Semiconductor Physics Division Fachverband Halbleiterphysik (HL ...

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Semiconductor Physics Division (HL) Thursday Hoffmann 1 , Shalini Gupta 2 , Ian T. Ferguson 2 , Martin Röver 3 , Dong-Du Mai 3 , and Angela Rizzi 3 — 1 Institut für Festkörperphysik, Technische Universität Berlin, Germany — 2 School of Electrical and Computer Engineering, Georgia Institute of Technology, Atlanta, USA — 3 IV. Physikalisches Institut and Virtual Institute of Spin Electronics (VISel), Georg-August Universität Göttingen, Germany Gd doped GaN is of high interest for spintronics because it shows a ferromagnetic behavior with a high magnetic moment per Gd atom as well as good conductivity at RT. To investigate the coupling mechanism behind the strong magnetization we examined MOCVD and MBE grown samples of epitaxial layers of this diluted magnetic semiconductor containing Gd at concentrations ranging from 10 18 cm −3 to 10 21 cm −3 . p-type and n-type co-doping allowed the investigation of the effect of the position of the Fermi level. We present high resolution photo luminescence (PL) spectra of a doublet peak at 1.7876 eV with a FWHM of 40 µeV in all Gd doped samples which we associate with an internal Gd 3+ transition. The relatively long time constant of 3.5 ms found in time resolved PL experiments confirms the attribution to an intra f-shell transition. Excitation spectra of this luminescence reveal efficient excitation bands between 2.0 and 2.6 eV. The results are discussed in terms of a bound state that might play a significant role in the ferromagnetic behavior reported for GaGdN. 15 min. break HL 41.9 Thu 12:00 BEY 154 Swift heavy ion irradiation induced recrystallization of implanted GaN — •Anne-Katrin Nix 1 , Sven Müller 1 , Ulrich Vetter 1 , Christina Trautmann 2 , and Hans Hofsäss 1 — 1 II. Physikalisches Institut, Universität Göttingen, Germany — 2 Gesellschaft für Schwerionenforschung, Darmstadt, Germany Preparing GaN:Mg by ion implantation is a widely used technique of doping, but results in a high level of lattice defects. Thermal annealing can be used for recrystallization, but sample decomposition often hampers the annealing effect. Here, we present swift heavy ion irradiation as an alternative annealing method. Due to high electronic energy loss along the ion track, the sample is locally heated during a time span of 10 −12 seconds. Thus, surrounding material stays unaffected. Implanted GaN samples were irradiated with several ion species at different energies with the objective to vary the electronic energy loss. Directly after implantation and after irradiation, the photoluminescence excited with a laser emitting in the UV-regime was examined at low temperature. The obtained spectra are compared to well known spectra of GaN and GaN:Mg. An annealing effect is seen after 578 MeV Cr-irradiation. GaN samples were irradiated with 668 MeV Niions with varying fluences, obtaining a similar electronic energy loss compared to 578 MeV Cr-ions, with the goal to examine a fluence dependence of the annealing process. In addition, irradiation with light ions, and thus much lower electronic energy loss, was performed with the aim of analyzing a possible threshold value of the electronic energy loss. HL 41.10 Thu 12:15 BEY 154 Role of the parasitic Mg3N2 phase in post-growth activation of p-doped Mg:GaN — •Björn Lange, Christoph Freysoldt, and Jörg Neugebauer — Max-Planck-Institut für Eisenforschung, 40237 Düsseldorf, Germany A critical issue in further improving the efficiency of modern white light-emitting diodes is the rather limited p-type conductivity achievable in GaN. The limited doping efficiency is a direct consequence of the low solubility of Mg in GaN due to the formation of the parasitic Mg3N2 phase. While the presence of this phase is well known and HL 42: Interfaces/surfaces often unavoidable its consequences on the acceptor activation mechanism (H codoping with subsequent H removal) have not been studied so far. We have therefore studied the possibility to exploit these Mg3N2 inclusions as potential hydrogen traps by means of DFT calculations. For this, Mg3N2 has been modeled in the anti-bixbyite