />

environments. However, to support technical developments, basic physical investigation<br />

must be carried out on technologically advanced devices.<br />

The goal <strong>of</strong> the present work is to describe magnetoplasmon excitations in a twodimensional<br />

electron gas in a GaN/AlGaN heterostructure. Our experiments were carried<br />

out on samples that have been recently shown to exhibit gate-voltage tunable plasmon<br />

resonances at zero magnetic field (B) [1]. The samples were large-area (about<br />

1.6 mm×1.6 mm) FETs with the gate electrode composed <strong>of</strong> two interdigitated comb-like<br />

structures. Five transistors with the gate periodicity between 1.5µm and 4µm were processed<br />

on a high-quality GaN/AlGaN heterostructure. The metal grating served both as<br />

a gate electrode controlling the concetration <strong>of</strong> a two-dimensional electron gas (n) and as<br />

a coupler between magnetoplasmons and incident THz radiation.<br />

Transmission ( a.u. )<br />

ν FIR = 2.52THz<br />

−3.5 −3 −2.5 −2 −1.5 −1 −0.5 0<br />

Gate Voltage ( V )<br />

B = 8T<br />

B = 6T<br />

B = 4T<br />

B = 2T<br />

B = 0T<br />

Fig. 1: Transmission <strong>of</strong> 2.52 THz radiation<br />

through a GaN/AlGaN FET as a function <strong>of</strong><br />

VG and B. Minima correspond to magnetoplasmon<br />

resonances.<br />

Bothmagnetoresistanceand magnetotransmission<br />

measurements were carried<br />

out at 4.2 K and B up to 10 T as a function<br />

<strong>of</strong> the gate polarization (VG). Shubnikov-de<br />

Haas oscillations <strong>of</strong> the magnetoresistance<br />

allowed us to estimate n as<br />

a function <strong>of</strong> the gate polarization (VG)<br />

and then an n-dependent electron quantum<br />

relaxation time τ. We found that τ<br />

dependsinanon-monotonicwayonnand<br />

ranges from 0.03 ps to about 0.1 ps.<br />

An example <strong>of</strong> magnetotransmission<br />

data shown in Fig. 1 gives a clear evidence<strong>of</strong>excitation<strong>of</strong>magnetoplamonresonances<br />

tunable by B and VG. Magnetoplasmons<br />

frequency, ωmp, can only approximately<br />

be described by a classical<br />

formula ωmp = �<br />

ω2 c +ω2 p. Deviations<br />

from this dependence are explained as a result <strong>of</strong> interaction <strong>of</strong> plasmons in gated and<br />

ungated parts <strong>of</strong> the transistor channel.<br />

This work was partially supported by the JU ENIAC MERCURE project No. 220120.<br />

[1] A. V. Muravjov et al., Appl. Phys. Lett. 96, 042105 (2010).<br />


TuO2 _______________________________________________________________<br />

40th "Jaszowiec" <strong>2011</strong> International School and Conference on the Physics <strong>of</strong> Semiconductors<br />

Structural and electronic properties <strong>of</strong> functionalized graphene<br />

Karolina Milowska, Magdalena Birowska, and Jacek A. Majewski<br />

Faculty <strong>of</strong> Physics, University <strong>of</strong> Warsaw, ul. Hoża 69, PL-00-681 Warszawa, Poland<br />

We present extensive ab initio studies in the framework <strong>of</strong> the density functional theory<br />

(DFT) <strong>of</strong> graphene layers functionalized with simple molecules, which provide novel<br />

predictions for electronic structure <strong>of</strong> the functionalized graphene mono- and bilayers. The<br />

calculations reveal that the functionalization <strong>of</strong> graphene by simple molecules (e.g., such as<br />

OH, NH, and COOH) causes opening <strong>of</strong> the electronic band gap, just indicating new<br />

perspectives for design <strong>of</strong> field effect transistors (FETs) based on single and bilayer graphene.<br />

Remarkable electronic, mechanical and thermal properties <strong>of</strong> graphene have made it a<br />

promising candidate for a new generation <strong>of</strong> electronic devices [1]. However, the single and<br />

bilayer graphene have zero energy band gaps. Therefore, it hinders direct application <strong>of</strong> these<br />

systems in FETs. On the other hand, the functionalization <strong>of</strong> the graphene could change its<br />

electronic structure. Since quantitatively this effect is rather unknown, we have undertaken<br />

theoretical studies <strong>of</strong> this issue.<br />

In the present paper, we study the effects <strong>of</strong> functionalization <strong>of</strong> graphene with simple<br />

organic molecules (NHn, OH, and COOH) focusing on the stability and band gaps <strong>of</strong> the<br />

structures. We perform DFT calculations for graphene supercells with various numbers <strong>of</strong> the<br />

attached molecules. Employing the numerical package SIESTA, we completely optimize the<br />

geometry <strong>of</strong> the structures getting very good agreement with existing experimental data [2],<br />

and other theoretical calculations [3]. All studied groups bind covalently to the graphene layer<br />

implying sp 2 to sp 3 rehybridization and local deformation <strong>of</strong> the graphene plane. As example<br />

<strong>of</strong> our results, below we depict band<br />

structures <strong>of</strong> graphene layer<br />

functionalized with one single<br />

molecule per 3x3 graphene<br />

superlattice. The molecules are (a) -<br />

OH, (b) - NH, (c) – NH2, and (d) –<br />

COOH. The red dotted lines indicate<br />

band structure <strong>of</strong> pure graphene, the<br />

black ones the electronic structure <strong>of</strong><br />

functionalized graphene. In all cases,<br />

the zero band gap at K-point <strong>of</strong> pure graphene opens, being equal to 0.11, 0.12, 0.25, and 0.24<br />

eV for OH, NH, NH2, and COOH, respectively. Generally, the stability <strong>of</strong> the functionalized<br />

graphene layers decreases with the growing concentration <strong>of</strong> functionalizing molecules.<br />

Interestingly, for higher (1x1) concentrations <strong>of</strong> OH, we observe separation <strong>of</strong> O and H,<br />

which cover now other sides <strong>of</strong> the graphene layer. On the other hand, quite naturally, the<br />

band gap increases with the concentration <strong>of</strong> the attached molecules, e.g., in the case <strong>of</strong> 2x2<br />

cell functionalized with hydroxyl group (graphene’s surface is strongly bent), the band gap<br />

reaches 1.04 eV. All this shows clearly that the band gap <strong>of</strong> graphene layers can be tuned<br />

within fairly large range. Further, it turns out that the functionalized graphene layers exhibit<br />

magnetic moments, just implying some possible spintronic applications.<br />

This paper has been supported by the project “SiCMAT” (Innovative Economy, POIG.01.03.01-14-155/09-00).<br />

1. S. Park, and R. S Ru<strong>of</strong>f, Nature Nanotech., 4, 217 (2009).<br />

2. H. B. Su, R. J. Nielsen, A. C. T. van Duin, and W. A. Goddard, Phys. Rev. B 75 134107 (2007).<br />

3. Jia-An Yan, M. Y. Chou, Phys. Rev. B 82 125403 (2010).<br />


40th _______________________________________________________________<br />

"Jaszowiec" <strong>2011</strong> International School and Conference on the Physics <strong>of</strong> Semiconductors TuO3<br />

Analysis <strong>of</strong> the magnetic anisotropy in ultrathin GaMnAs<br />

O. Proselkov 1 , W. Stefanowicz 1 , S. Dobkowska 1 , J. Sadowski 1,2 ,<br />

T. Dietl 1,3 , M. Sawicki 1<br />

1 Institute <strong>of</strong> Physics, Polish Academy <strong>of</strong> Sciences, Warszawa, Poland<br />

2 MAX-Lab, Lund University, Lund, Sweden<br />

3 Institute <strong>of</strong> Theoretical Physics, University <strong>of</strong> Warsaw, Poland<br />

(Ga,Mn)As is the main test-bed material for testing concepts <strong>of</strong> future spintronics devices<br />

and its magnetic anisotropy is assumed to be reasonably well known both on the ground <strong>of</strong><br />

understanding <strong>of</strong> the experimental data and on the level <strong>of</strong> a description its microscopic<br />

origins. However, some very recent studies have brought new insights into the magnetic<br />

anisotropy <strong>of</strong> thin and ultrathin (Ga,Mn)As. Firstly, in very thin (Ga,Mn)As layers, despite an<br />

adequate high Mn concentration to assure their homogenous metallic character, a blocked<br />

superparamagnetic response has been found [1]. Then, it was postulated that the strength <strong>of</strong><br />

the ever-present in-plane uniaxial magnetic anisotropy was related to nanometer-scale ripples<br />

on the (Ga,Mn)As surface [2]. Furthermore, a collapse in the magnitude <strong>of</strong> the anomalous<br />

Hall effect was found for thin layers at low temperatures [3]. Finally, a cycloidal spin<br />

arrangement and the accompanying uniaxial in-plane anisotropy <strong>of</strong> diagonal [110]/[1-10]<br />

directions was suggested to form in very thin layers [4].<br />

As all these findings indicate that the long celebrated macrospin approach [5] to the<br />

phenomenological description <strong>of</strong> the magnetic anisotropy may cease its validity in ultrathin<br />

layers, an examination <strong>of</strong> the conditions <strong>of</strong> its applicability in this technological important<br />

case is timely and important. Simultaneously, means for assessment <strong>of</strong> the abovementioned<br />

concepts are to be defined and tested. For these purposes we investigate in greater detail<br />

temperature dependence <strong>of</strong> the magnetic anisotropy <strong>of</strong> 4 nm Ga0.925Mn0.075As, a layer that<br />

according do [1] exhibits two types <strong>of</strong> magnetic responses: a temperature reversible one<br />

typical for a long-range-coupled magnetization and an irreversible one possessing<br />

characteristics similar to a blocked superparamagnet. We find that we can self consistently<br />

describe our findings on the grounds <strong>of</strong> the macrospin model as in [5] only above a<br />

temperature at which signatures <strong>of</strong> blocked superparamagnetic response disappears (25 K in<br />

this case). Below, the same model yields conflicting results which depend on the magnetic<br />

history <strong>of</strong> the sample. However, if applied at this temperature range, this approach implies<br />

disappearance <strong>of</strong> magnetic anisotropy at temperatures corresponding to the mean blocking<br />

temperature <strong>of</strong> the superparamagnetic part <strong>of</strong> the layer and a change <strong>of</strong> sign <strong>of</strong> the cubic<br />

anisotropy at the lowest temperatures. The last findings point to a strong coupling between<br />

these two major constituencies <strong>of</strong> the magnetic response in such a layers.<br />

The work was supported in part by the European Research Council through the FunDMS<br />

Advanced Grant within the "Ideas" 7th Framework Programme <strong>of</strong> the EC, EC Network<br />

SemiSpinNet (PITN-GA-2008-215368) and Polish MNiSW 2048/B/H03/2008/34 grant.<br />

[1] M. Sawicki, et al., Nature Phys. 6, 22 (2010); S. Dobkowska, et al., this conference.<br />

[2] S. Piano, et al., arXiv:1010.0112.<br />

[3] D. Chiba et al., Phys. Rev. Lett. 104, 106601 (2010).<br />

[4] A. Werpachowska and T. Dietl, Phys. Rev. B 82, 085204 (2010).<br />

[5] K.-Y. Wang, et al., Phys. Rev. Lett., 95, 217204 (2005).<br />


TuO4 _______________________________________________________________<br />

40th "Jaszowiec" <strong>2011</strong> International School and Conference on the Physics <strong>of</strong> Semiconductors<br />

Spin-lattice relaxation <strong>of</strong> a single Mn 2+ ion in a CdTe quantum dot<br />

M. Goryca 1,2 , P. Plochocka 2 , P. Wojnar 3 , T. Kazimierczuk 1 , M. Potemski 2<br />

and P. Kossacki 1,2<br />

1 Institute <strong>of</strong> Experimental Physics, Faculty <strong>of</strong> Physics, University <strong>of</strong> Warsaw, Warsaw, Poland<br />

2 Grenoble High Magnetic Field Laboratory, CNRS, Grenoble, France<br />

3 Polish Academy <strong>of</strong> Sciences, Warsaw, Poland<br />

From the point <strong>of</strong> view <strong>of</strong> miniaturization <strong>of</strong> magnetic memory devices a quantum dot<br />

(QD) containing a single magnetic impurity is an interesting system. It has been demonstrated<br />

that it gives unique possibility to read and manipulate the electronic spin state <strong>of</strong> the single<br />

atom [1-3]. Although principles <strong>of</strong> using such system as a memory device are known, further<br />

studies <strong>of</strong> interaction <strong>of</strong> the magnetic ion with its neighborhood are still needed. Particularly a<br />

detailed knowledge about the spin-lattice relaxation is <strong>of</strong> a great importance, as this kind <strong>of</strong><br />

relaxation is a main factor limiting the information storage time.<br />

In this work we present spectroscopic studies <strong>of</strong> the spin-lattice relaxation <strong>of</strong> a single<br />

Mn 2+ ion embedded in a CdTe/ZnTe QD, performed in high magnetic field. The experiment<br />

was done in a micro-photoluminescence setup at temperature <strong>of</strong> about 15 K. The magnetic<br />

field up to 12 T was produced with a superconductive magnet and applied in the Faraday<br />

configuration, parallel to the growth axis <strong>of</strong> the sample. The sample contained a single layer<br />

<strong>of</strong> self-assembled QDs. The PL <strong>of</strong> the QDs was excited using a tunable dye laser in the range<br />

570−610 nm. The spin-conserving excitation transfer between two QDs [4] was used to inject<br />

spin-polarized excitons into the QD containing the Mn 2+ ion and to orient its spin [3].<br />

The spin-lattice relaxation time <strong>of</strong> the Mn 2+ ion was determined in two independent<br />

experiments. In the first one we measured the efficiency <strong>of</strong> the optical orientation <strong>of</strong> the Mn 2+<br />

ion [3] as function <strong>of</strong> excitation power and magnetic field. Thus we were able to examine in<br />

detail the competition between two processes: the optical orientation <strong>of</strong> the Mn spin by spinpolarized<br />

excitons injected into the QD and thermalization <strong>of</strong> the magnetic ion in external<br />

magnetic field caused by spin-lattice relaxation. When these processes exactly compensate<br />

each other, i.e. the spin orientation rate is exactly equal to the relaxation rate, then the Mn 2+ is<br />

completely depolarized. Thus the spin orientation rate can be estimated using the exciton<br />

injection rate to the QD and probability <strong>of</strong> the Mn spin flip per one exciton [3].<br />

In the second, time-resolved experiment we measured directly the Mn spin relaxation<br />

time. We used the same mechanism as previously to orient the magnetic ion. Once it reached<br />

the steady state we turned the excitation <strong>of</strong>f for a certain period. Then we turned the excitation<br />

on again to probe the Mn spin. By varying the length <strong>of</strong> the dark period in the range <strong>of</strong> 20 ns –<br />

500 µs we were able to measure the relaxation time in the wide range <strong>of</strong> magnetic fields.<br />

Both experiments gave the same results within experimental accuracy. The spin-lattice<br />

relaxation time <strong>of</strong> the Mn 2+ ion in the QD for low magnetic field (around 1 T) reaches<br />

hundreds <strong>of</strong> microseconds and is comparable to the value measured for bulk, diluted<br />

(Cd,Mn)Te material [5]. It is, however, decreasing rapidly for high magnetic field and at 12T<br />

reaches value <strong>of</strong> tens <strong>of</strong> nanoseconds, which is over two orders <strong>of</strong> magnitude smaller than the<br />

one observed for bulk material. Possible origins <strong>of</strong> this surprising difference include strain<br />

fields, usually high in structures containing QDs.<br />

[1] L. Besombes et al., Phys. Rev. Lett. 93, 207403 (2004).<br />

[2] C. Le Gall et al, Phys. Rev. Lett. 102, 127402 (2009).<br />

[3] M. Goryca et al, Phys. Rev. Lett. 103, 087401 (2009).<br />

[4] T. Kazimierczuk et al, Phys. Rev. B 79, 153301 (2009).<br />

[5] T. Strutz et al., Phys. Rev. Lett. 68, 3912 (1992).<br />


40th _______________________________________________________________<br />

"Jaszowiec" <strong>2011</strong> International School and Conference on the Physics <strong>of</strong> Semiconductors TuO5<br />

Magnetophotoluminescence <strong>of</strong> CdTe/ZnTe Quantum Dots:<br />

G-factor and Diamagnetic Shift Variation in a Single Dot<br />

Tomasz Kazimierczuk 1 , Mateusz Goryca 1,2 , Piotr Wojnar 3 ,<br />

Andrzej Golnik 1 , Piotr Kosacki 1,2<br />

1 Institute <strong>of</strong> Experimental Physics, Faculty <strong>of</strong> Physics,<br />

University <strong>of</strong> Warsaw, ul. Hoża 69, PL-00-681 Warsaw, Poland<br />

2 LNCMI-CNRS (UJF, UPS, INSA), BP 166, 38042 Grenoble Cedex 9, France<br />

3 Institute Physics, Polish Academy <strong>of</strong> Science,<br />

Al. Lotników 32/46, PL-02-668 Warsaw, Poland<br />

Typically, self-assembled quantum dots (QDs) are characterized by strong confining<br />

potential, which defines carrier wavefunctions. A flat shape <strong>of</strong> such QDs lifts the degeneracy<br />

between light and heavy holes in favor <strong>of</strong> the latter ones. Due to the selection rules, the<br />

recombination occurs between an electron and a hole <strong>of</strong> opposite spin directions. Thus, in the<br />

magnetic field along growth direction, all transitions with carriers in the ground state exhibit<br />

Zeeman given by a difference between electron and hole Landé factors. Particularly, the same<br />

Zeeman splitting should be observed for X, X + , X - , and XX transitions. Such a predictiction<br />

was tested already in early stages <strong>of</strong> QD research demonstrating equality between Landé<br />

factors for neutral and charged exciton with precision better than 1% [1].<br />

In this work we present a comparison <strong>of</strong> magnetic field influence on various excitonic<br />

transitions in single CdTe/ZnTe QDs. The experiments included PL measurements in<br />

magnetic field up to 28T applied either<br />

perpendicular or parallel to the growth direction.<br />

Recorded data allowed us to extract two basic<br />

parameters: excitonic Landé factor related to linear<br />

Zeeman effect and diamagnetic shift related to<br />

quadratic field dependence. Contrary to the picture<br />

described previously, we discovered a clear<br />

systematic difference between g-factors inferred<br />

from X, X + , and X - transitions in Faraday<br />

configuration (Fig. 1). Landé factor inferred from<br />

the XX transition follows exactly values related to<br />

X transition, as in both cases the spectral effect is<br />

related to splitting <strong>of</strong> the same state (being final or<br />

initial one respectively). The average difference<br />

between X + and X - g-factors yields 21%.<br />

Similar but smaller difference between g-factors inferred from different excitonic<br />

transitions was found in the in-plane field geometry. In such a geometry, qualitatively similar<br />

behavior was reported for pyramidal InGaAs/AlGaAs QDs [2] and interpreted in terms <strong>of</strong><br />

Coulomb correlations. In our case, such an interpretation is supported by analysis <strong>of</strong><br />

diamagnetic coefficients. The lack <strong>of</strong> significant systematic difference in diamagnetic shift<br />

allows us to reject an alternative explanation <strong>of</strong> g-factor variation by increase/decrease <strong>of</strong><br />

barrier penetration by weakly confined hole.<br />

[1] J. J. Finley et al., Phys. Rev. B 66, 153316 (2002).<br />

[2] D. Y. Oberli et al., Phys. Rev. B 80, 165312 (2009).<br />

107<br />

g-factor <strong>of</strong> X + , X - , and XX<br />

3.0<br />

2.5<br />

2.0<br />

1.5<br />

XX<br />

X +<br />

X -<br />

1.0<br />

1.0 1.5 2.0 2.5 3.0<br />

g-factor <strong>of</strong> X<br />

Fig. 1. Correlation between Landé factors<br />

inferred from different excitonic transitions<br />

in 24 different dots.

TuO6 _______________________________________________________________<br />

40th "Jaszowiec" <strong>2011</strong> International School and Conference on the Physics <strong>of</strong> Semiconductors<br />

ControloverCdTeQuantumDotsEmissionusingZnTe-based<br />

MicropillarCavities<br />

T.Jakubczyk 1 ,W.Pacuski 1 ,A.Golnik 1 ,C.Kruse 2 ,D.Hommel 2 ,<br />

H.Hilmer 3 ,R.Schmidt-Grund 3 and J.A.Gaj 1<br />

1 Institute<strong>of</strong>ExperimentalPhysics,University<strong>of</strong>Warsaw,ul.Hoża69,00-681Warsaw,<br />

Poland<br />

2 Institute<strong>of</strong>SolidStatePhysics,University<strong>of</strong>Bremen,Postfach330440,D-28334,<br />

Bremen,Germany<br />

3 InstituteforExperimentalPhysicsII,UniversitätLeipzig,Linnéstr.5,D-04103,<br />

Leipzig,Germany<br />

Pillarmicrocavitiescontainingquantumdots(QDs)werereportedassystemsenablingcontroloverspatial,energeticandtemporalaspects[1]<strong>of</strong>lightemission.Interms<strong>of</strong><br />

applicationsinquantuminformationprocessing[2],coherentcouplingbetweenlightand<br />

matterobtainedinsuchsystemsis<strong>of</strong>primaryimportance.Inthiscontextanextension<br />

<strong>of</strong>theresearchfromtheGaAs-basedsystemstowiderband-gapII-VIbasedonesispromising.Therobustexcitonicstatesandastrongercarrierconfinementinthesematerials<br />

extendthefunctionalityrange<strong>of</strong>suchQDsystemstohighertemperatures.<br />

Inthiscommunicationwereportamicrophotoluminescencestudy<strong>of</strong>pillarcavities<br />

basedonsuperlattice-baseddistributedBraggreflectors(DBRs)[3],lattice-matchedto<br />

ZnTe.Micropillars<strong>of</strong>diametersrangingfrom0.7to5µmwereetchedbyfocusedion<br />

beam(FIB)out<strong>of</strong>aplanarmicrocavitybasedontheseDBRsandcontainingasingle<br />

plane<strong>of</strong>CdTe/ZnTequantumdots.<br />

Boththeemission<strong>of</strong>asingleQDaswellasemission<strong>of</strong><br />

anensemble<strong>of</strong>themwasusedtoexaminedifferentproperties<strong>of</strong>thestructures.Wepresentresultsprovingtheability<strong>of</strong>ZnTe-basedpillarstoenhancetheQDsemission(see<br />

Figure).Furthermore,wepresenttheirabilitytoguidethis<br />

emissionresultinginanenhancedcollectionrate.Thesefeaturesstronglyindicatethepossibility<strong>of</strong>constructingefficientsinglephotonsourcesbasedonCdTeQDs.<br />

Toinhibittheunwantedemissionintotheleakymodes<br />

anewapproachwasused.AlateralDBRmade<strong>of</strong>yttriastabilizedzirconiaandaluminawassuccessfullydeposited.Wepresentadetailedphotoluminescencestudy<strong>of</strong>theobtainedstructures.<br />

[1]J.M.Gérard,B.Sermage,B.Gayral,B.Legrand,E.Costard,andV.Thierry-Mieg,<br />

Phys.Rev.Lett.81,1110<br />

(1998).<br />

[2]J. P. Reithmaier, G. S ֒ ek, A. Löffler, C. H<strong>of</strong>mann,<br />

S.Kuhn,S.Reitzenstein,L.V.Keldysh,V.D.Kulakovskii,T.L.Reinecke,andA.Forchel,Nature432,197<br />

(2004).<br />

[3]W.Pacuski,C.Kruse,S.Figge,andD.Hommel,Applied<br />

PhysicsLetters94,191108(2009).<br />


40th _______________________________________________________________<br />

"Jaszowiec" <strong>2011</strong> International School and Conference on the Physics <strong>of</strong> Semiconductors TuP1<br />

Exciton and Mn spin dynamics in a nonresonantly coupled pair<br />

<strong>of</strong> (Cd,Mn)Te quantum dots<br />

T. Smoleński 1 , ̷L. Cywiński 2 , M. Goryca, 1 and P. Kossacki 1<br />

1 Institute <strong>of</strong> Experimental Physics, Faculty <strong>of</strong> Physics, University <strong>of</strong> Warsaw, ul. Ho˙za<br />

69, 00-681 Warsaw, Poland<br />

2 Institute <strong>of</strong> Physics, Polish Academy <strong>of</strong> Sciences, Al. Lotników 32/46, 02-668 Warsaw,<br />

Poland<br />

It has been shown recently that a spin <strong>of</strong> a single Mn ion residing inside a quantum<br />

dot can be optically oriented by circularly polarized light creating spin-polarized excitons<br />

in the dot [1,2]. In case <strong>of</strong> experiments in which the absoprtion line corresponding to a<br />

specific spin state <strong>of</strong> the Mn was excited resonantly [2], it was argued that the spin relaxation<br />

<strong>of</strong> holes (due to phonon scattering and spin-orbit interaction, leading to transitions<br />

between bright and dark exciton states seen in experiment in Ref. [2]) can explain the<br />

existence <strong>of</strong> the Mn optical orientation [3].<br />

In experiment from Ref. [1] the spin-polarized excitons were created in the Mn free<br />

dot which was nonresonantly coupled to an adjacent dot [4] containing a single Mn spin<br />

[1,5]. The exciton (X) tunnels out <strong>of</strong> the first dot in a few ps [4], but the timescale on<br />

which it relaxes to its ground state in the second dot (thereby lowering its energy by<br />

≈ 0.2 eV) is not precisely know. Depending on whether this relaxation is fast or slow<br />

compared to the dynamics <strong>of</strong> the exciton coupled to the Mn spin, different states <strong>of</strong> the<br />

X+Mn system are expected to be realized after the relaxation. In order to model both the<br />

dynamics <strong>of</strong> the photoluminescence from the second dot, and the Mn spin dynamics (due<br />

to the sp-d exchange coupling with the exciton), we use the Lindblad generator formalism<br />

to describe all the relaxation process: the transition from the first dot to the ground<br />

state <strong>of</strong> the second dot, the radiative recombination, and the carrier spin relaxation. In<br />

this way we model both the coherent and the incoherent the processes leading to the<br />

brightening <strong>of</strong> dark excitons [5], possible population trapping in the dark state, and Mn<br />

optical orientation.<br />

This work was supported by InTechFun (Grant No. POIG.01.03.01-00-159/08), by the<br />

Homing and the Start programmes <strong>of</strong> the Foundation for Polish Science supported by the<br />

EEA Financial Mechanism, and by the Polish Ministry <strong>of</strong> Science and Higher Education<br />

as research grants in years 2010-<strong>2011</strong>.<br />

[1] M. Goryca, T. Kazimierczuk, M. Nawrocki, A. Golnik, J.A. Gaj, P. Kossacki, P. Wojnar,<br />

and G. Karczewski, Phys. Rev. Lett. 103, 087401 (2009).<br />

[2] C. Le Gall, L. Besombes, H. Boukari, R. Kolodka, J. Cibert, and H. Mariette, Phys.<br />

Rev. Lett. 102, 127402 (2009).<br />

[3] ̷L. Cywiński, Phys. Rev. B 82, 075321 (2010).<br />

[4] T. Kazimierczuk, J. Suffczyński, A. Golnik, J.A. Gaj, P. Kossacki, and P. Wojnar,<br />

Phys. Rev. B 79, 153301 (2009).<br />

[5] M. Goryca, P. Plochocka,T. Kazimierczuk, P. Wojnar, G. Karczewski, J.A. Gaj, M. Potemski,<br />

and P. Kossacki Phys. Rev. B 82, 165323 (2010).<br />


TuP2 _______________________________________________________________<br />

40th "Jaszowiec" <strong>2011</strong> International School and Conference on the Physics <strong>of</strong> Semiconductors<br />

Three-dimensional anisotropy studies <strong>of</strong> CdTe quantum dots<br />

J. Kobak 1 , W. Pacuski 1 , T. Kazimierczuk 1 , J. Suffczy ski 1 , T. Jakubczyk 1 , A. Golnik 1 ,<br />

P. Kossacki 1 , M. Nawrocki 1 , J.A. Gaj 1 , C. Kruse 2 , D. Hommel 2<br />

1 Institute <strong>of</strong> Experimental Physics, Faculty <strong>of</strong> Physics, University <strong>of</strong> Warsaw,<br />

ul. Ho a 69, PL-00-681 Warszawa, Poland<br />

2 Institute <strong>of</strong> Solid State Physics, University <strong>of</strong> Bremen, PO.Box 330 440, D-28334 Bremen,<br />

Germany<br />

In standard microphotoluminescence study <strong>of</strong> quantum dots (QDs) optical axis is<br />

oriented parallel to the growth axis <strong>of</strong> the sample. Therefore polarization <strong>of</strong> emitted photons is<br />

perpendicular to the growth axis <strong>of</strong> the sample. In this work we present the results <strong>of</strong> microand<br />

macro-photoluminescence study <strong>of</strong> QDs in a configuration with optical axis perpendicular<br />

to the growth axis. Such measurements were possible since the samples were cleaved and<br />

their edge was exposed. In our measurements we explore both polarizations parallel and<br />

perpendicular to the growth axis.<br />

We studied selforganized CdTe QDs in ZnTe matrix grown by molecular beam<br />

epitaxy (MBE). Two techniques were used in order to form dots from thin CdTe layer. In the<br />

first technique the CdTe thin layer was covered by amorphous tellurium at a low temperature.<br />

Subsequently amorphous tellurium was evaporated at growth temperature before overgrowing<br />

with ZnTe. In the second technique, the CdTe thin layer was overgrown by ZnTe with high<br />

Zn content, without changing the temperature. For spectroscopy samples were placed under<br />

the microscope lens which position was controlled by three xyz piezo translation stages.<br />

Whole system was immersed in superfluid helium in a magneto-optical cryostat. Lines<br />

associated to the single QDs we identified with use <strong>of</strong> single photon correlation<br />

measurements.<br />

Two emission bands were observed in the macro-photoluminescence measurements.<br />

Band seen at lower energy was attributed to quantum dots. This band is strongly polarized in<br />

direction perpendicular to the growth axis <strong>of</strong> the sample. Such effect is expected for heavy<br />

hole excitons. The band located at higher energy was identified as coming from the wetting<br />

layer. It shows a small degree <strong>of</strong> linear polarization with a stronger component in direction<br />

parallel to the growth axis.<br />

In micro-photoluminescence measurements we have found that more than 99% <strong>of</strong><br />

QDs show the complete polarization perpendicular to the growth axis. This is in agreement<br />

with macro-photoluminescence results and with prediction done for heavy hole excitons.<br />

Surprisingly, we have found also sharp emission lines exhibiting a weak polarization degree.<br />

Moreover, for them the stronger polarization was parallel to the growth axis. Apart from<br />

polarization properties, such lines exhibit typical properties <strong>of</strong> QDs: excitation power<br />

dependence and temporal evolution such as biexciton – exciton cascade observed in photon<br />

correlation measurements.<br />

We have done magneto-optical measurements to get a deeper understanding <strong>of</strong> the<br />

observed unusual polarization anisotropy. We applied two configurations: with a magnetic<br />

field in a direction parallel or perpendicular to the growth axis. Results <strong>of</strong> the polarization<br />

resolved magnetooptical measurements are discussed in terms <strong>of</strong> multi-axis anisotropy <strong>of</strong><br />

QDs.<br />


40th _______________________________________________________________<br />

"Jaszowiec" <strong>2011</strong> International School and Conference on the Physics <strong>of</strong> Semiconductors TuP3<br />

Probing electron concentration in epitaxial graphene<br />

using Raman spectroscopy<br />

K.Grodecki 1,2 , W.Strupinski 2 , A.Wysmołek 1 , R.Stępniewski 1 , and J.M.Baranowski 2,1<br />

1 Faculty <strong>of</strong> Physics, University <strong>of</strong> Warsaw, Hoża 69, 00-681 Warsaw, Poland<br />

kacper.grodecki@fuw.edu.pl<br />

2 Institute <strong>of</strong> Electronic Materials Technology, Wólczynska 133, 01-919 Warsaw, Poland<br />

Over the past years, a lot <strong>of</strong> attention has been attracted to graphene. Its unique<br />

properties could make it the successor to silicon in electronic applications. Now one <strong>of</strong> the<br />

essential problems that must be solved in graphene technology is accurate control <strong>of</strong> the<br />

carrier concentration in the structure. Within this communication we would like to let know<br />

that this problem can be effectively solved by the use <strong>of</strong> Raman spectroscopy. The Raman<br />

spectra <strong>of</strong> graphene-like structures consist <strong>of</strong> several bands. The most prominent among them<br />

are so called G and 2D bands. The position <strong>of</strong> the 2D band is predominantly strain sensitive<br />

whereas the position <strong>of</strong> the G band depends both on the intensity and the doping [1, 2].<br />

In this communication we show how frequency measurements <strong>of</strong> G and 2D-band may<br />

be used to evaluate free carrier concentration in graphene layer. In order to validate this<br />

method we have examined a set <strong>of</strong> graphene samples with known electron concentration.<br />

The measured graphene layers were grown on Si face <strong>of</strong> 4H-SiC on-axis substrates by<br />

chemical vapor deposition technique [3]. As determined from the Hall measurements the<br />

electron concentration <strong>of</strong> different samples remained between 6x10 12 and 2.4x10 13 cm -2 .<br />

Micro-Raman measurements were performed at room temperature using single grating<br />

spectrometer equipped with longpass filters and CCD camera. The size <strong>of</strong> exciting light spot<br />

from 532nm Nd:YAG continuous wave laser at the sample surface was approximately 2mm<br />

in diameter.<br />

The micro-Raman measurements performed on the samples with different electron<br />

concentration revealed pronounced shifts <strong>of</strong> the G and 2D band positions. Basing on the<br />

strain dependences <strong>of</strong> these bands from literature data [2] the electron concentration induced<br />

shift was calculated. This result - 1cm -1 /10 12 cm 2 - remains in compliance with the<br />

measurements performed within this concentration range for a gated structure freestanding<br />

graphene measurements [1]. This proves that Raman spectroscopy can be very useful as a fast<br />

contactless method which allows to determinate the local carrier concentration and strain (up<br />

to micrometer scale) in graphene structures.<br />

[1] A. Das, S. Pisana, B.Chakraborty, S. Piscanec, S. K. Saha, U. V. Waghmare, K. S.<br />

Novoselov, H. R. Krishnamurthy, A. K. Gaim, A. C. Ferrari and A. K. Sood, Nature<br />

Nanotechnology 3, 210 (2008)<br />

[2] N. Ferralis, Journal <strong>of</strong> Materials Science 45, 19, (<strong>2011</strong>)<br />

[3] W. Strupinski, K. Grodecki, A. Wysmolek, R. Stepniewski, T. Szkopek, P. E. Gaskell, A.<br />

Gruneis, D. Haberer, R. Bozek, J. Krupka, and J. M. Baranowski, NANO Letters (<strong>2011</strong>) DOI:<br />

10.1021/nl200390e<br />


TuP4 _______________________________________________________________<br />

40th "Jaszowiec" <strong>2011</strong> International School and Conference on the Physics <strong>of</strong> Semiconductors<br />

Dynamical control <strong>of</strong> excitonic fine structure with nanomechanical strain<br />

M. Zielinski 1 , G. W. Bryant 2 , N. Malkova 2 , J. Sims 3 , W. Jaskolski 1 , J. Aizupura 4<br />

1 <strong>Instytut</strong> <strong>Fizyki</strong> UMK, ul. Grudziądzka 5, 87-100 Toruń, Poland<br />

2 Atomic Physics Division and Joint Quantum Institute, NIST, 100 Bureau Drive,<br />

Gaithersburg, Maryland 20899-8423, USA<br />

3 Information Technology Laboratory, NIST, 100 Bureau Drive, Gaithersburg, Maryland<br />

20899-8423, USA<br />

4 Centro Mixto de Fisica de Materiales CSIC-UPV/EHU and Donostia International Physics<br />

Center, Paseo Manuel Lardizabal 4, Donostia-San Sebastian 20018, Spain<br />

We show how nanomechanical strain [1,2] can be used to achieve dynamical control<br />

<strong>of</strong> electronic and optical properties <strong>of</strong> semiconductor quantum dots, including exciton fine<br />

structure <strong>of</strong> single quantum dots and charge distributions in double quantum dots with<br />

potential applications in quantum dot based schemes for entangled photon-pair generation and<br />

manipulating interacting qubits for quantum information processing.<br />

We use atomistic tight-binding theory [1-4] to describe the response <strong>of</strong> confined states<br />

in a self-assembled quantum dot embedded in a nanomechanical cantilever. The internal strain<br />

due to the lattice mismatch, the external strain and the internal readjustment to minimize the<br />

applied strain must all be accounted for to model correctly the strain effects [1,2]. We show<br />

that applied strain can be used to manipulate the fine structure splitting <strong>of</strong> mechanoexcitons<br />

by distorting electron and hole charge distributions and rotating hole orientation. Electrons<br />

and hole levels and charge distributions can shift together or in opposite directions depending<br />

on how the strain is applied. This gives control for tailoring band gaps and optical response.<br />

The strain can also be used to transfer electrons and holes between vertically or laterally<br />

coupled dots, giving a mechanism for manipulating transition strengths and interacting qubits<br />

for quantum information processing.<br />

We demonstrate that nanomechanical strain tunes the exchange splittings and rotates<br />

the polarization <strong>of</strong> mechanoexcitons, providing energy and phase control <strong>of</strong> excitons. We<br />

further demonstrate that nanomechanical strain is a tool to shift electron and hole levels,<br />

transfer carriers between dots, manipulate mechanoexciton shape, orientation, fine-structure<br />

splitting and optical transitions rates.<br />

[1] G.W. Bryant, M. Zielinski, N. Malkova, J. Sims, W. Jaskolski and J. Aizpurua, Phys. Rev.<br />

Lett. 105, 067404 (2010).<br />

[2] G.W. Bryant, M. Zielinski, N. Malkova, J. Sims, W. Jaskolski, J. Aizpurua, prepared for<br />

publication in Phys. Rev. B<br />

[3] W. Jaskolski, M. Zielinski, G.W. Bryant, J. Aizpurua, Phys. Rev. B 74, 205309 (2006).<br />

[4] M. Zielinski, M. Korkusinski, and P. Hawrylak, Phys. Rev. B 81, 085301 (2010).<br />


40th _______________________________________________________________<br />

"Jaszowiec" <strong>2011</strong> International School and Conference on the Physics <strong>of</strong> Semiconductors TuP5<br />

Non-Markovian noise at the Fermi-edge singularity in quantum<br />

dots<br />

Katarzyna Roszak 1 and Tomáˇs Novotn´y 2<br />

1 Institute <strong>of</strong> Physics, Wroc̷law University <strong>of</strong> Technology, 50-370 Wroc̷law, Poland<br />

2 Department <strong>of</strong> Condensed Matter Physics, Faculty <strong>of</strong> Mathematics and Physics,<br />

Charles University, 12116 Prague, Czech Republic<br />

The Fermi-edge singularity is a phenomenon originating from the interaction <strong>of</strong> conduction<br />

electrons with localised perturbations and is characterised by a power-law divergence.<br />