structure. The calculated structural properties are in good agreement with available experimental data. Based on these studies hydrogen has been explored in various positions and charge states. Our results show significantly higher H solubilities compared to GaN. Further a strongly bound N-H complex is identified which is more stable than the Mg-H complex in GaN. The implications for Mg activation in Mg:GaN above the Mg solubility limit will be discussed. HL 41.11 Thu 12:30 BEY 154 Growth modes of thick InGaN films on GaN — •Martin Leyer, André Kruse, Joachim Stellmach, Markus Pristovsek, and Michael Kneissl — Technische Universität Berlin, Institut für Festkörperphysik, Hardenbergstr. 36, 10623 Berlin We have investigated the growth of thick (d > 10 nm) InGaN layers grown by metal-organic vapour phase epitaxy (MOVPE). The composition was controlled by varying the growth temperature between 700 ◦ C and 850 ◦ C. The indium content and the strain was determined by X-ray diffraction. We observed two distinct peaks in the ω-2Θ scans for most of the samples. For lower growth temperatures a relaxed top layer was found in reciprocal space maps. For higher growth tempratures a lateral decomposed top layer was observed. The variation of the indium content with growth temperature was exponential. We propose the following model: In a first step a smooth strained InGaN wetting layer is grown in the Stranski-Krastanov growth mode. Then a transition from 2D to 3D growth takes place. For growth temperatures below 750 ◦ C a rough layer is grown pseudomorphic up to the crititcal layer thickness and subsequently relaxes. For growth temperatures above 750 ◦ C a strained, but laterally inhomogeneous layer is grown. For the strained layer the activation energy for indium incorporation was ∼2.2 eV and for the relaxed layer ∼0.6 eV. Growth rate(s) and the critical thickness(es) were obtained by in-situ ellipsometry. HL 41.12 Thu 12:45 BEY 154 Katalysator- und maskenfreies Wachstum von GaN- Nanosäulen — •Gerd Kunert 1 , Timo Aschenbrenner 1 , Carsten Kruse 1 , Stephan Figge 1 , Detlef Hommel 1 , Marco Schowalter 2 und Andreas Rosenauer 2 — 1 Institut für Festkörperphysik - Halbleiterepitaxie - Universität Bremen, 28359 Bremen, Otto-Hahn Allee, NW1 — 2 Institut für Festkörperphysik - Transmissionselektronenmikroskopie - Universität Bremen, 28359 Bremen, Otto-Hahn Allee, NW1 In den letzten Jahren gab es umfangreiche Untersuchungen in Bezug auf Galliumnitrid basierten Nanosäulen. Viele Herstellungsmethoden verwenden dabei katalysatorinduziertes Wachstum oder eine Maskierung der Oberfläche. In diesem Beitrag wird ein Verfahren präsentiert, was die Schwierigkeiten dieser Methoden in Bezug auf Materialqualität umgeht. In einem ersten Schritt wird die Saphir-Oberfläche mit r-Flächenorientierung mit Hilfe metallorganischer Gasphasenepitaxie nitridiert. Es bilden sich AlN Inseln auf dem Substrat. In einem zweiten Schritt wird das Wachstum mittels MBE durchgeführt. Das Verfahren führt zu um 28 ◦ zur Oberflächennormalen geneigten Nanokristallen. Deren sehr gute kristalline Qualität wurde durch TEM- Untersuchungen gezeigt. Zeitgleich zum Wachstum der Nanokristalle wächst eine kompakte, zweidimensionale Schicht Galliumnitrid, die zur Kontaktierung späterer Bauelemente genutzt werden kann. Es wurden Nanosäulen hergestellt mit einer Länge von bis zu 3µm. Deren Dichte lässt sich zwischen 10 7 cm −2 und 10 9 cm −2 variieren. Time: Thursday 9:30–10:45 Location: POT 51 HL 42.1 Thu 9:30 POT 51 How a hydrogen passivated surface could appear to be metallic: the story of the 3C-SiC (001) 3x2 surface — •Peter Deak, Balint Aradi, and Thomas Frauenheim — Bremen Center for Computational Materials Science, University of Bremen, Am Fallturm 1, D-28359 Bremen, Germany Photo electron and scanning tunneling spectroscopy has revealed partially filled states near the conduction band edge of the silicon rich (3x2)-reconstructed (001) surface of cubic SiC, after exposure to atomic hydrogen. These were attributed to a row of unsaturated singly occupied Si dangling bonds by experimentalists, and to a defect band due to a row of Si-H-Si tri-center bonds by theorists. Here we show that the surface is, in fact, completely passivated by hydrogen (no partially filled defect band) but, in doped samples, the accumulation of conduc-