It was first predicted theoretically for X-ray absorption in metals [1] and later<br />

verified experimentally [2]. The theory then developed [1,3] has been used to describe<br />

other situations involving a similar Hamiltonian and leading to the same power-law divergence,<br />

such as resonant tunnelling through localised levels. The proper description <strong>of</strong><br />

the transport set-up is done by an extension <strong>of</strong> the original Fermi-edge singularity model,<br />

where no charge transfer between the continuum and the localized level(s) is considered,<br />

to the interacting resonant level model, which has served recently as an important benchmark<br />

for novel quantum transport techniques [4].<br />

The Fermi-edge singularity in transport through quantum dots occurs in the regime<br />

where the energy <strong>of</strong> the quantum dot level(s) is similar to that <strong>of</strong> one <strong>of</strong> the leads. The<br />

change <strong>of</strong> occupation in the local level(s) during the tunnelling process leads to sudden<br />

changes in the scattering potential and hence, to truncated singular behaviour <strong>of</strong> the<br />

current through the quantum dot at resonance and power-law dependence <strong>of</strong> the current<br />

away from resonance [5,6]. As shown in recent experiments [6], noise in this regime<br />

displays characteristic behaviour (including a super-Poissonian maximum) which cannot<br />

be accounted for by the Markovian theory. Since the Fermi-edge singularity transport<br />

setup involves both many-body correlations and quantum coherence it is reasonable to<br />

expect large non-Markovian corrections in the singularity region. We show that this is<br />

indeed the case and that including the non-Markovian effects allows us to account for all<br />

<strong>of</strong> the qualitative features <strong>of</strong> the noise.<br />

[1] G. D. Mahan, Phys. Rev. 163, 612 (1967).<br />

[2] P. H. Citrin, Phys. Rev. B 8, 5545 (1973).<br />

[3] G. D. Mahan, Many-Particle Physics (Kluwer, New York, 2000).<br />

[4] E. Boulat, H. Saleur, and P.Schmitteckert, Phys. Rev. Lett.101, 140601 (2008).<br />

[5] H. Frahm, C. von Zobeltitz, N. Marie, and R. J. Haug, Phys. Rev. B74,035329 (2006).<br />

[6] N. Marie, F. Hohls, T. Lüdtke, K.Pierz, R. J. Haug, Phys. Rev. B 75, 233304 (2007).<br />


TuP6 _______________________________________________________________<br />

40th "Jaszowiec" <strong>2011</strong> International School and Conference on the Physics <strong>of</strong> Semiconductors<br />

Transport and Spin Properties <strong>of</strong> CdTe/CdMgTe Quantum Point Contacts<br />

M. Czapkiewicz, J. Wróbel, V. Kolkovsky, P. Nowicki, M. Aleszkiewicz, M. Wiater and<br />

T. Wojtowicz<br />

<strong>Instytut</strong> <strong>Fizyki</strong> <strong>PAN</strong>, Warszawa, Poland<br />

The spin properties <strong>of</strong> one-dimensional (1D) semiconductor devices are more strongly influenced<br />

by electron-electron interactions as compared to two-dimensional (2D) systems. Up to<br />

now, however, studies <strong>of</strong> spin characteristics <strong>of</strong> 1D transport channels are almost exclusively<br />

devoted to III-V heterostructures and quantum wells [1]. Here we report on fabrication and<br />

low temperature magnetotransport measurements <strong>of</strong> quantum point contacts, patterned from ntype<br />

CdTe/CdMgTe modulation doped quantum well. We expect that the many-body effects are<br />

quite important in this material since the effective mass is larger and the dielectric constant is<br />

smaller as compared to GaAs.<br />

Two-terminal quantum point contacts<br />

have been made <strong>of</strong> high quality 2D electron<br />

gas with concentration n2D= 4.6×10 11 cm −2<br />

and mobility µ = 0.2×10 6 cm 2 /Vs. The<br />

nanojunctions <strong>of</strong> length L≈0.2 µm and lithographic<br />

width Wlith = 0.45±0.01 µm are<br />

patterned by e-beam lithography and deepetching<br />

techniques. The carrier density in the<br />

device is controlled by means <strong>of</strong> V-shaped<br />

side gates which are separated from the constriction<br />

area by narrow etched grooves, see<br />

Fig. 1. The differential conductances G have<br />

been measured in a He-3 cryostat as a function<br />

<strong>of</strong> dc source-drain excitation and inplane<br />

magnetic field by employing a standard<br />

low-frequency lock-in technique.<br />

SG<br />

5<br />

10<br />

15<br />

µm<br />

etched grooves<br />

SG<br />

Figure 1: Atomic force microscopy image<br />

<strong>of</strong> quantum point contact formed by chemical<br />

etching. The width <strong>of</strong> nano-junction is controlled<br />

by side gates (SG).<br />

Data show that G is quantized vs gate voltage, however, conductance steps are reduced<br />

down to≈ 0.6×(2e 2 /h) and superimposed on reproducible conductance fluctuations (UCF’s).<br />

We have also found, that the quantization/UCF patterns change considerably when voltages are<br />

applied asymmetrically to the side gates. Therefore, we argue that transport in studied devices<br />

is strongly influenced by short range potential fluctuations present in the junction area. However,<br />

in spite <strong>of</strong> a disorder, we have been able to observe the Zeeman splitting <strong>of</strong> 1D transport<br />

channels at milikelvin temperatures. By comparing obtained splittings with the nonlinear conductance<br />

data we have been able to determine the effective Landé factor in the constriction. We<br />

have found that |g ∗ | = 1.7±0.1 in remarkable agreement with bulk value reported for CdTe,<br />

independently on 1D level index. We conclude that there is no enhancement <strong>of</strong> electronic g factor.<br />

This is an unexpected result, since such enhancement, attributed to exchange interaction,<br />

was always observed for quantum point contacts and quantum wires made <strong>of</strong> GaAs [2].<br />

We acknowledge the support from the Polish Ministry <strong>of</strong> Science and Higher Education,<br />

project number N202/103936 and partial support by the European Union within European Regional<br />

Development Fund (grant Innovative Economy POIG.01.01.02-00-008/08).<br />

[1] K.-F. Berggren and M. Pepper, Phil. Trans. R. Soc. A 368, 1141 (2010).<br />

[2] K. J. Thomas et al., Phys.Rev. B 68, 4846 (1998).<br />


40th _______________________________________________________________<br />

"Jaszowiec" <strong>2011</strong> International School and Conference on the Physics <strong>of</strong> Semiconductors TuP7<br />

The Role <strong>of</strong> Strong Coupling in the Superradiance <strong>of</strong> Ensembles<br />

<strong>of</strong> Quantum Dots.<br />

Micha̷l Kozub, Pawe̷l Machnikowski<br />

1 Institute <strong>of</strong> Physics, Wroc̷law University <strong>of</strong> Technology, 50-370 Wroc̷law, Poland<br />

We present a theoretical description <strong>of</strong> spontaneous emision from a dense ensemble<br />

<strong>of</strong> self-assembled quantum dots (QDs) in the limit <strong>of</strong> weak excitation. We show that<br />

the decay rate <strong>of</strong> the spontaneous emission from the system depends on the number <strong>of</strong><br />

collectively emitting dots, thus confirming the experimental conclusion [1] on the appearance<br />

<strong>of</strong> collective (superradiant) emission in the inhomogeneous ensamble <strong>of</strong> interacting<br />

dots. However our simulations indicate that interactions over a common electromagnetic<br />

reservoir are not sufficiently strong to account for the observed increase <strong>of</strong> the decay rate.<br />

We therefore introduce additional terms in the interaction potential to make up for the<br />

discrepancy between the experiment and our simulations.<br />

In the experiment [1], the decay rate <strong>of</strong> the luminescence signal was analysed as a<br />

function <strong>of</strong> the size <strong>of</strong> mesas cut out in a planar ensemble <strong>of</strong> QDs, that is, <strong>of</strong> the number<br />

<strong>of</strong> dots collectively interacting with the electromagnetic field. The observed decay <strong>of</strong> luminescence<br />

after a resonant excitation had an approximately exponential character with<br />

the decay rate increasing as the mesas became larger. This was interpreted as a collective<br />

effect, due to long range coupling between the dots.<br />

In this work we model the ensemble <strong>of</strong> QDs as a system <strong>of</strong> interacting and collectively<br />

emitting two-level atoms. In analogy with the experimental procedure, we study the<br />

emission <strong>of</strong> systems <strong>of</strong> dots with randomly selected transition energies that are randomly<br />

placed (at a fixed density) on squares <strong>of</strong> different sizes. The evolution <strong>of</strong> the system<br />

is simulated using the equations derived in the Power-Zinau-Wooley picture and in the<br />

Markov approximation, which corresponds to the method <strong>of</strong> Ref. [2], generalized to nonidentical<br />

emitters placed in a dielectric medium. We make the additional assumption that<br />

at most one exciton is present in the system in the initial state, coherently delocalized<br />

over all the dots (which corresponds to resonant excitation with a very short pulse). We<br />

calculate the time dependence <strong>of</strong> the average number <strong>of</strong> excitons in the dots for system<br />

parameters corresponding to the experimental situation [1] and show that the population<br />

decay indeed has an approximately exponential character with rates very close to<br />

those observed experimentally. This result could not be achieved for dipole-coupled dots,<br />

without taking into account another type <strong>of</strong> QD interaction. We show that including a<br />

short range tunnel coupling between the dots leeds to an increase <strong>of</strong> the recombination<br />

rate which is indistinguishable from that obtained for an artificially enhanced long-range<br />

dipole coupling. In this way we show that the coupling between the quantum dot emitters<br />

and the electromagnetic vacuum cannot be held fully responisble for the variation <strong>of</strong> the<br />

emission rate as proposed in [1].<br />

[1] M. Scheibner et al., Nature Physics, 3, 106 (2007).<br />

[2] R. H. Lehmberg, Phys. Rev. A, 2, 883 (1970).<br />


TuP8 _______________________________________________________________<br />

40th "Jaszowiec" <strong>2011</strong> International School and Conference on the Physics <strong>of</strong> Semiconductors<br />

Photoluminescence Linewidth Analysis<br />

<strong>of</strong> Single CdMnTe Quantum Dots<br />

M. Szymura 1;2 , Ł. Kłopotowski 2 , P. Wojnar 2 , K. Fronc 2 , T. Kazimierczuk 3 ,<br />

G. Karczewski 2 and T. Wojtowicz 2<br />

1 Department <strong>of</strong> Mathematics and Natural Sciences,<br />

Cardinal Stefan Wyszyński University, Warsaw, Poland<br />

2 Institute <strong>of</strong> Physics, Polish Academy <strong>of</strong> Sciences, Warsaw, Poland<br />

3 Faculty <strong>of</strong> Physics, University <strong>of</strong> Warsaw, Warsaw, Poland<br />

Quantum dots (QDs) doped with transition metal ions are interesting from the point <strong>of</strong> view <strong>of</strong><br />

both basic studies and applications in quantum information processing. Introducing e.g. Mn,<br />

substantially changes magnetooptical properties <strong>of</strong> QDs. In particular, the photoluminescence<br />

(PL) linewidth (γ) for a CdTe QD is about 200 μeV, while for CdMnTe with 20% <strong>of</strong><br />

manganese increases by a factor <strong>of</strong> 100. In this report, we present a comprehensive study <strong>of</strong><br />

the PL linewidth performed on a large number <strong>of</strong> dots differing with respect to size and<br />

manganese concentration.<br />

Two kinds <strong>of</strong> structures with CdMnTe QDs were investigated: those with ZnTe barrier and<br />

with ZnMnTe barrier. All quantum dots were grown by molecular beam epitaxy (MBE) on<br />

GaAs substrate. A 4 μm thick buffer CdTe layer was first deposited. Next a ZnTe (and<br />

ZnMnTe) barrier layer was grown. QDs were developed from 6 monolayers <strong>of</strong> CdMnTe. PL<br />

measurements were performed as a function <strong>of</strong> the magnetic field up to 5T at a temperature <strong>of</strong><br />

2K. We study samples with manganese concentration between 1% to 20%.<br />

The presence <strong>of</strong> manganese ions caused giant Zeeman splitting <strong>of</strong> electronic states <strong>of</strong> QDs.We<br />

-<br />

analyzed two circular polarizations. In practice, the transitions in polarization quickly<br />

disappear as a result <strong>of</strong> efficient spin relaxation between the Zeeman split subbands. We<br />

reproduce the transition energies with a Brillouin function, which allows us to evaluate<br />

manganese concentration and temperature.<br />

Broadening <strong>of</strong> the PL transitions from Mn-containig QDs is due to magnetization fluctuations.<br />

Therefore, γ depends on magnetic field, QD volume and manganese concentration and<br />

temperature [1]. At the absence <strong>of</strong> magnetic field, for a QD with 1.5% manganese we find γ<br />

≈4.5 meV. As the magnetic field is increased, γ decreases by about a factor <strong>of</strong> 2 in 5T. This<br />

decrease manifests the freezing <strong>of</strong> the magnetic fluctuations in increased magnetic field. We<br />

reproduce this narrowing <strong>of</strong> the linewidth with magnetic field with a formula derived from<br />

fluctuation dissipation theorem [1]. We obtain a good agreement between the results <strong>of</strong><br />

calculations and experiment. Moreover, the above analysis allows us to evaluate the QD<br />

volume. We find that QDs emitting in higher energies have smaller volume, as expected.<br />

[1] G. Bacher et al. Phys. Rev. Lett. 89, 127201 (2002).<br />


40th _______________________________________________________________<br />

"Jaszowiec" <strong>2011</strong> International School and Conference on the Physics <strong>of</strong> Semiconductors TuP9<br />

Statistical Study <strong>of</strong> the Inter-Dot Excitation Transfer in CdTe/ZnTe<br />

Quantum Dots.<br />

Maciej Koperski 1 , Tomasz Kazimierczuk 1 , Mateusz Goryca 1 , Andrzej Golnik 1 ,<br />

Jan A. Gaj 1 , Michał Nawrocki 1 , Piotr Wojnar 2 and Piotr Kossacki 1<br />

1 Institute <strong>of</strong> Experimental Physics, Faculty <strong>of</strong> Physics, University <strong>of</strong> Warsaw,<br />

Hoża 69, 00-681 Warsaw, Poland<br />

2 Polish Academy <strong>of</strong> Sciences, Lotników 32/46, 02-668 Warsaw, Poland<br />

Photoluminescence (PL) experiments are the most basic tool for characterization <strong>of</strong><br />

semiconductor quantum dots (QDs). They provide broad range <strong>of</strong> information on the<br />

excitonic complexes, such as neutral excitons (X), trions (X + , X - ) and biexcitons (XX),<br />

confined in a zero-dimensional system. Measurements <strong>of</strong> PL spectra as a function <strong>of</strong><br />

excitation laser energy is known as photoluminescence excitation technique (PLE). Features<br />

in PLE spectra are usually related to excited exciton states or phonon-replica. Additionally, in<br />

the system <strong>of</strong> self organized CdTe/ZnTe quantum dots, very sharp resonances at high energy<br />

are <strong>of</strong>ten observed. They were attributed to the efficient excitation transfer between<br />

neighboring dots [1]. Diverse experiments have been reported, taking advantage <strong>of</strong> the<br />

excitation transfer, such as polarization conversion [1] and orientation <strong>of</strong> the magnetic ion in<br />

dots with single manganese atom [2]. Despite extensive studies, the detailed mechanism <strong>of</strong> the<br />

inter-dot tunneling remains not entirely clear.<br />

Here we present the results <strong>of</strong> detailed statistical analysis <strong>of</strong> the energy distribution <strong>of</strong><br />

resonances detected in PLE spectra. We studied MBE grown samples containing single layer<br />

<strong>of</strong> self organized CdTe/ZnTe quantum dots. The samples were placed in a microphotoluminescence<br />

setup at low temperature (1.5 - 15 K). The PL was excited using a tunable<br />

dye laser in the range 570−610 nm. During measurements we have selected more than 200<br />

single quantum dots for which exciton complexes (X, X+, X- and XX) were identified, and<br />

for which the sharp resonances related to inter-dot transfer were observed. Each resonance<br />

correspond to the absorption with creation <strong>of</strong> neutral exciton, after which the exciton is<br />

transferred to the second dot and subsequently recombines. The difference between energies<br />

<strong>of</strong> neutral excitons in the two dots varied in our experiments between 80 meV and 300 meV<br />

and was mainly limited by the experimental setup. This distance is larger than typical distance<br />

between s-shell and p-shell groups <strong>of</strong> lines which yields about 50meV in our case. We<br />

analyzed the occurrence probability versus the energy distance between neutral exciton line in<br />

the emitting dot and the exciton energy in the absorbing dot (resonance energy).<br />

This analysis revealed relatively flat distribution <strong>of</strong> the resonances, which shows that<br />

levels participating in the tunneling are not collected in separate groups (p-shell, d-shell etc.).<br />

This might be interpreted as a result <strong>of</strong> smearing discrete distribution into continues one due<br />

to the statistical scatter between different dots or significant broadening <strong>of</strong> the excited states<br />

in each emitting dot. In any case, the observed flat distribution suggest that the density <strong>of</strong><br />

states participating in the tunneling is flat or the tunneling occurrence is not limited by the<br />

presence <strong>of</strong> excited states in the emitting dot.<br />

[1] T. Kazimierczuk, J. Suffczyński, A. Golnik, J. A. Gaj, P. Kossacki and P. Wojnar, Phys.<br />

Rev. B 79, 153301 (2009).<br />

[2] M. Goryca, T. Kazimierczuk, M. Nawrocki, A. Golnik, J. A. Gaj, P. Kossacki, P. Wojnar<br />

and G. Karczewski, Phys. Rev. Lett. 103, 087401 (2009)<br />


TuP10 _______________________________________________________________<br />

40th "Jaszowiec" <strong>2011</strong> International School and Conference on the Physics <strong>of</strong> Semiconductors<br />

Phonon influence on the weak measurement <strong>of</strong> double quantum<br />

dot spin states.<br />

̷L. Marcinowski 1 , M. Krzy˙zosiak 1,2 , K. Roszak 1 , P. Machnikowski 1 ,<br />

R. Buczko 3 , and J. Mostowski 3<br />

1 Institute <strong>of</strong> Physics, Wroc̷law University <strong>of</strong> Technology, 50-370 Wroc̷law, Poland<br />

2 Beijing University <strong>of</strong> Technology, 100124 Beijing, China<br />

3 Institute <strong>of</strong> Physics, Polish Academy <strong>of</strong> Sciences, 02-668 Warsaw, Poland<br />

The occupation <strong>of</strong> a double quantum dot (DQD) can be measured by coupling one <strong>of</strong> the<br />

dots to a quantum point contact (QPC) and monitoring the current flowing through the<br />

QPC [1]. This originates form the interaction between the quantum dot and QPC electrons<br />

which shifts the energy level in the QPC depending on the dot state influencing the<br />

tunneling rate through the point contact. A single tunneling event is a weak measurement<br />

<strong>of</strong> the dot and provides a small amount <strong>of</strong> information about the system state. A series<br />

<strong>of</strong> tunneling events constitutes a continous measurement process in which the descrete<br />

current yields a growing amount <strong>of</strong> information on the system state, but also gradually<br />

destroys quantum coherence and localizes the state, consistently with the von Neumann<br />

projection principle.<br />

The same measurement scheme can be used to distinguish two-electron spin-singlet<br />

and spin-triplet states in DQDs [2]. This is based on Pauli exclusion: when the spin-triplet<br />

state requires the electrons to be localized in different dots (only QD ground states are<br />

taken into account), double dot occupation is allowed for the spin-singlet state. Hence,<br />

monitoring the fluctuations in the QPC current allows one to perform a measurement in<br />

the singlet-triplet basis.<br />

In this work we study the effect <strong>of</strong> phonon-induced transitions which are present in this<br />

solid state system [3] on the QPC current and the measurement-influenced evolution <strong>of</strong> the<br />

DQD state. In the strong bias regime, when the measurement is possible in the absence<br />

<strong>of</strong> phonons [2], phonon-induced relaxation leads to a reduction <strong>of</strong> the double occupation,<br />

and hence, to quenching <strong>of</strong> the current response. In the weak bias case the energy transfer<br />

from the tunneling electron is insufficient to excite the QD to a doubly occupied state<br />

and the phonon reservoir serves as a source <strong>of</strong> noise which makes the excitation possible;<br />

this is reflected in the current trace.<br />

[1] T. M. Stace and S. D. Barrett, Phys. Rev. Lett. 92, 136802 (2004).<br />

[2] S. D. Barrett and T. M. Stace, Phys. Rev. B 73, 075324 (2006).<br />

[3] K. Roszak and P. Machnikowski, Phys. Rev. B 80, 195315 (2009).<br />


40th _______________________________________________________________<br />

"Jaszowiec" <strong>2011</strong> International School and Conference on the Physics <strong>of</strong> Semiconductors TuP11<br />

Influence<strong>of</strong>CarrierTrappingontheOpticalProperties<strong>of</strong><br />

InAs/InPQuantumDashes<br />

P.Kaczmarkiewicz,A.Musiał,G.Sęk,P.Podemski,J.Misiewicz,<br />

P.Machnikowski<br />

Institute<strong>of</strong>Physics,WrocławUniversity<strong>of</strong>Technology,50-370Wrocław,Poland<br />

Wemodeltheopticalproperties<strong>of</strong>highlyanisotropicInAs/InPquantumdots,also<br />

referredtoasquantumdashes(QDashes)[1]. Thosestructuresarecharacterizedbya<br />

non-uniformshapeandpresence<strong>of</strong>widthfluctuations[2]whichcanprovideadditional<br />

confinementinthesystem. Trapping<strong>of</strong>carriersinthepotentialfluctuationsleadsto<br />

quantitativeandqualitativechangeintheopticalproperties<strong>of</strong>thesystem.Theexciton<br />

groundstateexhibitsQD-like(stronglyconfined[3])propertieswhichcanexplainthe<br />

temperaturedependence<strong>of</strong>thedegree<strong>of</strong>linearpolarization<strong>of</strong>emittedradiation.<br />

WeshowhowthetransitionratesdependonQDashshapeparameters(amplitude<br />

andposition<strong>of</strong>thewidening)andcalculateemissionspectraforsingleQDashesandfor<br />

ensembles<strong>of</strong>QDashes. Wealsostudythepolarizationproperties<strong>of</strong>radiationemitted<br />

bythesystem. WeconfirmthataQDashwidthfluctuationleadstostrongtrapping<br />

<strong>of</strong>carriers,whichresultsinloweringtheanisotropy<strong>of</strong>anexcitongroundstateinspite<br />

<strong>of</strong>strongelongation<strong>of</strong>thewholestructure. Suchacharacter<strong>of</strong>theconfiningpotential<br />

manifestsitselfintheexperimentalresultsbyaloweredvalue<strong>of</strong>thedegree<strong>of</strong>linear<br />

polarizationatlowtemperaturesandlowexcitationconditions.<br />

Theinvestigation<strong>of</strong>theopticalproperties<strong>of</strong>QDashesisimportantfromthepoint<strong>of</strong><br />

view<strong>of</strong>possibleapplications.IthasbeenshownthatQDashesmayhaveseveraladvantagesoverothernanostructures,especiallyinlaserapplicationsfortelecommunication[1].<br />

Understanding<strong>of</strong>thepolarizationpropertiesandthereforecarrierconfinementregimeis<br />

alsoimportantforconstructingpolarization-insensitiveopticalamplifiers[4].<br />

Webaseourmodeling<strong>of</strong>theelectron-holesystemontheeffectivemassmethodwith<br />

light-heavyholebandmixingtakenintoaccount[5].Theholestatesareassumedtoconsist<br />

mostly<strong>of</strong>aheavy-holecomponentwithsmalladmixture<strong>of</strong>light-holestates.Thislightholeadmixturedependsontheanisotropy<strong>of</strong>theholewavefunctionviathemomentum<br />

dependence<strong>of</strong>therelevantelement<strong>of</strong>the k·pHamiltonian. Interferencebetweenthe<br />

radiationemittedbythelight-andheavy-holecomponentsleadstotheappearance<strong>of</strong><br />

apreferredpolarizationaxis,paralleltothestructureelongation. Thepresence<strong>of</strong>an<br />

additionalconfinementduetotheshapefluctuationdecreasestheanisotropy<strong>of</strong>anexciton<br />

groundstateandreducestheDOP.Inourmodelingweconsiderthecase<strong>of</strong>asingle<br />

excitonconfinedinaQDash,whichcorrespondstoexperimentalmeasurementsinthe<br />

weakexcitationregime. Coulombcorrelationsareincludedwithintheconfigurationinteractionscheme.<br />

[1]J.P.Reithmaier,G.Eisenstein,A.Forchel,Proc.IEEE95,1779(2007).<br />

[2]H.Deryetal.,J.Appl.Phys.95,6103(2004).<br />

[3]G.Sęketal.,J.Appl.Phys.105,086104(2009).<br />

[4]P.Podemskietal.,Appl.Phys.Lett.93,171910(2008).<br />

[5]P.Kaczmarkiewiczetal.,ActaPhys.Pol.A(inprint).<br />


TuP12 _______________________________________________________________<br />

40th "Jaszowiec" <strong>2011</strong> International School and Conference on the Physics <strong>of</strong> Semiconductors<br />

CdSe/ZnS Colloidal Quantum Dots with Alloyed Core/Shell Interfaces:<br />

a Photoluminescence Dynamics Study<br />

Konrad Dziatkowski 1,2 , Daniel Ratchford 1 , Thomas Hartsfield 1 , Xiaoqin Li 1 ,<br />

Yan Gao 3 and Zhiyong Tang 3<br />

1 Department <strong>of</strong> Physics, University <strong>of</strong> Texas at Austin, Austin TX 78712, USA<br />

2 Institute <strong>of</strong> Experimental Physics – University <strong>of</strong> Warsaw, 00-681 Warsaw, Poland<br />

3 National Center for Nanoscience and Technology, 100190 Beijing, P. R. China<br />

The colloidal semiconducting nanocrystals – and quantum dots (QDs) in particular –<br />

have been a subject to extensive studies driven by their promising photonic or photovoltaic<br />

applications. There are numerous features <strong>of</strong> colloidal QDs, to mention only long term<br />

photostability, that make them superior to other single emitters, e.g dye molecules.<br />

Nevertheless, their optical properties are not yet optimal from the viewpoint <strong>of</strong> technological<br />

utilization. One <strong>of</strong> the main unresolved issues with respect to colloidal QDs is fluctuations<br />

<strong>of</strong> photoluminescence (PL), or so called “blinking”. Despite intense experimental<br />

and theoretical research effort, the detailed microscopic mechanism <strong>of</strong> PL intermittency<br />

remains obscure, and so far no single theoretical model accounts for all observations [1].<br />

The most comprehensive explanation postulates that when a QD is charged, a non-radiative<br />

Auger ionization suppresses PL and makes the emitter appearing dark [2]. Recently,<br />

the diminishing <strong>of</strong> Auger process and, in consequence, quenching <strong>of</strong> blinking was observed<br />

in selenium-based ternary QDs, even though these QDs possessed extra charge [3].<br />

Subsequent theoretical study claimed this observation to be a general feature <strong>of</strong> the emitters<br />

with smoothed quantum confinement potential, since under such conditions the transition<br />

matrix element for the Auger recombination is greatly reduced [4].<br />

This paper reports a comprehensive study <strong>of</strong> PL dynamics in chemically-synthesized<br />

CdSe/ZnS QDs with alloyed core/shell interfaces. Due to the s<strong>of</strong>tening <strong>of</strong> the structural<br />

interface between the core and the shell such emitters manifest a quantum yield <strong>of</strong> up<br />

to 80 % [5]. The employed technique <strong>of</strong> time-correlated single photon counting enabled<br />

measurements <strong>of</strong> the decay <strong>of</strong> exciton luminescence from both the ensemble and individual<br />

QDs. For decreasing emission wavelength, the ensemble data revealed systematic increase<br />

<strong>of</strong> total decay rates with greater variation, as it is expected that smaller emitters are more<br />

influenced by the surface/interface trap states. In experiments performed on single QDs,<br />

the PL trajectories exhibited familiar two-state blinking pattern, despite alloying between<br />

CdSe and ZnS. We suggest that in core/shell QDs with a large band gap <strong>of</strong>fset, as in case<br />

<strong>of</strong> CdSe/ZnS system, the compositionally graded energy pr<strong>of</strong>ile at the interface may not<br />

be smooth enough to suppress non-radiative Auger recombination and prevent blinking.<br />

[1] P. Frantsuzov, M. Kuno, B. Janko, and R. A. Marcus, Nature Physics 4, 519 (2008).<br />

[2] A. L. Efros and M. Rosen, Physical Review Letters 78, 1110 (1997).<br />

[3] X. Wang, X. Ren, K. Kahen, M. A. Hahn, M. Rajeswaran, S. Maccagnano-Zacher,<br />

J. Silcox, G. E. Cragg, A. L. Efros, and T. D. Krauss, Nature 459, 686 (2009).<br />

[4] G. E. Cragg and A. L. Efros, Nano Letters 10, 313 (2010).<br />

[5] W. K. Bae, K. Char, H. Hur, and S. Lee, Chemistry <strong>of</strong> Materials 20, 531 (2008).<br />


40th _______________________________________________________________<br />

"Jaszowiec" <strong>2011</strong> International School and Conference on the Physics <strong>of</strong> Semiconductors TuP13<br />

Spectroscopy <strong>of</strong> Indirect Excitons in Vertically Stacked CdTe<br />

Quantum Dot Structures<br />

K. Kukliński, ̷L. K̷lopotowski, K. Fronc, P. Wojnar, T. Wojciechowski,<br />

M. Czapkiewicz, J. Kossut, G. Karczewski, and T. Wojtowicz<br />

Institute <strong>of</strong> Physics, Polish Academy <strong>of</strong> Sciences, Al. Lotników 32/46,<br />

02–668 Warsaw, Poland<br />

We show that by means <strong>of</strong> an electric field we can tune the energy levels in vertical<br />

single quantum dot (QD) pairs and study transitions related to recombination <strong>of</strong> indirect<br />

excitons.<br />

Embedding QDs in vertical field effect structures enables electrical control <strong>of</strong> the QDs<br />

charge state and allows to study properties related to Coulomb interactions. There are<br />

many reports on studies <strong>of</strong> direct excitons in single CdTe/ZnTe QDs. However, for this<br />

material system, no data is reported on indirect excitons, i.e. excitons related to recombination<br />

<strong>of</strong> an electron and a hole localized in two separate QDs. Obtaining a controlled<br />

coupling between two QDs is interesting from the point <strong>of</strong> view <strong>of</strong> basic research and for<br />

realization <strong>of</strong> a quantum gate, the building block <strong>of</strong> a quantum computer.<br />

In this communication, we present the results <strong>of</strong> spectroscopic studies <strong>of</strong> indirect excitons<br />

in vertically stacked QD structures. The QDs were embedded in the insulating<br />

region <strong>of</strong> a p–i–Schottky diode grown by molecular beam epitaxy on a GaAs substrate.<br />

The layer sequence was the following: a 4-µm thick p–doped ZnTe buffer, a 100 nm <strong>of</strong><br />

undoped ZnTe, two QD layers separated by a 4 or 8 nm ZnTe spacer, a 50 nm thick<br />

ZnTe cap and a 50 nm ZnMgTe blocking barrier. QDs were formed from a 2D CdTe layer<br />

six monolayers thick. Ohmic contacts were established to the p–type ZnTe and a Al/Au<br />

Schottky contacts were deposited on top <strong>of</strong> the sample. Single QD pairs were accessed<br />

through 200 nm diameter shadow mask apertures. The photoluminescence (PL) signal<br />

was excited with a 532 nm laser beam focused to a 2 µm spot with a microscope objective.<br />

PL spectra were measured at 10 K as a function <strong>of</strong> a bias voltage.<br />

At zero bias we observe PL transitions from single pairs <strong>of</strong> QDs. We identify charge<br />

states from uncoupled dots. As the bias is increased, transitions shift due to the quantum<br />

confined Stark effect. Evidence <strong>of</strong> the presence <strong>of</strong> indirect excitons in the measured PL<br />

spectra are transitions strongly shifting in the electric field, as a consequence <strong>of</strong> greater<br />

value <strong>of</strong> dipole moment compared with direct excitons. We observe two types <strong>of</strong> lines:<br />

these for which Stark shift is about 0.5–1 meV/V and these with 4–7 meV/V. In accordance<br />

with the above conclusion, we identify the first transitions as related to direct<br />

excitons, and the latter as indirect. We assume that the dipole moment <strong>of</strong> the indirect excitons<br />

is pind = ed, where d is the spacer layer thickness. Therefore, the Stark shift <strong>of</strong> the<br />

indirect exciton allows us to calibrate the electric field in our structure. We find that this<br />

field is a factor <strong>of</strong> four smaller than expected from a capacitor formula: F = (U−Ubi)/W,<br />

where U and Ubi is the applied and built–in voltage, respectively, and W is the width <strong>of</strong><br />

the intrinsic region. Having calibrated F, we find that the dipole moment for the direct<br />

exciton pdir ∼ 1 nm, thus pind ≈ 4(8)pdir. With decreasing reverse bias we observe both<br />

the red– and blue–shifted indirect exciton transitions. We assume that due to strain–<br />

enhanced nucleation, the QDs in the top layer are larger than the bottom dots. Basing<br />

on the band pr<strong>of</strong>iles <strong>of</strong> our structures, we conclude that the blue–shifted transitions are<br />

related to the recombination <strong>of</strong> the electron and hole localized in the top and the bottom<br />

dot, respectively. In turn, red–shifted indirect excitons are associated with the recombination<br />

<strong>of</strong> the electron and hole in the bottom and the top dot, respectively.<br />


TuP14 _______________________________________________________________<br />

40th "Jaszowiec" <strong>2011</strong> International School and Conference on the Physics <strong>of</strong> Semiconductors<br />

Collective spontaneous emission from pairs <strong>of</strong> quantum dots:<br />

the role <strong>of</strong> coupling and system geometry<br />

Wildan Abdussalam, Anna Sitek, Pawe̷l Machnikowski<br />

Institute <strong>of</strong> Physics, Wroc̷law University <strong>of</strong> Technology, 50-370 Wroc̷law, Poland<br />

We study the role <strong>of</strong> various types <strong>of</strong> coupling between semiconductor quantum dots<br />

(QDs) in the collective spontaneous emission <strong>of</strong> double dot systems. Excitons delocalized<br />

in closely spaced QDs recombine in a different way than in a single QD due to collective<br />

interaction <strong>of</strong> the two emitters with the quantum radiation field [1]. While for noninteracting<br />

dots this collective effect appears only for almost identical dots (with the<br />

transition energies within the emission line width), coupling between the dots leads to<br />

accelerated or slowed down emission even for dots with different transitions energies,<br />

which is manifested in the linear and non-linear response from these systems [2].<br />

In this contribution, we study the spontaneous emission from an exciton confined in<br />

a double quantum dot. We focus on the similarities and differences between the cases <strong>of</strong><br />

radiative (long-range, dipole) and tunnel coupling between the excitons in the dots. We<br />

show that for strictly identical dots the oscillating nature <strong>of</strong> the dipole coupling on long<br />

distances leads to non-monotonic dependence <strong>of</strong> the radiative decay rate on the inter-dot<br />

separation, which is not present in the case <strong>of</strong> tunnel coupling. However, for a double<br />

dot system with a realistic, technologically feasible mismatch <strong>of</strong> transition energies, the<br />

collective effects disappear completely well before these oscillations become relevant. In<br />

this case, there is no qualitative difference between the two physically different coupling<br />

types and the system dynamics in the two cases becomes indistinguishable for appropriate<br />

values <strong>of</strong> the coupling parameters.<br />

We believe that these findings may shed some light on the interpretation <strong>of</strong> the experiment<br />

[3] in which enhanced emission was observed in a quantum dot ensemble in which<br />

the dipole coupling energies on the typical inter-dot distances were much smaller than the<br />

average transition energy mismatch between the dots.<br />

[1] A. Sitek, P. Machnikowski, Phys. Rev. B 75, 035328 (2007).<br />

[2] A. Sitek, P. Machnikowski, Phys. Rev. B 80, 115319 (2009); 80, 115301 (2009).<br />

[3] M. Scheibner, T. Schmidt, L. Worschech, A. Forchel, G. Bacher, T. Passow, D. Hommel,<br />

Nature Physics 3, 106 (2007).<br />


40th _______________________________________________________________<br />

"Jaszowiec" <strong>2011</strong> International School and Conference on the Physics <strong>of</strong> Semiconductors TuP15<br />

Tunneling Transfer Protocol in a Quantum Dots Chain Immune<br />

to Inhomogeneity<br />

Kamil Korzekwa and Pawe̷l Machnikowski<br />

Institute <strong>of</strong> Physics, Wroc̷law University <strong>of</strong> Technology, 50-370 Wroc̷law, Poland<br />

We propose a quantum dot (QD) implementation <strong>of</strong> a quantum state transfer channel.<br />

The proposed channel consists <strong>of</strong> N vertically stacked QDs with the nearest neighbour<br />

tunnel coupling, placed in an axial electric field. The QD chain isdoped with oneelectron.<br />

The terminal QDs are assumed to provide a stronger binding for the electron. In this<br />

way, the states with electron on the terminal dots are energetically separated from other,<br />

but remain indirectly coupled via the QD chain. The inhomogeneity <strong>of</strong> the QD chain<br />

is included by using normal distribution for the values <strong>of</strong> tunnel couplings and electron<br />

energy levels. By applying an external electric field along the chain, the terminal QDs<br />

are brought to resonance, which leads to an efficient occupation transfer.<br />

Our proposal requires a global electric field as the only control means, therefore it<br />

satisfies the requirement <strong>of</strong> simplicity. We study the conditions <strong>of</strong> getting the quantum<br />

state transfer and compare the results with the previously proposed protocol [1]. We show<br />

that in our model the transfer fidelity for a homogenous QD chain is independent <strong>of</strong> the<br />

chains length. We show also that controlling the global electric field is enough to make<br />

the system resistant to inhomogeneity <strong>of</strong> both QD energies and tunnel coupling. Within<br />

our model we use the real QD parameters and show that almost perfect tunneling transfer<br />

is possible.<br />

Sinceourprotocolfulfillsthesimplicityrequirementitisexperimentallyfeasible. Since<br />

thetransferisindependent<strong>of</strong>theQDinhomogeneityitcouldbeusedtotransferexcitation<br />

in a real QD chain.<br />

a)<br />

|P| 2<br />

1<br />

0.75<br />

0.5<br />

0.25<br />

0<br />

0 25 50 75 100<br />

t [ns]<br />

b)<br />

|P| 2<br />

1<br />

0.75<br />

0.5<br />

0.25<br />

0<br />

0 25 50 75 100<br />

Evolution <strong>of</strong> the occupation <strong>of</strong> on the initial QD (solid line), the final QD (dashed line) and the<br />

QDs inside the chain (dotted line) <strong>of</strong> 10 QDs spin chain. Electron energies in the QD’s normally<br />

distributed with standard deviation ∆ = 10meV and tunnel couplings J = 10meV. Terminal<br />

QDs energy E = 50meV a) No external electric field; b) Terminal QDs tuned to resonance by<br />

an external field.<br />

[1] A. Wójcik, T. ̷Luczak, P. Kurzyński, A. Grudka, T. Gdala, and M. Bednarska, Phys.<br />

Rev. A 72, 034303<br />

123<br />

t [ns]

TuP16 _______________________________________________________________<br />

40th "Jaszowiec" <strong>2011</strong> International School and Conference on the Physics <strong>of</strong> Semiconductors<br />

Thermal Quenching <strong>of</strong> Photoluminescence from epitaxial InGaAs/GaAs<br />

Quantum Dots with High Lateral Aspect Ratio<br />

Anna Musiał 1 , Grzegorz S k 1 , Aleksander Mary ski 1 , Paweł Podemski 1 ,<br />