Semiconductor Physics Division (HL) Thursday tion band electrons at a polar surface can explain the experimental observations. HL 42.2 Thu 9:45 POT 51 Control of Donor Charge States with the Tip of a Scanning Tunnelling Microscope — •K. Teichmann 1 , M. Wenderoth 1 , S. Loth 1 , R. G. Ulbrich 1 , J. K. Garlef 2 , A. P. Wijnheijmer 2 , and P. M. Koenraad 2 — 1 IV. Physikalisches Institut, Georg-August- Universität Göttingen — 2 PSN, Eindhoven University of Technology, the Netherlands The functionality of nanoscale semiconductor devices crucially depends on details of the electrostatic potential landscape on the atomic scale and its microscopic response to external electric fields. We report here an investigation of charge state switching of buried single Si donors in 6 · 10 18 cm −3 n-doped GaAs with scanning tunnelling microscopy (STM) under UHV conditions at 5K. The effect of tip induced band bending (TIBB) through the freshly cleaved (110)-surface was used to change the charge state of individual donors from neutral to positively charged and reverse. Scanning tunnelling spectroscopy (STS) revealed a ring like feature around each donor center. The ring radius depends on tip bias voltage [1] . The charge state of each donor in the random arrangement of dopants was in most cases unambiguously fixed by the extension of the tip-induced space charge cloud, which was located under the tip and controlled by the applied voltage. For certain geometric configurations the system showed bi- (or multi-) stable behaviour, this lead to dynamic flickering of the ionization sequence. This work was supported by DFG SFB 602 and DFG SPP 1285. [1] PRL 101, 076103 (2008) HL 42.3 Thu 10:00 POT 51 Application of catalytic nanoparticles to wide bandgap semiconductor surfaces — •Susanne Schaefer 1 , Sonja Wyrzgol 2 , Yizhen Wang 1 , Johannes Lercher 2 , and Martin Stutzmann 1 — 1 Walter Schottky Institut, Technische Universität München, Am Coulombwall 3, 85748 Garching, Germany — 2 Technische Chemie 2, Technische Universität München, Lichtenbergstr. 4, 85747 Garching, Germany Wide bandgap semiconductors are investigated as materials for the electronic control of catalytic reactions via metal nanoparticles. To conduct test reactions like selective hydrogenation, platinum nanoparticles are applied to the semiconductor surface. GaN substrates, which were grown by MOCVD as well as PAMBE, were used as nanoparticle support. The nanoparticles were prepared by two methods: spincoating with polymer-encapsulated Pt nanoparticles and evaporation of Pt at elevated temperatures. For polyvinyl-pyrolidone (PVP)coated nanoparticles, an avarage size of 2.4 nm was observed with TEM. The PVP-coated particles were applied to the semiconductor support via spin coating and activated by oxygen plasma. For particles applied by evaporation, Pt layers with a nominal thickness of 0.2-5 nm were deposited, as determined by EDX. Under defined heating and gas flow, the platinum atoms coalesce to particles. Particle sizes and distributions were investigated with AFM. For testing the electronic properties of these semiconductor-metal interfaces, Schottky HL 43: Photonic crystals I diodes were processed with standard lithography. UI-characteristics were measured for various particle sizes. HL 42.4 Thu 10:15 POT 51 Surface states and origin of the Fermi level pinning on non-polar GaN(1100) surfaces — •Lena Ivanova 1 , Svetlana Borisova 2 , Holger Eisele 1 , Mario Dähne 1 , Ansger Laubsch 3 , and Philipp Ebert 2 — 1 Institut für Festkörperphysik, Technische Universität Berlin, 10623 Berlin, Germany — 2 Institut für Festkörperforschung, Forschungszentrum Jülich GmbH, 52425 Jülich, Germany — 3 OSRAM Opto-Semiconductors GmbH, 93055 Regensburg, Germany Group-III nitrides raised considerable attraction because of their ideal properties for green, blue, and ultraviolet laser and LED devices. One particular challenge of the epitaxial growth is the impurity, dopant, and defect incorporation during growth, which often depends on the position of the Fermi level at the growth