Janusz Andrzejewski 1 and Jan Misiewicz 1<br />

Andreas Löffler 2 , Sven Höfling 2 , Stephan Reitzenstein 2 , Johann Peter Reithmaier 2,*<br />

and Alfred Forchel 2<br />

1 Institute <strong>of</strong> Physics, Wrocław University <strong>of</strong> Technology, Wrocław, Poland<br />

2 Technische Physik, Universität Würzburg, Wilhelm Conrad Röntgen-Center for Complex<br />

Material Systems, Am Hubland, D-97074 Würzburg, Germany<br />

Self-assembled In0.3Ga0.7As quantum dots (QDs) [1] have already been demonstrated as<br />

good candidates for testing QED phenomena due to enhanced exciton oscillator strength [2]<br />

being a result <strong>of</strong> increased structure volume and weaker carrier confinement [3].<br />

This report concerns temperature dependence <strong>of</strong> photoluminescence (PL) in both<br />

ensemble and single QD regime. At 5K, PL spectra consist <strong>of</strong> two bands related to the wetting<br />

layer (WL) and QD ensemble, separated by rather small energy (~ 35 meV) indicated shallow<br />

confining potentials for holes and electrons. At low temperatures WL emission dominates and<br />

there is a rather insignificant carrier transfer to the dots because most <strong>of</strong> the WL states are<br />

localized [3]. At higher temperatures (> 30K) carriers are released becoming extended into the<br />

WL plane and carrier redistribution increases the QD emission with respect to the WL one.<br />

Thermal quenching <strong>of</strong> PL from QD ensemble reveals activation energy <strong>of</strong> about 30 meV<br />

corresponding well with the confinement energy identifying a primary carrier loss mechanism<br />

– release <strong>of</strong> carriers to WL. This has also been confirmed at the single QD level. The<br />

activation energy varies between 15-30 meV reflecting the dot size and hence carrier<br />

confinement distribution. In case <strong>of</strong> single QD the impact <strong>of</strong> WL is stronger, because the<br />

probability that a carrier returns to the same dot and emits is lower than the probability <strong>of</strong><br />

carrier to get back to any <strong>of</strong> the dots contributing to the ensemble PL. In a very few cases a<br />

fingerprint <strong>of</strong> 2 nd significantly smaller activation energy is observed, suggesting that carrier<br />

localization inside those structures is not present or localization energy is very low (below 1<br />

meV - the resolution <strong>of</strong> the method) meaning that at low temperatures excitons are always<br />

extended over the entire QD and experience large coherence volume. This was further<br />

supported by polarization-resolved temperature dependent PL to probe the symmetry <strong>of</strong><br />

confining potential by obtaining the degree <strong>of</strong> linear polarization. It is low at 5K (~6%) and<br />

decreases slightly with temperature due to emission from higher energy states reflecting lower<br />

effective anisotropy [4]. This can be understood as a result <strong>of</strong> very weak confining potential<br />

for electrons in agreement with 8 band kp calculations giving DOP values below 10%.<br />

[1] A. Löffler, J. P. Reithmaier, A. Forchel, A. Sauerwald, D. Peskes, T. Kümmell and G.<br />

Bacher, J. Cryst. Growth 286, 6 (2006).<br />

[2] J. P. Reithmaier, G. S k, A. Löffler, C. H<strong>of</strong>mann, S. Kuhn, S. Reitzenstein, L. Keldysh,<br />

V. Kulakovskii, T. Reinecke and A. Forchel, Nature 432, 197 (2004).<br />

[3] G. S k, A. Musiał, P. Podemski, M. Syperek, J. Misiewicz, A. Löffler, S. Heling, L.<br />

Worschech and A. Forchel, J. Appl. Phys. 107, 96106 (2010).<br />

[4] P. Kaczmarkiewiecz, A. Musiał, G. S k, P. Podemski, P. Machnikowski and J. Misiewicz,<br />

Acta Phys. Pol. A (<strong>2011</strong>), in press.<br />

* Currently at: Institute <strong>of</strong> Nanostructure Technologies and Analytics, Technische Physik, Universität Kassel<br />


40th _______________________________________________________________<br />

"Jaszowiec" <strong>2011</strong> International School and Conference on the Physics <strong>of</strong> Semiconductors TuP17<br />

Excitonic Magnetoabsorption <strong>of</strong> Cylindrical Quantum Disks<br />

P. Schillak and G. Czajkowski<br />

Department <strong>of</strong> Theoretical Physics, University <strong>of</strong> Technology and Life Sciences<br />

Al. Pr<strong>of</strong>. S. Kaliskiego 7, 85-789 Bydgoszcz, Poland<br />

Potential applications <strong>of</strong> semiconducting nanostructures in novel optoelectronic devices<br />

gain the interest in the optical properties <strong>of</strong> quantum disks, quantum wires and the<br />

other low-dimensional objects. Here we investigate the influence <strong>of</strong> excitonic effects on<br />

the optical properties <strong>of</strong> quantum disks in the external magnetic field. We propose the<br />

theoretical approach which enables to calculate the optical functions for low-dimensional<br />

structureswithcylindricalsymmetryexposedtotheexternalmagneticfielddirectedalong<br />

the symmetry axis. We consider an interacting electron-hole pair inside the semiconducting<br />

nanostructure with hard-wall potential <strong>of</strong> finite height (different for an electron and<br />

a hole) at the boundaries. The different anisotropic effective masses are taken inside and<br />

outside the structure. The novelty <strong>of</strong> our approach is that the six-dimensional eigenvalue<br />

problem is transformed into the equivalent eigenvalue problem given by the system <strong>of</strong> the<br />

coupled two-dimensional second order differential equations. Differential equations are<br />

solved numerically. As an example, we present in Fig. 1 the magnetic field dependent<br />

excitonic energy spectrum for the InP disk with the radius R = 8 nm and the height<br />

d = 4 nm. Additionally, numerical results are compared with experimental data taken<br />

from the paper by Dewitz et al. [1]. Having the energies and wave functions, we apply<br />

the real density matrix approach [2,3]<br />

which gives the optical functions including<br />

magnetoabsorption and the<br />

magnetic field dependent dielectric<br />

tensor for light- and heavy-hole excitons.<br />

In this approach the linear<br />

response is described by the set <strong>of</strong><br />

coupledequationsfortwo-pointcorrelation<br />

function Y(re,rh) and the<br />

Maxwellianfieldequation. TheconstitutiveequationforY(re,rh)hasa<br />

form <strong>of</strong> Schrödinger’s equation with<br />

a source term, which reflects the<br />

light-matter interaction. Numerical<br />

computations were performed<br />

for In0.55Al0.45As/Al0.35Ga0.65 and<br />

CdSe quantum disks. Our numerical<br />

results are in a good agreement<br />

with experimental data.<br />

relative energy [meV]<br />

120<br />

100<br />

80<br />

60<br />

40<br />

20<br />

0<br />

0 20 40 60<br />

magnetic field B [T]<br />

Figure 1: The calculated (full lines) and measured<br />

(circles) relative positions <strong>of</strong> energy levels<br />

as functions <strong>of</strong> the applied magnetic field.<br />

[1] C. v. Dewitz, F. Hatami, M. Millot, J. M. Broto, J. Léotin, and W. T. Masselink,<br />

Appl. Phys. Letters 95, 151105 (2009).<br />

[2] G. Czajkowski, F. Bassani, and L. Silvestri, Rivista del Nuovo Cimento, 26(5-6), pp.<br />

1-150 (2003).<br />

[3] P. Schillak and G. Czajkowski, phys. stat. sol. (c), 5 2495 (2008).<br />


TuP18 _______________________________________________________________<br />

40th "Jaszowiec" <strong>2011</strong> International School and Conference on the Physics <strong>of</strong> Semiconductors<br />

Spontaneous emission from double quantum dots: collective<br />

effects and carrier-phonon kinetics<br />

Pawe̷l Karwat, Anna Sitek, Pawe̷l Machnikowski<br />

Institute <strong>of</strong> Physics, Wroc̷law University <strong>of</strong> Technology, 50-370 Wroc̷law, Poland<br />

We study theoretically the evolution <strong>of</strong> luminescence from a double quantum dot in the<br />

presence <strong>of</strong> carrier-phonon interaction. We show that the spontaneous emission from<br />

this system is determined by the interplay <strong>of</strong> collective effects, which are due to the<br />

interaction between the two emitters and their common radiative reservoir, and phononinduced<br />

transitions (typically on a much shorter time scale).<br />

The time-resolved luminescence from double quantum dots is an interesting subject <strong>of</strong><br />

study as it yields information on the carrier kinetics under the joint action <strong>of</strong> two reservoirs:<br />

electromagnetic vacuum to which the exciton ultimately decays in the spontaneous<br />

emission (radiative recombination) process and the lattice excitations which may dephase<br />

the charge states and lead to transitions between the confined states. One <strong>of</strong> the experiments<br />

[1] revealed surprising temperature dependence <strong>of</strong> the luminescence life time which<br />

first grows as the temperature is increased and then decreases. This enhancement <strong>of</strong> the<br />

exciton lifetime with increasing temperature is in striking contrast to the single dot case<br />

where the life time drops down with temperature, which can easily be associated with<br />

various possible thermally activated non-radiative channels.<br />

In this presentation, we model the system kinetics using a Markovian Lindblad generator<br />

for the spontaneous emission process [2] and a non-Markovian time-convolutionless<br />

dissipator that accounts for the phonon-related kinetics [3]. We show that the quasiequilibrium<br />

distribution <strong>of</strong> the occupations <strong>of</strong> the two lowest energy eigenstates sets up<br />

within several picoseconds after the excitation. Since one <strong>of</strong> these states is more active<br />

optically (“brighter”) this leads to temperature dependence <strong>of</strong> the emission rate and to<br />

non-monotonic dependence <strong>of</strong> the luminescence life time similar to that observed in the<br />

experiment.<br />

[1] C. Bardot, M. Schwab, M. Bayer, S. Fafard, Z. Wasilewski, P. Hawrylak, Phys. Rev.<br />

B 72, 035314 (2005).<br />

[2] A. Sitek, P. Machnikowski, Phys. Rev. B 80, 115319 (2009).<br />

[3] P. Machnikowski, K. Roszak, A. Sitek, Acta Phys. Pol. A 116, 818 (2009).<br />


40th _______________________________________________________________<br />

"Jaszowiec" <strong>2011</strong> International School and Conference on the Physics <strong>of</strong> Semiconductors TuP19<br />

Cathodoluminescence studies <strong>of</strong> the II – VI semiconducting quantum dots<br />

grown by molecular beam epitaxy<br />

P. Łach 1 , A. Reszka 1 , G. Karczewski 1 , P. Wojnar 1 , T. Wojtowicz 1 , A. Kamińska 1 ,<br />

and A. Suchocki 1,2<br />

1<br />

- Institute <strong>of</strong> Physics, Polish Academy <strong>of</strong> Sciences, al. Lotników 32/46, 02-668, Warsaw,<br />

Poland<br />

2<br />

- Institute <strong>of</strong> Physics, Kazimierz Wielki University, Weyssenh<strong>of</strong>fa 11, Bydgoszcz 85-072,<br />

Poland<br />

Self-assembled quantum dots (QDs) as zero-dimensional structures are very<br />

intensively studied optically and electrically on account <strong>of</strong> their shape and small sizes. Our<br />

QDs consisted <strong>of</strong> II-VI compounds were grown on (100)-oriented GaAs substrate by Stranski-<br />

Krastanov mode by molecular beam epitaxy method (MBE). The obtained objects have<br />

diameter and height <strong>of</strong> the order <strong>of</strong> 10 - 40 nm and 1 - 14 nm, respectively. The QDs were<br />

embedded in a ZnTe matrix with thickness <strong>of</strong> about 1.5 µm and they were composited <strong>of</strong> 6<br />

monolayers. A whole was covered by a thin cap (100 nm). The concentration <strong>of</strong> the QDs was<br />

equal to 10 9 – 10 10 <strong>of</strong> QDs per 1 cm 2 .<br />

We have measured the cathodoluminescence (CL) spectra <strong>of</strong> such structures as a<br />

function <strong>of</strong> the temperature. The luminescence was excited by electron beam with<br />

accelerating voltage <strong>of</strong> the order <strong>of</strong> 5 – 15 kV. This experiment was carried out for CdTe,<br />

CdMnTe and CdSe QDs. We have checked behaviour <strong>of</strong> the CL signals in different places on<br />

these samples. This dependence shows certain non-uniform distribution <strong>of</strong> QDs in the<br />

examined samples. With increase <strong>of</strong> the temperature the spectra became considerably wider<br />

and their intensities decreased. It results from excitons redistribution between QDs (the<br />

carriers were intercepted from a wetting layer to a bigger QD which emits a light in lower<br />

energies). The possible mechanisms <strong>of</strong> the temperature luminescence quenching are nonradiative<br />

exciton recombination due to the Auger effect and disappearance <strong>of</strong> the quantum<br />

confinement at higher temperatures. We discuss the both mechanisms in this paper.<br />

The examples <strong>of</strong> the results <strong>of</strong> the photoluminescence (PL) and cathodoluminescence (CL)<br />

measurements (on the left) and the temperature dependences for CdTe QDs (on the right) are<br />

presented below for comparison.<br />

CL intensity [au]<br />

PL intensity [au]<br />

40000<br />

30000<br />

20000<br />

10000<br />

0<br />

20K<br />

2,00 2,05 2,10 2,15 2,20 2,25 2,30<br />

50000<br />

40000<br />

30000<br />

20000<br />

10000<br />

0<br />

Energy [eV]<br />

70K<br />

60K<br />

50K<br />

40K<br />

30K<br />

1,95 2,00 2,05 2,10 2,15 2,20 2,25 2,30 2,35<br />

Energy [eV]<br />

T=60K<br />

T=57K<br />

T=53K<br />

T=48K<br />

T=43K<br />

T=34K<br />

T=28K<br />

T=20K<br />

T=11K<br />

CL intensity [au]<br />

14000<br />

12000<br />

10000<br />

8000<br />

6000<br />

4000<br />

2000<br />

10 20 30 40 50 60 70<br />

Temperature [K]<br />

10 20 30 40 50 60<br />

In this case we have measured the PL and CL spectra to about 100K. The CdMnTe and CdSe<br />

QDs have given the CL signals even over 200K.<br />

Acknowledgement: This work was partially supported by the grant <strong>of</strong> the Polish Ministry <strong>of</strong><br />

Science and Higher Education for years 2007-<strong>2011</strong>.<br />

127<br />

PL intensity [au]<br />

14000<br />

12000<br />

10000<br />

8000<br />

6000<br />

4000<br />

2000<br />

Temperature [K]

TuP20 _______________________________________________________________<br />

40th "Jaszowiec" <strong>2011</strong> International School and Conference on the Physics <strong>of</strong> Semiconductors<br />

Raman spectroscopy <strong>of</strong> CdTe/ZnTe quantum dots<br />

E. Zielony 1 , E. Popko 1 , Z. Gumienny 1 , P. Kamyczek 1 , A. Henrykowski 1 , J. Jacak 1 ,<br />

G.Karczewski 2<br />

1 Institute <strong>of</strong> Physics, Wroclaw University <strong>of</strong> Technology, Wybrzeze Wyspianskiego 27,<br />

50-370 Wroclaw, Poland<br />

2 Institute <strong>of</strong> Physics, Institute <strong>of</strong> Physics Polish Academy <strong>of</strong> Science, al. Lotnikow 32/46,<br />

02-668 Warsaw, Poland<br />

Semiconducting low-dimensional structures <strong>of</strong> CdTe quantum dots embedded in ZnTe matrix<br />

have been investigated by micro-Raman spectroscopy. A reference ZnTe sample (without<br />

dots) was also studied for comparison. Both samples were grown by molecular beam epitaxy<br />

technique on the p-type GaAs substrate. The QD sample consists <strong>of</strong> 3µm thick p + -type<br />

ZnTe:N buffer deposited on the p-type GaAs substrate, 1µm undoped ZnTe, a layer <strong>of</strong> CdTe<br />

quantum dots and 0.3µm <strong>of</strong> undoped ZnTe cap. The CdTe self assembled quantum dots were<br />

grown in the atomic layer epitaxy (ALE) growth mode by deposition <strong>of</strong> six monolayers <strong>of</strong><br />

CdTe. The process <strong>of</strong> QD formation was induced by covering the CdTe layers with an<br />

amorphous tellurium layer and its subsequent thermal desorption [1]. The presence <strong>of</strong><br />

quantum dot structure was confirmed by photoluminescence measurements. The Raman<br />

measurements have been performed at room temperature with the help <strong>of</strong> the T64000 Jobin<br />

Ivon spectrometer configured in the triple subtractive mode <strong>of</strong> operation. The samples were<br />

excited by an Ar 2+ laser working at a wavelength <strong>of</strong> 514.5 nm. The Raman spectra have been<br />

recorded for different acquisition parameters <strong>of</strong> the measurement. For the reference and QD<br />

sample localized longitudinal (LO) phonons <strong>of</strong> 210cm -1 wavenumber associated with the<br />

ZnTe layer [2] are observed. In the case <strong>of</strong> QD sample another broad band corresponding to<br />

the LO CdTe phonon related to the QD-layer appears at a wavenumber <strong>of</strong> 160 cm -1 [2]. Such<br />

behavior does not exhibit the Raman spectra for the reference sample. For both samples<br />

additionally tellurium - related peaks at wavenumbers around 120 cm -1 and 140 cm -1 are<br />

detected. It has been noticed that the intensity <strong>of</strong> CdTe and ZnTe LO phonon peaks decreases<br />

with increasing time <strong>of</strong> a laser beam exposition while the signal associated with Te - like<br />

Raman peaks increases. The latter is caused by the laser damage in the ZnTe layer followed<br />

by the formation <strong>of</strong> Te aggregates on the ZnTe surface. The Raman measurements confirm<br />

the presence <strong>of</strong> CdTe layer <strong>of</strong> quantum dots in the investigated material.<br />

[1] P. Wojnar, J. Suffczynski, K. Kowalik, A. Golnik, M. Aleszkiewicz, G. Karczewski, and<br />

J. Kossut, Nanotechnology 19, 235403 (2008).<br />

[2] V. S. Vinogradov, et al., Physics <strong>of</strong> the Solid State, Vol. 50, No. 1, 2008.<br />


40th _______________________________________________________________<br />

"Jaszowiec" <strong>2011</strong> International School and Conference on the Physics <strong>of</strong> Semiconductors TuP21<br />

Quantum interference in charge transport via the quantum dots<br />

coupled between the metallic and superconducting leads<br />

J. Barański and T. Domański<br />

Institute <strong>of</strong> Physics, M. Curie Sk̷lodowska University, 20-031 Lublin, Poland<br />

We analyze the effects <strong>of</strong> quantum interference observable in the Andreev current<br />

induced through the double quantum dots coupled between an isotropic superconductor<br />

and metallic leads. Superconducting properties are transfered on such nanostructures<br />

due to the proximity effect and they have qualitative influence on the nonequillibrium<br />

phenomena, especially in a regime <strong>of</strong> the subgap bias voltage |V | ≤ ∆/|e|. We will<br />

discuss condictions necesarry for observing the Fano resonances and shall try to account<br />

for their interplay with the strong correlation effects.<br />


TuP22 _______________________________________________________________<br />

40th "Jaszowiec" <strong>2011</strong> International School and Conference on the Physics <strong>of</strong> Semiconductors<br />

Bound Magnetic Polaron Molecule in Diluted Magnetic<br />

Semiconductors within the Heitler-London Approximation<br />

Henryk Bednarski 1 and Jozef Spa̷lek 2<br />

1 Centre <strong>of</strong> Polymer and Carbon Materials, Polish Academy <strong>of</strong> Sciences, 34 M.<br />

Curie-Sk̷lodowska Str., 41-819 Zabrze, Poland<br />

2 Marian Smoluchowski Institute <strong>of</strong> Physics, Jagiellonian University, Reymonta 4,<br />

30-059 Kraków, Poland<br />

Recently developed by us a complete microscopic theory <strong>of</strong> the bound magnetic polaron<br />

molecule (BMPM) in diluted magnetic semiconductors [1] is reformulated in a reduced<br />

spin-state space. Namely, we solve the BMPM problem by taking into account the<br />

Heitler-London form <strong>of</strong> the two-electron wave function in the four dimensional spin-state<br />

space (comprising one singlet and three triplet states) and include the thermodynamic<br />

fluctuations <strong>of</strong> the magnetization due to the localized 3d spins in the Gaussian form.<br />

In this manner, the role <strong>of</strong> covalency effects can be clearly identified and its influence<br />

analysed in detail. Moreover, within this formulation the quantum aspect <strong>of</strong> the BMPM<br />

problem is also accounted for, as it results from quantum-mechanical indistinguishability<br />

<strong>of</strong> the two impurity electrons and is contained in the antisymmetric nature <strong>of</strong> their<br />

two-electron wave function. Therefore, the formulation <strong>of</strong> the BMPM theory provides us<br />

with the opportunity <strong>of</strong> studying the physical origin <strong>of</strong> the competition between ferroand<br />

antiferro-magnetic interactions <strong>of</strong> the impurity pair <strong>of</strong> electrons in diluted magnetic<br />

semiconductors.<br />

[1] H. Bednarski, J. Spa̷lek, arXiv:0912.0662v4, submitted to Phys. Rev. B.<br />


40th _______________________________________________________________<br />

"Jaszowiec" <strong>2011</strong> International School and Conference on the Physics <strong>of</strong> Semiconductors TuP23<br />

Intersubband polaritons in strongly pumped microcavities<br />

M. Zału ny and A. Kozłowski<br />

Institute <strong>of</strong> Physics, M. Curie-Skłodowska University, Pl. M. Curie-Skłodowskiej 1, 20-031<br />

Lublin, Poland<br />

It is well known that resonant microstructures have an important role in the<br />

enhancement <strong>of</strong> nonlinear optical processes in multiple quantum well (MQWs) structures. So<br />

far most research has concentrated on the nonlinear effects connected with excitonic coupling<br />

to the microcavity (MC) mode [1]. In our recent paper [2] we have discussed theoretically the<br />

nonlinear intersubband response <strong>of</strong> the MQWs embedded in MCs. The simplest case, when<br />

strong probe wave acts alone, has been considered. It has been shown, that in the strongcoupling-regime<br />

the saturation effect leads to the evolution <strong>of</strong> the absorption spectra from a<br />

pair <strong>of</strong> simple harmonic oscillators to highly anharmonic ones.<br />

The experimental and theoretical results reported in [1] indicate that in MCs the pumpprobe<br />

nonlinear excitonic effects [leading to the formation <strong>of</strong> the so-called Mollow absorption<br />

spectrum (MAS) [3]] are much more easily observed due to the enhancement <strong>of</strong> the optical<br />

field by the MC. Stimulated by this fact in this paper we discuss theoretically the problem <strong>of</strong><br />

the formation <strong>of</strong> the intersubband MAS in the MQW-MC systems.<br />

Like in our previous paper, we employ a semiclassical approach based on the transfer<br />

matrix formalism (TMF) and the so-called sheet model [2,4] (The sheet, modeling the optical<br />

response <strong>of</strong> the QW, is characterized by the 2D conductivity.) For simplicity we neglect the<br />

influence <strong>of</strong> the electron-electron interaction on the nonlinear intersubband response. It has a<br />

good justification when the surface electron concentration in QW is relatively small (�10¹¹<br />

cm²) and the wavelength <strong>of</strong> incident radiation is not too large ( �10 m) [5]. To achieve a<br />

sufficiently strong coupling between intersubband excitation and incident radiation the<br />

experimentally studied systems usually contain a large number <strong>of</strong> QWs. Practically, the QWs<br />

occupy almost the whole space between mirrors. Unfortunately, in such type <strong>of</strong> systems,<br />

contrary to those studied in [1], the strength <strong>of</strong> the optical field acting on each QW is<br />

different. The numerical simulations reported in this paper are performed taking into account<br />

also the above mentioned effect.<br />

The calculations <strong>of</strong> the angle resolved (probe) absorption spectra are decomposed into<br />

two sequential stages. First, the pump wave propagation in the system is described neglecting<br />

the energy transfer between the (strong) pump and (weak) probe waves. The spatial pump<br />

field distribution in the MC-MQW system is calculated taking into account the saturation<br />

effect. The case when the response to pump beam has bistable character [2] is also considered.<br />

At second stage, the linear problem <strong>of</strong> the probe beam propagation in the system is solved<br />

taking into account fact the that the Mollow 2D (intersubband) conductivity <strong>of</strong> each QW [3,6]<br />

depends on the pump field intensity, at the position <strong>of</strong> the QW, determined in the first stage.<br />

For better understanding <strong>of</strong> the role <strong>of</strong> the spatial variation <strong>of</strong> the pump field in the MC we<br />

also present a simplified approach based on a Fabry-Perot model and the “mean field”<br />

approximation [2].<br />

[1] B. Beveaurd, et al. CR Acad. Sci. IV-Phys., 2, 1439 (2001) and references therein.<br />

[2] M. Zału ny and C. Nalewajko, J. Appl. Phys. 107, 123106 (2010).<br />

[3] B. R. Mollow, Phys. Rev. 188,1969 (1969); Phys. Rev. A 5, 227 (1972).<br />

[4] M. Zału ny and C. Nalewajko, J. Appl. Phys. 99, 026104 (2006.<br />

[5] R. A. Kaindl, et al., Phys. Rev. B 62, 161308(R) (2001).<br />

[6] P. Meystre and M. Sargent III, Elements <strong>of</strong> quantum optics (Springer, Berlin, 1990).<br />


TuP24 _______________________________________________________________<br />

40th "Jaszowiec" <strong>2011</strong> International School and Conference on the Physics <strong>of</strong> Semiconductors<br />

Magneto-Optical Studies <strong>of</strong> Narrow Band-Gap Heterostructures with Type<br />

II Quantum Dots InSb in an InAs Matrix<br />

Mikhail S. Mukhin 1 , Yakov V. Terent’ev 1 , Leonid E. Golub 1 , Mikhail O.Nestoklon 1 ,<br />

Boris Ya. Meltser 1 , Alexey N. Semenov 1 , Victor A. Solov’ev 1 , Alla A. Sitnikova 1 , Alexey A.<br />

Toropov 1 and Sergey V. Ivanov 1<br />

1 I<strong>of</strong>fe Physical-Technical Institute <strong>of</strong> Russian Academy <strong>of</strong> Sciences, 26 Polytekhnicheskaya,<br />

St Petersburg 194021, Russian Federation<br />

Spin is the only electron internal degree <strong>of</strong> freedom, and utilizing it in the new<br />

generation <strong>of</strong> semiconductor devices is the main goal <strong>of</strong> semiconductor spintronics. Today<br />

spintronics focuses mainly on diluted magnetic semiconductors (DMS) where ferromagnetism<br />

and giant Zeeman splitting can be obtained due to exchange interaction between free carriers<br />

and Mn ions. Most <strong>of</strong> the work has been focused on II-VI DMS such as CdMnTe or ZnMnSe<br />

and some others. Enhanced magnetic properties <strong>of</strong> these materials exist only at cryogenic<br />

temperatures due to paramagnetic behavior <strong>of</strong> the magnetic ions. A lot <strong>of</strong> recent investigations<br />

have been focused on doping III-V semiconductors into a DMS state. Although III-V DMS<br />

demonstrate higher Curie temperature, Mn is much less soluble here than in II-VI<br />

semiconductors [1]. Another issue is that in III-V compounds magnetic doping harmfully<br />

affects emission properties and, moreover, changes conductivity to p-type. We have proposed<br />

a new approach to the problem based on using narrow band-gap III-V compounds possessing<br />

the largest intrinsic electronic g-factor, which are InSb and InAs [2].<br />

Present research is focused on spin-related phenomena in low-dimensional InSb/InAs<br />

heterostructures and continues our preliminary studies [3]. Magneto-optical investigations <strong>of</strong><br />

type-II InSb QDs in an InAs matrix and their theoretical consideration are reported. In(Sb,As)<br />

heterostructures incorporating monolayer-scale InSb insertions in the InAs matrix, possessing<br />

high recombination efficiency, were fabricated by MBE. Circularly polarized<br />

photoluminescence has been measured in the<br />

polarization, %<br />

100<br />

80<br />

60<br />

40<br />

20<br />

0<br />

-20<br />

T=2K<br />

B=4T<br />

0,1 1 10<br />

Excitation Intensity, W/cm 2<br />

Faraday geometry, allowing direct probing <strong>of</strong> the<br />

electron spin state. Experiments were carried out in<br />

the temperature range from 2 to 140 K and<br />

excitation density 0.1 – 20 W/cm 2 . It was found that<br />

the polarization degree varies form 100% -minus<br />

up to 15% -plus, depending on the excitation<br />

intensity and temperature. The physical model <strong>of</strong> the<br />

phenomenon is developed, based on the selection<br />

rules for optical transitions involving heavy holes,<br />

dependence <strong>of</strong> the oscillator strength on the excitation density, inherent for indirect in space<br />

optical transitions that only are allowed here, and an analysis <strong>of</strong> equilibrium population <strong>of</strong><br />

energy levels. The detailed calculation <strong>of</strong> the energy spectrum <strong>of</strong> charge carries, performed<br />

within a tight-binding approximation, is also presented. The results <strong>of</strong> the theoretical<br />

treatment are in good agreement with experimental data. The simulation <strong>of</strong> the experimental<br />

data revealed that the oscillator strength <strong>of</strong> the optical transitions <strong>of</strong> electrons with the spin<br />

oriented either along or against the magnetic field vector differs by approximately 1,8 times.<br />

[1] T.T. Chen, C.H. Chen, W.S. Su et al., J. Appl. Phys. 93, 9655 (2002).<br />

[2] Ya.V. Terent’ev, A.A. Toropov, B.Y. Meltser et al., Semiconductors 44, 194 (2010).<br />

[3]Ya.V. Terent’ev, O.G. Lyublinskaya, A.A. Toropov et al., Semiconductors 43, 635 (2009).<br />


40th _______________________________________________________________<br />

"Jaszowiec" <strong>2011</strong> International School and Conference on the Physics <strong>of</strong> Semiconductors TuP25<br />

Phonon-assisted tunneling between hole states in double<br />

quantum dots<br />

Krzyszt<strong>of</strong> Gawarecki, Pawe̷l Machnikowski<br />

Institute <strong>of</strong> Physics, Wroc̷law University <strong>of</strong> Technology, 50-370 Wroc̷law, Poland<br />

In this work, we present a theoretical analysis <strong>of</strong> hole states in double quantum dots<br />

(DQDs). The system under consideration, consist two, coupled, vertically stacked dots<br />

with one trapped hole. We study hole states in a realistic model which takes into account<br />

the system geometry and strain in the DQD. In present work, we also investigate<br />

phonon-related processes in such a system. We derive phonon-assisted relaxation between<br />

two lowest hole states and compare those results with the previous one for electrons [1,2].<br />

The relaxation rates are calculated as a function <strong>of</strong> external axial electric field. Carrierphonon<br />

couplings via both deformation potential and piezoelectric interactions can be<br />

predominant for different parameters. As shown previously [3] the ground state in such<br />

molecule can be antibonding [3], which corresponds to recent experiments[4]. In consequence,<br />

for some distance between dots (relevant to ground state bonding–antibonding<br />

transition) relaxation rate can be very small.<br />

DQDs, can be used for designing quantum-coherent devices, including spin-based<br />

quantum bits. Information can be encoded on carrier states in such structures. However,<br />

phonon-related processes are inevitable in a cristal environment and may limit the<br />

feasibility <strong>of</strong> implementing quantum control in these systems. In consequence, it is essential<br />

to understand not only the spectral properties <strong>of</strong> various kinds <strong>of</strong> semiconductor<br />

nanostructures but also the relevant phonon-induced processes.<br />

Our model [1] is based on a k·p approximation for a single particle in a strained selfassembled<br />

structure. The local band structure in a such a system is obtained from the<br />

8-band Hamiltonian with strain-induced terms (Bir-Pikus Hamiltonian).We solve the 4x4<br />

part (light- and heavy holes with two possible spin orientation) <strong>of</strong> this hamiltonian. Interation<br />

with the conduction band, was included via quasi-degenerate perturbation theory.<br />

Next, we calculate the the valence band edges and the effective mass for heavy and light<br />

holes as a function <strong>of</strong> the spatial position. The hole wave functions are calculated within<br />

a multi-component envelope function approximation combined with a Ritz variational<br />

method. Finally, the Fermi golden rule is used to obtain the relaxation rates between the<br />

lowest energy eigenstates.<br />

[1] K. Gawarecki, M. Pochwa̷la, A. Grodecka-Grad and P. Machnikowski,<br />

Phys. Rev. B 81, 245312 (2010).<br />

[2] K. Gawarecki, P. Machnikowski, Acta Phys. Pol. A (in press), arxiv:1007.1577 (2010).<br />

[3] J.I. Climente et al., Phys. Rev. B 78, 115323 (2008).<br />

[4] M.F. Doty et al., Phys. Rev. Lett. 102, 047401 (2009).<br />


TuP26 _______________________________________________________________<br />

40th "Jaszowiec" <strong>2011</strong> International School and Conference on the Physics <strong>of</strong> Semiconductors<br />

Intermediate band formation for electrons and holes in an<br />

inhomogeneous chain <strong>of</strong> quantum dots<br />

Igor Bragar, Krzyszt<strong>of</strong> Gawarecki, Pawe̷l Machnikowski<br />

Institute <strong>of</strong> Physics, Wroc̷law University <strong>of</strong> Technology, 50-370 Wroc̷law, Poland<br />

We study the electron and hole states <strong>of</strong> a chain <strong>of</strong> non-identical, vertically stacked quantum<br />

dots (QDs). It has been proposed [1] that inserting such a structure in the intrinsic<br />

region <strong>of</strong> a p-i-n junction solar cell could increase the photocurrent due to sequential<br />

absorption <strong>of</strong> low energy (below-bandgap) photons. While this idea has been gaining<br />

growing experimental support in the recent years [2], it still seems unclear whether the<br />

underlying physics corresponds to the intermediate band concept [3] as the band formation<br />

in a quantum dot chain may be strongly suppressed by the energy inhomogeneity.<br />

In this presentation we use a simple model <strong>of</strong> a tunnel-coupled chain <strong>of</strong> dots containing<br />

one electron or one hole. By using the energy and coupling parameters obtained from<br />

realistic k.p calculations [4], we are able to relate the spectral properties <strong>of</strong> the chain as<br />

well as the localization properties <strong>of</strong> the carrier wave functions to the actual geometry <strong>of</strong><br />

the system (dot sizes and inter-dot separations). We study how the pseudo-band formed<br />

<strong>of</strong> the ground states confined in the QDs disintegrates upon increasing the inhomogeneity<br />

<strong>of</strong> the electron and hole energies. We discuss the impact <strong>of</strong> localization on the interband<br />

absorption from the pseudo-band to extended (bulk) states, which is a prerequisite for the<br />

functionality <strong>of</strong> the photovoltaic device. We formulate the conditions for the strength <strong>of</strong><br />

the coupling (hence the dot separation) as compared to the degree <strong>of</strong> the inhomogeneity<br />

that must be satisfied in order for the system to support delocalized states.<br />

[1] V. Aroutiounian, S. Petrosyan, A. Khachatryan, K. Touryan, J. Appl. Phys. 89, 2268<br />

(2001).<br />

[2] Y. Okada, T. Morioka, K. Yoshida, R. Oshima, Y. Shoji, T. Inoue, T. Kita, J. Appl.<br />

Phys. 109, 024301 (<strong>2011</strong>).<br />

[3] A. Luque and A. Martí, Phys. Rev. Lett. 78, 5014 (1997).<br />

[4] K. Gawarecki, M. Pochwa̷la, A. Grodecka-Grad, P. Machnikowski, Phys. Rev. B 81,<br />

245312 (2010).<br />


40th _______________________________________________________________<br />

"Jaszowiec" <strong>2011</strong> International School and Conference on the Physics <strong>of</strong> Semiconductors TuP27<br />

Modelling <strong>of</strong> 1D-2D structural transitions in small Yukawa<br />

clusters<br />

Olga Rancova, Egidijus Anisimovas<br />

Department <strong>of</strong> Theoretical Physics, Vilnius University, Saul˙etekio al. 9, LT-10222<br />

Vilnius, Lithuania<br />

We study structural transitions that take place in the simplest finite-size systems and<br />

are analogous to phase transitions in the macroscopic limit [1]. The model system consists<br />

<strong>of</strong> a small number <strong>of</strong> identical interacting charged particles in an anisotropic trap, where<br />

the particles form stable ordered structures known as Wigner crystals [2]. These are<br />

observed in diverse systems as electrons on liquid He, ions in traps, 2D semiconductor<br />

quantum dots, dusty plasmas, macroscopic model systems [3]. Recent developments <strong>of</strong><br />

experimental techniques allowed direct investigation <strong>of</strong> structural and dynamic properties<br />

<strong>of</strong> systems consisting <strong>of</strong> just a few or several tens <strong>of</strong> particles under different external<br />

conditions that can be modelled numerically.<br />

In present research we model 1D-2D dimensional structural transition caused by decreasing<br />

anisotropy <strong>of</strong> the confining potential that is one <strong>of</strong> the simplest cases <strong>of</strong> continuous<br />

phase transitions. Such dimensional transitions called ”zigzag transitions” are studied<br />

experimentally and modelled numerically [4, 5].<br />

The model system consists <strong>of</strong> N identical charged particles interacting through pairwiseYukawapotentialVin<br />

= r −1 exp(−κr)withscreeningparameterκandconfinedby2D<br />

asymmetric parabolic potential Vc = x 2 +α 2 y 2 . The potential interaction energy strongly<br />

dominates over the kinetic energy. Thus, we are concerned with the minimization <strong>of</strong> the<br />

total potential energy. α > 1 squeezes the confining potential inducing structural transitions<strong>of</strong>thesystem.<br />

ThenumericaltechniquecombiningtheMetropolisprocedureandthe<br />

Newtonoptimizationmethodisusedtoobtaintheenergeticallyfavourableconfigurations.<br />

The calculations are performed for different κ changing the value <strong>of</strong> α.<br />

In present research we investigate the simplest systems up to N = 9 undergoing<br />

1D-2D structural transition. Changing the potential parameter α gradually we explore<br />

dependence <strong>of</strong> the order parameter <strong>of</strong> the system 〈y〉, defined as average system width<br />

in the anisotropy dimension, on the potential anisotropy parameter looking for evidences<br />

<strong>of</strong> the phase transition. When α reaches a critical value αc the system order parameter<br />

drops to zero and we obtain a power-law behaviour <strong>of</strong> the order parameter 〈y〉 ∝ (αc−α) γ<br />

in the vicinity <strong>of</strong> this critical point (γ is a critical exponent). We find that in this area<br />

γ ≈ 0.5 in all cases investigated. Whereas [5] suggests γ = 0.387 in N = 5,κ = 3 case.<br />

We obtain that the value <strong>of</strong> the critical exponent changes when the potential anisotropy<br />

parameter value departs from the critical point.<br />

Acknowledgments O. Rancova is funded by Lithuanian Science Council (grant No.<br />

SF-PD-2009-08-17-0054).<br />

[1] O. Rancova, E. Anisimovas, and T. Varanavičius, Phys. Rev. E 83, 036409 (<strong>2011</strong>).<br />

[2] E. Wigner, Phys. Rev. 46, 1002 (1934); C. C. Grimes and G. Adams, Phys. Rev.<br />

Lett. 42, 795 (1979); E. Y.Andrei, G. Deville, D. C. Glattli, F. I. B. Williams, E.Paris,<br />

and B. Etienne, Phys. Rev. Lett. 60, 2765 (1988).<br />

[3] A. V. Filinov et.al., Phys. Rev. Lett. 86, 3851 (2001); M. Saint Jean et.al., Europhys.<br />