surface. For the non-polar GaN surfaces only little is known about the exact positions of the surface states and thus their possible influence on the Fermi energy. Therefore, we investigated GaN(1100) cleavage surfaces by crosssectional scanning tunneling microscopy and spectroscopy [1]. We identified the energy positions and types of surface states as well as the origin of the Fermi level pinning on GaN(1100) cleavage surfaces. It is found that both the N and Ga derived intrinsic dangling bond surface states are outside of the fundamental band gap. The observed Fermi level pinning 1.0 eV below the conduction band edge is attributed to the high step and defect density at the surface but not to intrinsic surface states. [1] L. Ivanova et al., APL 93, 192110 (2008). This work is supported by the DFG. HL 42.5 Thu 10:30 POT 51 Initial stages of GaN(0001)-2x2 - oxidation — •Pierre Lorenz 1 , Richard Gutt 2 , Juergen A. Schaefer 1 , and Stefan Krischok 1 — 1 Institute of Physics and Institute of Micro- und Nanotechnologies, Technical University Ilmenau, P.O. Box 100565, D- 98684 Ilmenau, Germany — 2 Fraunhofer Institute for Applied Solid State Physics, Tullastr. 72, 79108 Freiburg, Germany We studied the initial oxidation stages of 2x2 reconstructed Ga-face GaN(0001) grown in-situ by PAMBE. The oxidation process was characterized using X-ray and ultraviolet photoelectron spectroscopy (XPS, UPS), as well as reflection high energy electron diffraction (RHEED). In particular, the evolution of the valence band structure, the work function and the core levels of gallium and nitrogen as well as the increase of the oxygen O 1s emission were studied in combination with the corresponding RHEED pattern as a function of oxygen exposure. The clean GaN(0001)-2x2 surface exhibits two surface states at 2 eV (S1) and 3.5 eV (S2) below the Fermi level. The exposure to O2 results in two well pronounced valence band structures at binding energies of about 6 eV and 11 eV, respectively, which are caused by the adsorbed oxygen. The 2x2 reconstruction as well as the S1 state disappear rapidly, revealing an extremely high reactivity of the as grown GaN surface, whereas the S2 state vanishes considerably slower. Time: Thursday 9:30–12:30 Location: POT 151 HL 43.1 Thu 9:30 POT 151 Electro-optical modulator in a polymer- infiltrated silicon slotted photonic crystal waveguide heterostructure resonator — •Jan Hendrik Wülbern, Alexander Petrov, and Manfred Eich — Institut für Optische und Elektronische Materialen, Technische Universität Hamburg-Harburg, 21073 Hamburg We present a novel concept of a compact, ultra fast electro-optic modulator, based on photonic crystal resonator structures that can be realized in two dimensional photonic crystal slabs of silicon as core material employing a nonlinear optical polymer as infiltration and cladding material. The novel concept is to combine a photonic crystal heterostructure cavity with a slotted defect waveguide. The photonic crystal lattice can be used as a distributed electrode for the application of a modulation signal. An electrical contact is hence provided while the optical wave is kept isolated from the lossy metal electrodes. Thereby, well known disadvantages of segmented electrode designs such as excessive scattering are avoided. The optical field enhancement in the slotted region increases the nonlinear interaction with an external electric field resulting in an envisaged switching voltage of less than 1 V at modulation speeds up to 100 GHz. HL 43.2 Thu 9:45 POT 151 Room Temperature Tuning of Photonic Crystal Cavities — •Karoline A. Piegdon 1,2 , Heiner Matthias 3 , Heinrich-S. Kitzerow 3 , Dirk Reuter 4 , and Cedrik Meier 2 — 1 University of Duisburg-Essen, Physics Department — 2 University of Paderborn, Nanophotonics and Nanomaterials — 3 University of Paderborn, Physical Chemistry — 4 Ruhr University of Bochum Photonic crystal (PC) cavities are in the focus of interest due to the strong light-matter interaction, that makes them suitable for quantum electrodynamics in cavities containing semiconductor quantum dots (QDs). Recent experiments with single QDs coupled to a high
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