Lett. 55, 45 (2001); D. Block et.al., Phys. Plasmas 15, 040701 (2008).<br />

[4] A. Melzer, Phys. Rev. E 73, 056404 (2006).<br />

[5] T. E. Sheridan and K. D. Wells, Phys. Rev. E 81, 016404 (2010).<br />


TuP28 _______________________________________________________________<br />

40th "Jaszowiec" <strong>2011</strong> International School and Conference on the Physics <strong>of</strong> Semiconductors<br />

EfficientCoulombmixingbetweensingle-and<br />

biexcitonstatesinInAsnanocrystals<br />

P.Kowalski,P.Machnikowski<br />

Institute<strong>of</strong>Physics,WrocławUniversity<strong>of</strong>Technology,50-370Wrocław,Poland<br />

Multipleexcitongeneration(impactionization)innanocrystalsisaprocess,inwhich<br />

absorption<strong>of</strong>asinglephotonleadstogeneration<strong>of</strong>multipleelectron-holepairs. This<br />

effecthasbeenobservedexperimentally[1,2]andmayincreasetheefficiency<strong>of</strong>solarcells.<br />

Whileearlierpapersreportedquantumefficienciesreaching700%[3],laterreportssuggestthatthosefigureswereoverestimatedandtheactualyielddoesnotexceed170%[4].<br />

Thiscontroversymightberesolvediftheprocessisbetterunderstoodtheoretically.<br />

Inourpreviouswork[5]wehaveshownthatcoherenttwo-pairgenerationwithasingle<br />

absorbedphotonispossibleduetoCoulombindirectmixing<strong>of</strong>single-exciton(onepair)<br />

andbiexciton(two-pair)configurations.However,forlowenergystatesthisprocessisin<br />

generalveryinefficientsincethetypicalmagnitude<strong>of</strong>theCoulombmatrixelement(tens<br />

<strong>of</strong>meV)ismuchsmallerthantheaverageseparationbetweentheenergylevels(hundreds<br />

<strong>of</strong>meV).<br />

Forhighenergystatesthespectrum<strong>of</strong>two-pairconfigurationsismuchmoredense.In<br />

thatrangetypicalseparation<strong>of</strong>statescoupledtoasinglepairisontheorder<strong>of</strong>100meV<br />

(forthenanocrystalradius<strong>of</strong>7nm).Thestrength<strong>of</strong>thecouplingstillreaches100meV.<br />

Hence,weexpectstrongmixingbetweenthe1-pairand2-pairconfigurationswhichcan<br />

leadtoamoreefficientbiexcitongeneration.<br />

[1]R.D.Schaller,V.I.Klimov,Phys.Rev.Lett.92,186601(2004).<br />

[2]R.D.Schaller,J.M.Pietryga,V.I.Klimov,NanoLett.7,3469(2007).<br />

[3]R.D.Schaller,M.Sykora,J.M.Pietryga,V.I.Klimov,NanoLett.6,424(2006).<br />

[4]M.TuanhTrinhet.al.,NanoLett.8,1713(2008).<br />

[5]P.Kowalski,P.Machnikowski,ActaPhys.Pol.114,1187(2008).<br />


40th _______________________________________________________________<br />

"Jaszowiec" <strong>2011</strong> International School and Conference on the Physics <strong>of</strong> Semiconductors TuP29<br />

Strongly disordered wetting layer in the InAs/GaAs self-assembled<br />

quantum dots system<br />

K. Gołasa 1 , M. Molas 1 , J. Borysiuk 1 , Z. R. Wasilewski 2 and A. Babiński 1<br />

1 Faculty <strong>of</strong> Physics, University <strong>of</strong> Warsaw, Hoża 69, 00-681 Warszawa, Poland<br />

2 Institute for Microstructural Sciences, National Research Council, Ottawa, Canada<br />

A wetting layer (WL) is an inevitable result <strong>of</strong> the formation <strong>of</strong> quantum dots (QDs) in<br />

the Stranski–Krastanow growth mode. It is usually assumed that the WL is a thin, highly<br />

strained quantum well, on which three-dimensional islands grow. However, in real systems<br />

the morphology <strong>of</strong> the WL can be more complex. In particular substantial indium fluctuations<br />

in the WL, which give rise to confinement <strong>of</strong> carriers have recently been identified [1].<br />

In this communication we address the question <strong>of</strong> growth conditions necessary to form<br />

such strongly disordered WL. We investigate two samples A and B grown with different<br />

parameters using molecular beam epitaxy. While growing both samples, a substrate rotation<br />

was stopped during the InAs layer deposition in order to obtain a gradient <strong>of</strong> InAs coverage<br />

across the wafer. The amount <strong>of</strong> deposited InAs was chosen in such a way as to have a<br />

continuous variation <strong>of</strong> the dot density, starting with just InAs WL to fairly high density <strong>of</strong><br />

InAs QDs on each wafer. In the reference sample A, grown in usual conditions, such the<br />

procedure resulted in an expected evolution <strong>of</strong> the low-temperature photoluminescence across<br />

the wafer. Low InAs coverage resulted in the WL-related emission centred at 1.44 eV, which<br />

disappeared from the spectrum with increasing intensity <strong>of</strong> the QDs emission due to<br />

increasing density <strong>of</strong> QDs grown. In the<br />

sample B the scenario was different. The<br />

lowest InAs coverage resulted in QDs<br />

emission but no emission from the WL (see<br />

lowest spectrum in Fig. 1). At the expected<br />

energy range <strong>of</strong> the WL-emission several<br />

sharp lines due to recombination <strong>of</strong> confined<br />

single excitons and biexcitons were<br />

observed. At higher InAs coverage a weak<br />

emission related to a thin WL (denoted with<br />

an arrow in Fig. 1) was observed, which<br />

disappeared at even higher InAs coverage<br />

leaving just the QDs emission in the<br />

1,25 1,30 1,35 1,40 1,45<br />

spectrum. The observed behaviour suggests that the WL does not form a continuous twodimensional<br />

layer before the onset <strong>of</strong> the QDs formation in the sample B. Instead, twodimensional<br />

platelets form, which can confine electrons and holes. With higher InAs coverage<br />

the platelets merge giving rise to usually observed WL. Our conclusions are supported with<br />

results <strong>of</strong> transmission electron microscopic study <strong>of</strong> the samples morphology.<br />

We present details <strong>of</strong> the growth procedure and discus their effect on the morphology<br />

<strong>of</strong> the investigated samples. We also discuss possible applications <strong>of</strong> self-assembled QDs<br />

grown with no continuous WL.<br />

[1] A. Babiński, J. Borysiuk, S. Kret, M. Czyż, A. Golnik, S. Raymond and Z. R. Wasilewski,<br />

Appl. Phys. Lett. 92, 171104 (2008).<br />

137<br />

PL intensity [arb.u.]<br />

Energy [eV]<br />

WL<br />

Fig.1. The μ-PL spectra from three positions <strong>of</strong><br />

the investigated sample.<br />

InAs nominal coverage

TuP30 _______________________________________________________________<br />

40th "Jaszowiec" <strong>2011</strong> International School and Conference on the Physics <strong>of</strong> Semiconductors<br />

Optical transformation <strong>of</strong> zero-dimensional confinement in the<br />

CdTe/CdMgTe multiple quantum wells<br />

M. Molas 1,* , K. Gołasa 1 , J. Łusakowski 1 , T.Wojtowicz 2 , and A. Babi ski 1<br />

1 Faculty <strong>of</strong> Physics, University <strong>of</strong> Warsaw, Ho a 69, 00-681 Warszawa, Poland<br />

2 Institute <strong>of</strong> Physics, Polish Academy <strong>of</strong> Sciences, Al. Lotników 32/46, 02-668 Warszawa, Poland<br />

Zero-dimensional confinement in semiconductor structures, which is crucial for many optoelectronic<br />

applications, may result from several factors influencing potential energy <strong>of</strong> carriers. Most intensely<br />

investigated are quantum dots (QDs) formed in Stranski-Krastanov growth mode [1,2] or due to composition<br />

fluctuations in thin quantum wells (QWs) [3]. There are, however, systems in which the confinement results<br />

from a more complicated potential energy pattern [4,5]. In this communication we report on QDs which in our<br />

opinion fall to the latter category as they can be transformed by optical illumination.<br />

An active part <strong>of</strong> the investigated structure consisted <strong>of</strong> 20 CdTe QWs, each <strong>of</strong> a 16 nm width,<br />

separated by a 48 nm-wide Cd0.8Mg0.2Te barriers. Both the QWs and the barriers were uniformly doped with 2 ×<br />

10 16 cm -3 iodine atoms acting as donors. Broad optical emission band related to the QWs was observed around<br />

1.6 eV. Moreover, a few discrete emission lines were observed at a lower energy. A more detailed analysis <strong>of</strong><br />

these lines, based on excitation power dependence <strong>of</strong> their intensity and polarization-sensitive measurements <strong>of</strong><br />

their optical anisotropy, allowed for their attribution to 0D-confined neutral excitons and biexcitons.<br />

A characteristic feature <strong>of</strong> the observed single-dot spectra was their instability against a high excitation powerdensity.<br />

Series <strong>of</strong> spectra in excited with a relatively low (I = 0.1 W) excitation power-density separated by 2<br />

min exposure to a high power light (I = 350 W) are shown in Fig. 1. It can be appreciated that the spectra<br />

change as a result <strong>of</strong> a high intensity illumination (see Fig.1 a - c) and a new lineshape, stable against subsequent<br />

high-density illumination was obtained (see Fig. 1 c - d).<br />

An analysis <strong>of</strong> the observed metastable behaviour <strong>of</strong> the spectra led us to conclusion that the<br />

confinement <strong>of</strong> carriers, responsible for the spectra, results partially from intrinsic electric fields due to ionized<br />

Luminescence (a.u.)<br />

d)<br />

c)<br />

b)<br />

a)<br />

1.535 1.540 1.545 1.550<br />

Energy (eV)<br />

1.555 1.560 1.565<br />

Figure 1. The -PL spectra from one position <strong>of</strong> the<br />

investigated sample<br />

donors and partially due to local potential<br />

fluctuations inherent to the QW growth process.<br />

Existence <strong>of</strong> such mixed QDs (MQDs) have<br />

previously been proposed on the basis <strong>of</strong> farinfrared<br />

spectroscopic measurements <strong>of</strong> the<br />

investigated structure [4, 5]. A large (from 20<br />

eV to nearly 120 eV) dispersion <strong>of</strong> the optical<br />

anisotropy <strong>of</strong> excitons confined in the MQDs<br />

suggests a substantial asymmetry <strong>of</strong> the<br />

respective lateral potential. The observed<br />

transform-ation <strong>of</strong> MQDs is proposed to be<br />

related to ionization <strong>of</strong> donors which changes<br />

the potential fluctuations pattern. A longer<br />

exposure to a high intensity light leads to<br />

formation <strong>of</strong> a different potential fluctuations<br />

pattern which is reflected by a different<br />

luminescence lineshape. Discussion <strong>of</strong> obtained<br />

results in terms <strong>of</strong> potential fluctuations in doped QW is presented.<br />

Partial financial support by Polish Ministry <strong>of</strong> Higher Education contract N N515071937<br />

is acknowledged.<br />

[1] M. Bayer, et al, Phys. Rev. B 65, 195315 (2002).<br />

[2] G. Karczewski, et al, Appl. Phys. Lett. 74, 3011 (1999).<br />

[3] L. Besombes, et al, phys. stat. sol. (a) 178, 197 (2000).<br />

[4] K. Karpierz, et al, Acta Phys. Polonica A 112, 237 (2007).<br />

[5] K. Nogajewski, et al, Acta Phys. Pol. A 114, 1259 (2008).<br />

* corresponding author : maciej.molas@fuw.edu.pl<br />


40th _______________________________________________________________<br />

"Jaszowiec" <strong>2011</strong> International School and Conference on the Physics <strong>of</strong> Semiconductors TuP31<br />

OpticalProperties<strong>of</strong>InGaAsQuantumDotson(100)GaAs<br />

SubstratesFormedbyDropletEpitaxy<br />

MichałKozub 1 ,AnnaMusiał 1 ,GrzegorzSęk 1 ,JanMisiewicz 1 ,<br />

VerenaZürbig 2 andJohannPeterReithmaier 2<br />

1 Institute<strong>of</strong>Physics,WrocławUniversity<strong>of</strong>Technology,50-370Wrocław,Poland<br />

2 Institute<strong>of</strong>NanostructureTechnologiesandAnalytics,TechnischePhysik,Universität<br />

Kassel,Heinrich-Plett-Str.40,34132Kassel,Germany<br />

Thepresence<strong>of</strong>awettinglayer(WL)inalasingsemiconductormicrostructureintroducesanaditionalandunwantedchannelforcarrierlossandnonradiativerelaxation,<br />

significantlyreducingtheefficiency<strong>of</strong>thedevice. However,inacommonIII-Vsystem<br />

grownbyself-assemblytheexistence<strong>of</strong>thewettinglayerisanintrinsicproperty<strong>of</strong>the<br />

growthmodeandisunlikelytobeavoided.Because<strong>of</strong>theWLsdeterioratinginfluence,<br />

techniques<strong>of</strong>growth<strong>of</strong>lowdimensionalstructuresaimingtoreducetheWLorevento<br />

allowthegrowth<strong>of</strong>nanostructureswithouttheWLatallarebeingsought.<br />

Weattemptasystematicstudy<strong>of</strong>theinfluence<strong>of</strong>temperatureandflowrateonthe<br />

formation<strong>of</strong>awettinglayerinanInGaAs/GaAsquantumdot(QD)systemgrownby<br />

the’dropletepitaxy’method[1,2],andfurtherhowthegrowthconditionchangesaffect<br />

theopticalproperties<strong>of</strong>theentirestructure.SeveralQDsamplesgrownattemperatures<br />

between 410and 500 o Candgrowthrates<strong>of</strong>80-110nm/hhavebeeninvestigatedby<br />

photoluminescence(PL)andcontactlesselectroreflectance(CER).Incase<strong>of</strong>thelatter,<br />

duetoenhancedsensitivity<strong>of</strong>themodulationtechniqueitwaspossiblet<strong>of</strong>ollownot<br />

onlytheQDs,butalsotheWLsopticaltransitionsalongthevariatinggrowthcondition<br />

parameters.Ourfindingssuggestthatwithdecreasingthesubstratetemperature,surface<br />

diffusioncanbeefficientlyreducedthusenablingustohindertheassembly<strong>of</strong>thewetting<br />

layer. TheWL-relatedopticaltransitionsarewellresolvedathighertemperatures(i.e.<br />

500 o C),furtherfollowedbyacorrespondingCERsignalintensitydropandbroadening,<br />

t<strong>of</strong>inallydisappearfortemperaturesaslowas 410 o C.Weassumethatthedecrement<strong>of</strong><br />

growthtemperature(and/orgrowthrate)renderstheWLdiscontinuousandcausesitto<br />

coverasmallfraction<strong>of</strong>thesurfaceonlyforthelowesttemperatures(growthrates).<br />

Asboththechangesinthegrowthconditionsanddisappearance<strong>of</strong>theWLareexpectedtoaffecttheopticalproperties<strong>of</strong>quantumdots,theQDopticaltransitionshave<br />

alsobeeninvestigatedvsthetechnologicalparameters. First<strong>of</strong>all,thecombination<strong>of</strong><br />

PLandCERallowedustodeterminethetransitionenergiesrelatedtoQDs.Then,we<br />

derivetherespectiveactivationenergiesfromPLthermalquenchingmeasurementsina<br />

temperaturerangefrom5to300Kinordertodetectareduction<strong>of</strong>thermally-induced<br />

losseswithdiminishingWL.Eventually,thechangesinthesurfacedensity<strong>of</strong>thedots<br />

havebeenstudiedbymeans<strong>of</strong>PLwiththespatialresolution<strong>of</strong>about1micrometer,<br />

whichallowedthedisclosure<strong>of</strong>thesubstructure<strong>of</strong>emissionpeaksfromtheensemble<br />

relatedtosinglequantumdotemissionlinesoreventhesplit<strong>of</strong>spectraintosharplines<br />

relatedtosingledots.<br />

[1]N.Koguchi,S.Takahashi,T.Chikyow,J.CrystalGrowth111,688(1991).<br />

[2]V.Zrbig,A.Gushterov,L.Lingys,M.Benyoucef,J.P.Reithmaier,Int.MBEConf.,<br />

Berlin,Germany(August,2010).<br />


TuP32 _______________________________________________________________<br />

40th "Jaszowiec" <strong>2011</strong> International School and Conference on the Physics <strong>of</strong> Semiconductors<br />

Magnetooptical study <strong>of</strong> excitons confined in potential fluctuations in<br />

the CdTe/CdMgTe quantum well<br />

M. Molas 1,* , K. Gołasa 1 , T. Kazimierczuk 1 , J.Łusakowski 1 , T.Wojtowicz 2 ,<br />

and A. Babi ski 1<br />

1 Faculty <strong>of</strong> Physics, University <strong>of</strong> Warsaw, ul. Ho a 69, 00-681 Warszawa, Poland<br />

2 Institute <strong>of</strong> Physics, PAS, Al. Lotników 32/46, 02-668 Warszawa, Poland<br />

The low dimensional systems based on quantum dots (QDs) are intensely investigated<br />

for their applications in efficient light sources and detectors tuned in a wide range <strong>of</strong><br />

wavelengths from near infrared to ultraviolet. A three-dimensional confinement <strong>of</strong> excitons<br />

can be achieved in self-assembled QDs [1], on fluctuations at interfaces <strong>of</strong> thin quantum<br />

wells (QWs) or in disordered thin QWs [2, 3]. However, it was recently shown that also in<br />

wider single or multiple doped QWs local potential fluctuations can also lead to the QD-like<br />

confinement [4, 5].<br />

We report on magneto-optical properties <strong>of</strong> sharp excitonic photoluminescence (PL)<br />

lines related potential fluctuations, which are observed from a sample with a single QW <strong>of</strong> a<br />

modulated thickness. An active part <strong>of</strong> the sample consisted <strong>of</strong> a 30.1 nm wide CdTe QW,<br />

sandwiched between two Cd0.8Mg0.2Te barriers. A 5 nm wide iodine doping layer was added<br />

in the Cd0.8Mg0.2Te barrier. Results <strong>of</strong> low-temperature micro-PL measurements, performed<br />

in magnetic field are presented.<br />

Optical emission from the QW was observed in energy range between 1.81 eV to<br />

1.83 eV, which reflected the change in the QW thickness. At lower energy, at several<br />

positions on the sample a few discrete emission lines were observed. The effect <strong>of</strong> sample<br />

temperature, magnetic field, and detection polarisation was studied. Neutral biexcitons and<br />

excitons confined to a single fluctuation-related QD were identified. We found that the<br />

optical anisotropy <strong>of</strong> observed excitonic features ranged from about 10 eV to nearly<br />

160 eV, which suggests a broad distribution <strong>of</strong> lateral potential asymmetry. Energy<br />

dispersion <strong>of</strong> neutral exciton states amounts to 22 meV, which suggests a substantial<br />

distribution <strong>of</strong> the confining potential. The values <strong>of</strong> effective excitonic Lande factor <strong>of</strong><br />

excitons ranged from |g*| = 1.8 to 2.<br />

We propose to explain our observations in a model <strong>of</strong> potential fluctuations in the QW<br />

resulting from a strongly disordered layer <strong>of</strong> iodine-doping. In regions <strong>of</strong> the highest iodine<br />

surface density the effect <strong>of</strong> electrostatic potential on the QW is the strongest. Confined<br />

electronic and hole states in the QW in such regions are strongly affected by the Quantum<br />

Confined Stark Effect (QCSE), which leads to a local decrease <strong>of</strong> the QW band-gap. In areas<br />

with a lower iodine density the effect is weaker and the QW bandgap is only weakly affected.<br />

Fluctuations <strong>of</strong> the effective QW bandgap due to the QCSE resulting from fluctuations <strong>of</strong><br />

remote ionized iodine donor density are responsible in our opinion for observed QD-like<br />

confinement <strong>of</strong> electrons and holes in the investigated structure.<br />

[1] For review, see D. Bimberg et al, Quantum Dot Heterostructures (Wiley, New York, 1999).<br />

[2] L. Besombes, et al, Phys. Stat. Sol. (a) 178, 197 (2000).<br />

[3] D. Gammon, et al, Phys. Rev. Lett. 76, 3005 (1996).<br />

[4] K. Karpierz, et al, Acta Phys. Pol. A 112, 237 (2007).<br />

[5] K. Nogajewski, et al, Acta Phys. Pol. A 114, 1259 (2008).<br />

* corresponding author : maciej.molas@fuw.edu.pl<br />


40th _______________________________________________________________<br />

"Jaszowiec" <strong>2011</strong> International School and Conference on the Physics <strong>of</strong> Semiconductors TuP33<br />

Mixed Correlation Phases In Elongated Quantum Dots<br />

A. Ballester 1 , J. M. Escartín 2 , J. L. Movilla 1 , M. Pi 2 , and J. Planelles 1<br />

1 Departament de Química Física I Analítica, Universitat Jaume I, Box 224, E-12080,<br />

Castelló, Spain<br />

2 Departament ECM, Facultat de Física, IN2UB, Universitat de Barcelona, E-08028,<br />

Barcelona, Spain<br />

Nanorods (NRs) are an elongated variant <strong>of</strong> quantum dots (QDs) with a highly<br />

anisotropic confining potential. These nanostructures constitute the bridge between zerodimensional<br />

QDs and one-dimensional quantum wires (QWs). Nanodumbbells (NDs) are<br />

another type <strong>of</strong> elongated structures. These structures are NRs sandwiched between two<br />

spherical caps, typically <strong>of</strong> a different material. The theoretical study <strong>of</strong> colloidal<br />

semiconductor NRs and NDs is <strong>of</strong> particular interest because <strong>of</strong> the ability to synthesize these<br />

kinds <strong>of</strong> nanostructures with precise size and shape control [1,2].<br />

The unusual long length <strong>of</strong> the NRs leads to characteristic pr<strong>of</strong>iles <strong>of</strong> the electron<br />

density distribution in the NR growth direction, which depend on the ratio among the NR<br />

length and the number <strong>of</strong> electrons populating the conduction band <strong>of</strong> the nanostructure. In<br />

the case <strong>of</strong> diluted N-electron NRs, the particles are distributed in an ordered way forming a<br />

Wigner crystal [3]. The Coulomb energy predominates so that correlations dominate the<br />

electronic structure. In the opposite limit, the particles interact weakly and the kinetic energy<br />

dominates the Coulomb repulsion. Then, the so-called Fermi-liquid phase appears [4]. The<br />

transition to the Fermi liquid goes through an intermediate phase (called charge-density wave<br />

within the local spin-density-functional theory) which can be considered as a partially melted<br />

Wigner molecule [5]. In this contribution, we present a comprehensive study on the electron<br />

density distribution in both, NRs under applied electric fields and nanodumbbells. We show<br />

that two <strong>of</strong> the above-mentioned phases may coexist simultaneously in the same structure –<br />

but in different regions – giving rise to new phases that we refer to as mixed correlation<br />

phases [6].<br />

[1] X. Peng, L. Manna, W. Yang, J. Wickham, E. Scher, A. Kadanich, and A. P. Alivisatos,<br />

Nature (London) 404, 59 (2000); S. Kan, T. Mokari, E. Rothenberg, and U. Banin, Nature<br />

Mater. 2, 155 (2003).<br />

[2] J. E. Halpert, V. J. Porter, J. P. Zimmer, and M. G. Bawendi, J. Am. Chem. Soc. 128, 12590<br />

(2006).<br />

[3] J. Planelles, M. Royo, A. Ballester, and M. Pi, Phys. Rev. B 80, 045324 (2009).<br />

[4] V. Filinov, M. Bonitz, and Yu. E. Lozovik, Phys. Rev. Lett. 86, 3851 (2001).<br />

[5] M. Koskinen, M. Manninen, and S. M. Reimann, Phys. Rev. Lett. 79, 1389 (1997).<br />

[6] A. Ballester, J. M. Escartín, J. L. Movilla, M. Pi, and J. Planelles, Phys. Rev. B 82, 115405<br />

(2010).<br />


TuP34 _______________________________________________________________<br />

40th "Jaszowiec" <strong>2011</strong> International School and Conference on the Physics <strong>of</strong> Semiconductors<br />

Simulation and Realization <strong>of</strong> Photonic Crystals in Light<br />

Emitting Devices<br />

Jakob Ebeling, Timo Aschenbrenner, Stephan Figge, and Detlef Hommel<br />

Institute <strong>of</strong> Solid State Physics, Univerity <strong>of</strong> Bremen, Germany<br />

Indium gallium nitride (InGaN) based light emitting diodes (LEDs) have been very<br />

successful in the blue and green spectral range. However, the light extraction efficiency<br />

<strong>of</strong> conventional devices suffers from the total internal reflection at the semiconductor-air<br />

interface due to the relatively high optical refractive index (GaN: n≈2.5) limiting the numerical<br />

extraction efficiency <strong>of</strong> conventional, planar LEDs based on GaN to about 4% [1].<br />

To overcome this issue, different concepts have been theoretically studied and partially<br />

realized. Among these are encapsulation with a lower index material, surface roughening,<br />

thin-film LEDs and structuring the surface with photonic crystal (PC) structures,<br />

which are periodic variations <strong>of</strong> the refractive index at the length scale <strong>of</strong> the emission<br />

wavelength. The most common concept <strong>of</strong> a one-dimensional PC is a distributed Bragg<br />

reflector (DBR).<br />

Additionally to LEDs, PCs can be used to improve the properties <strong>of</strong> laser diodes.<br />

Commonly known are vertical-cavity surface-emitting laser diodes with DBR mirrors,<br />

but also distributed feedback (DFB) and DBR-based edge emitting laser diodes have<br />

been realized [2][3].<br />

We present numerical simulations <strong>of</strong> different types <strong>of</strong> PCs and their realization on<br />

various structures. A one-dimensional photonic crystal grating was fabricated by focused<br />

ion beam (FIB) milling at the resonator ends <strong>of</strong> an edge-emitting GaN-based laser device<br />

forming a lateral GaN-air-DBR to enhance the reflectivity <strong>of</strong> the resonator mirrors.<br />

Optoelectronic measurements were made to compare devices with and without additional<br />

gratings.<br />

Moreover, green emitting InGaN-based quantum dot LED structures were grown on<br />

sapphire substrates with metal-organic vapour-phase epitaxy. Different two-dimensional<br />

PCs were structured by FIB milling as well as e-beam lithography with subsequent dry<br />

etching. The structure surfaces are characterized by scanning electron microscopy and<br />

atomic force microscopy and a comparison between the two technological methods is<br />

given. Optical measurements obtained by micro-photoluminescence are used to quantify<br />

the light extraction enhancement achieved by PC structuring. The results <strong>of</strong> the different<br />

structures are compared with each other and with previous simulations.<br />

[1] C. Wiesmann, K. Bergenek, N. Linder, and U.T. Schwarz, Laser & Photonics Review<br />

3, 3 (2009).<br />

[2] D. H<strong>of</strong>stetter, R.L. Thornton, L.T. Romano, D.P. Bour, M. Kneissl, and R.M. Donaldson,<br />

Appl.Phys.Lett. 73, 2158 (1998).<br />

[3] J. Cho, S. Cho, B.J. Kim, S. Chae, C. Sone, O.H. Nam, J.W. Lee, Y. Park, and T.I.<br />

Kim, Appl.Phys.Lett. 76, 12 (2000).<br />


40th _______________________________________________________________<br />

"Jaszowiec" <strong>2011</strong> International School and Conference on the Physics <strong>of</strong> Semiconductors TuP35<br />

Electro-optical characterization <strong>of</strong> Ti/Au-ZnTe Schottky diodes with CdTe<br />

quantum dots<br />

E. Zielony 1 , E. Popko 1 , Z. Gumienny 1 , G. Karczewski 2<br />

1 Institute <strong>of</strong> Physics, Wroclaw University <strong>of</strong> Technology, Wybrzeze Wyspianskiego 27,<br />

50-370 Wroclaw, Poland<br />

2 Institute <strong>of</strong> Physics, Institute <strong>of</strong> Physics Polish Academy <strong>of</strong> Science, al. Lotnikow 32/46,<br />

02-668 Warsaw, Poland<br />

In this study the deep level transient spectroscopy (DLTS) and micro-Raman spectroscopy<br />

techniques have been applied to investigate ZnTe (p-type) – Ti/Au Schottky diodes containing<br />

a layer <strong>of</strong> CdTe self-assembled quantum dots (SAQDs). The reference sample was the ZnTe –<br />

Ti/Au diode without dots. Both samples were grown by molecular beam epitaxy technique.<br />

The dots were formed in the atomic layer epitaxy (ALE) growth mode by deposition <strong>of</strong> three<br />

monolayers <strong>of</strong> CdTe. Micro-Raman measurements confirmed the presence <strong>of</strong> CdTe layer <strong>of</strong><br />

quantum dots in the investigated material. A broad band corresponding to the longitudinal<br />

(LO) CdTe phonon related to the QD-layer has been detected at a wavenumber <strong>of</strong> 160 cm -1<br />

[1] in the case <strong>of</strong> the QD sample. Another peak observed for the wavenumber <strong>of</strong> 210 cm -1 has<br />

been assigned to localized LO phonon associated with the ZnTe layer in both samples [1].<br />

Moreover two tellurium - related bands at wavenumbers around 120 cm -1 and 140 cm -1 have<br />

been identified [1]. DLTS measurements for the sample with QDs reveal the presence <strong>of</strong> two<br />

hole-related signals with thermal activation energies equal to EH1=0.2eV and EH2=0.4eV. For<br />

the reference ZnTe-Ti/Au diode only the H2 signal is observed. It may be concluded that the<br />

H1=0.2 eV level can be assigned to the hole emission from defects accompanying quantum<br />

dots formation. The 0.4eV trap is attributed to the ZnTe bulk material [2].<br />

[1] V. S. Vinogradov, et al., Physics <strong>of</strong> the Solid State, Vol. 50, No. 1, 2008.<br />

[2] E. Placzek-Popko, et al., Physica B 404 5173-5176 (2009).<br />


TuP36 _______________________________________________________________<br />

40th "Jaszowiec" <strong>2011</strong> International School and Conference on the Physics <strong>of</strong> Semiconductors<br />

Magneto-photoluminescence studies <strong>of</strong> many body scattering processes<br />

in two-dimensional hole gas<br />

J. Jadczak1 1 , L. Bryja 1 , A. Wojs 1 , G. Bartsch 2 , D.R. Yakovlev 2 , M. Bayer 2<br />

and M. Potemski 3<br />

1 Institute <strong>of</strong> Physics, Wroclaw University <strong>of</strong> Technology, Wroclaw, Poland<br />

2 Experimentelle Physik 2, Technische Universität Dortmund, D-44227 Dortmund, Germany<br />

3 Grenoble High Magnetic Field Laboratory, CNRS, Grenoble, France<br />

Fig. 1. PL vs magnetic field for<br />

the symmetric 15 nm QW.<br />

The 2D hole gas due to the much higher effective mass<br />

create a unique possibility to study a variety <strong>of</strong> many<br />

body scattering processes, also not observed in n-type<br />

systems. In this work we report on detailed lowtemperature<br />

(T=1.8÷4.2K), high-field (B≤28T), polarization-resolved<br />

magneto-photoluminescence (MPL) studies<br />

<strong>of</strong> 2DHG in GaAs quantum wells. In the symmetric 15<br />

nm wide well with a hole concentration p=1.5x10 11 /cm -2<br />

we detected in MPL spectra shake-up hole replicas <strong>of</strong><br />

positively charged exciton (trion) on acceptor bound [1]<br />

and free states (Fig. 1). The former one for the first time.<br />

In asymmetric samples with different width<br />

(w=18÷30nm) and concentrations (p=1.4÷2.3x10 11 /cm -2 ) we observed an opposite process<br />

with hole cyclotron replicas going to higher energies with the magnetic field. The most intriguing<br />

effect, never reported before, was the observation <strong>of</strong> a coupling between the free and<br />

acceptor bound positively charged excitons (Fig. 2). The two<br />

states: free and bound are brought into resonance by an exchange<br />

<strong>of</strong> a quantum <strong>of</strong> a hole cyclotron energy in a scattering<br />

process with surrounding 2D holes. For the 22 nm wide<br />

sample the resonance is observed in magnetic field around<br />

B=13T. Further to our previous studies [3] we observed, that<br />

the essential coupling occur between free trion - X + and a<br />

pair <strong>of</strong> hole cyclotron replicas <strong>of</strong> the acceptor bound trion -<br />

CR-AX + . The coupling can be observed in photoluminescence<br />

spectra due to a nearly energy degeneracy <strong>of</strong> all final<br />

states involve in a radiative recombination. Aided with numerical<br />

calculations [3] we have explained the existence <strong>of</strong><br />

two hole replicas as the transitions <strong>of</strong> bright and dark doublet<br />

states <strong>of</strong> AX + . The dark state is enabled in an optical<br />

transition by transfer <strong>of</strong> an access momentum to one <strong>of</strong> 2D<br />

Fig. 2. PL vs magnetic field<br />

for asymmetric 22 nm QW<br />

surrounding holes. In three coupling states model we have calculated energy and intensity <strong>of</strong><br />

all observed PL lines and we have obtained a very good agreement with experimental data.<br />

The coupling is observed in spite <strong>of</strong> very small coupling constants (determined from experiment)<br />

only because two replicas are involved.<br />

[1] L. Bryja et al., Phys. Rev. B 75, 035308 (2007).<br />

[2] J. Jadczak et al. Acta Physica Polonica (in press)<br />

[3] A. Wojs, Phys. Rev. B 76, 085344 (2007); Phys. Rev. Lett. 104, 086801 (2010).<br />


40th _______________________________________________________________<br />

"Jaszowiec" <strong>2011</strong> International School and Conference on the Physics <strong>of</strong> Semiconductors TuP37<br />

Norm. PhotoVoltaic Response (arb. Units)<br />

1.5<br />

1.0<br />

T = 10K<br />

Spin related effect in Si-MOSFETs THz Photoresponse<br />

Hadley Videlier 1 , Nina Dyakonova 1 , Frederic Teppe 1 , Cristophe Consejo 1 ,<br />

Wojciech Knap 1 , Jerzy Lusakowski 2 , Daniel Tomaszewski 3 , Jacek Marczewski 3 , Piotr<br />

Grabiec 3<br />

1 Laboratoire Charles Coulomb UMR 5221 CNRS- Université Montpellier 2, 34095<br />

Montpellier, France<br />

2 Institute <strong>of</strong> Experimental Physics, Warsaw University, Warsaw, Poland<br />

3 Institute <strong>of</strong> Electron Technology, Al. Lotnikow 32/46, 02-668 Warsaw, Poland<br />

Photoresponse to sub-terahertz and Terahertz (THz) radiations in Silicon Metal Oxide<br />

Semiconductor Field Effect Transistors (Si-MOSFETs) has been studied previously [1-3]. The<br />

obtained values <strong>of</strong> responsivity and Noise Equivalent Power have shown the potential <strong>of</strong> Si-<br />

MOSFETs as sensitive detectors at room temperature [1]. The photovoltaic response<br />

dependences on the gate length and the gate bias were in good agreement with the Dyakonov-<br />

Shur plasma wave detection theory [4]. The detection signal results from the rectification <strong>of</strong><br />

high frequency currents induced by the incident radiation in the transistor channel. The<br />

rectification takes place due to a nonlinear response <strong>of</strong> the gated two dimensional electron gas<br />

and a source-drain asymmetry. The main source <strong>of</strong> nonlinearity is the superposition <strong>of</strong> two<br />

radiation-induced effects: i) the modulation <strong>of</strong> the carrier density in the channel by variations<br />

<strong>of</strong> the gate potential and ii) current oscillations due to variations <strong>of</strong> the drain potential.<br />

Recently, monolithically integrated THz focal-plane arrays including antennas and amplifiers<br />

on a single silicon die has been designed to work at room temperature [5].<br />

In this work we have studied the effect <strong>of</strong> high magnetic field on the Si-MOSFETs<br />

photovoltaic response subjected to sub-terahertz radiation (185-300 GHz ) at low temperature.<br />

Frequency (GHz)<br />

400<br />

300<br />

200<br />

100<br />

0<br />

0 3 5 8 10 13 15<br />

B (Tesla)<br />

!" # $ %$" &$ ' (<br />

% ) * + ,$ -<br />

( . %$<br />

185 GHz<br />

In Figure 1, photovoltaic signal is presented at 10 K<br />

as a function <strong>of</strong> applied magnetic field for three<br />

different incident frequencies. A very well<br />

0.5<br />

pronounced structure shifts to higher fields by<br />

0 2 4 6 8 10<br />

Magnetic Field (T)<br />

12 14 16<br />

increasing incident frequency. The positions <strong>of</strong> peaks<br />

is plotted in the inset <strong>of</strong> Fig.1 where the straight line<br />

follows equation f = gµ BB<br />

/ h and g = 2 . The calculation is in good qualitative agreement<br />

with experiments indicating a possible link between the observed structures in the<br />

photovoltaic response and a spin related phenomenon. This effect has been studied as a<br />

function <strong>of</strong> different parameters as temperature and applied source-drain current. However, a<br />

full quantitative interpretation <strong>of</strong> our results requires more complete experimental and<br />

theoretical developments.<br />

[1] R. Tauk, F. Teppe, S; Boubanga, et al. Appl. Phys. Lett. 89, 253511 (2006)<br />

[2] W. Knap, F. Teppe, Y. Meziani, et al. Applied Physics Letters 85 (4) 675-677 (2004)<br />

[3] W. Stillman, M.S. Shur, D. Veksler, S. Rumyantsev and F. Guarin, El. Lett., V43 (2007)<br />

[4] M. Dyakonov and M. Shur. El. Dev., IEEE Transactions on, 43(3):380–387 (1996)<br />

[5] E. Ojefors et al., IEEE Journal <strong>of</strong>, 44(7):1968–1976 (2009)<br />


TuP38 _______________________________________________________________<br />

40th "Jaszowiec" <strong>2011</strong> International School and Conference on the Physics <strong>of</strong> Semiconductors<br />

THz emission related to hot plasmons and plasma wave instability in<br />

field effect transistors<br />

Nina Dyakonova 1,2 , Abdelouahad El Fatimy 3 , Yahya Meziani 4 , Dominique<br />

Coquillat 1,2 , Wojciech Knap 1,2 , Frederic Teppe 1,2 , Petre Buzatu 1,2 , Luca Varani 5 , Hugues<br />

Marinchio 5 , Jeremy.Torres 5 , Pilippe Nouvel 5<br />

1 Université Montpellier 2, Laboratoire Charles Coulomb UMR 5221, Montpellier, France<br />

2 CNRS, Laboratoire Charles Coulomb UMR 5221, Montpellier, France<br />

3 Cardiff School <strong>of</strong> Physics and Astronomy, Cardiff University, Cardiff, United Kingdom<br />

4 Departamento de Física Aplicada, Universidad de Salamanca, Salamanca, Spain<br />

5 IES, UMR CNRS 5214, Université Montpellier 2, Montpellier, France<br />

The plasma waves in two-dimensional channel can be generated thermally by hot electrons<br />

(hot plasmons) [1] or can be due to plasma wave instability [2]. The THz emission due to<br />

radiative decay <strong>of</strong> non resonant hot plasmons is characterized by broad non tunable spectrum<br />

and a smooth onset. The mechanism plasma wave instability in a field effect transistor was<br />