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Semiconductor Physics Tuesday HL 13.4 Tue 11:45 BEY 118 Optical investigation of spin polarization in semiconductor heterostructures — •Ulrich Niedermeier, Klaus Wagenhuber, Christian Gerl, and Werner Wegscheider — Institut für Experimentelle und Angewandte Physik, Universität Regensburg, 93040 Regensburg Creating spin polarization in low-dimensional semiconductor systems is essential for the realization of new concepts in the field of spintronics. It has been shown that in systems with lifted spin degeneracy due to spinorbit interaction a new class of spin-related effects can be observed [1]. While the spin-galvanic effect is referred to as a current induced by spin orientation we are investigating the reversed spin-galvanic effect, i.e. a spin orientation induced by a current. Recently, it has been demonstrated by means of optical interband spectroscopy that passing an electric current in lateral geometry through a two-dimensional hole system leads to an orientation of spins [2]. In order to reproduce this effect photoluminescence measurements of modulation doped p-type AlGaAs/GaAs heterojunctions under current flow are performed. The analysis of the circular polarization of the photoluminescence signal should give a lowlimit estimate of the spin polarization achieved. [1] S. D. Ganichev, W. Prettl, J. Phys.: Condens. Matter (Topical Review) 15, R935 (2003). [2] A. Yu. Silov et al., Appl. Phys. Lett. 85, 5929-5931 (2004). HL 13.5 Tue 12:00 BEY 118 Suppression of Spin Relaxation in n-InGaAs-Wires — •Alexander Holleitner — Center for NanoScience (CeNS), Munich, Germany The spin dynamics of electrons have been investigated in narrow twodimensional n-InGaAs channels as a function of the wire width [1]. We find that electron-spin relaxation times increase with decreasing channel width, in accordance with recent theoretical predictions [2]. Surprisingly, the suppression of the spin relaxation rate can be detected for widths that are an order of magnitude larger than the electron mean free path. We find the spin diffusion length and the wire width to be the relevant length scales for explaining these effects. We discuss to which extent confinement, spin-orbit coupling, and structural parameters such as strain explain the observed effects. For the presented work, we acknowledge financial support by AFOSR and ONR. [1] A.W. Holleitner, V. Sih, R.C. Myers, A.C. Gossard, and D.D. Awschalom in preparation (2005). [2] A.G. Malshukov, K.A. Chao, Phys. Rev. B 61, 2413 (2000). HL 13.6 Tue 12:15 BEY 118 Rashba Spin Splitting in GaN Heterostruckturen — •Wolfgang Weber 1 , S.D. Ganichev 1 , Z.D. Kvon 2 , V.V. Belkov 3 , L.E. Golub 3 , S.N. Danilov 1 , D. Weiss 1 , W. Prettl 1 , Hyun-Ick Cho 4 , and Jung-Hee Lee 4 — 1 Institut für Experimentelle und Angewandte Physik, Universität Regensburg, 93040 Regensburg — 2 Institut für Halbleiterphysik, Novosibirsk, 630090, Russland — 3 A. F. Ioffe Physikalisch-Technisches Institut, 194021 St. Petersburg, Russland — 4 Kyungpook Staatliche Universität, 1370, Sankyuk-Dong, Daegu 702-701, Korea The spin splitting in k-space of the conduction band of low-dimensional GaN-structures was experimentally proved. It is shown, that the excitation of (0001)-oriented GaN quantum wells with infrared or terahertz radiation causes the circular photogalvanic effect. From a microscopical point of view this effect is a consequence of spin-orbit-coupling, which removes the spin-degeneration of the carriers in k-space, and the optical selection rules. The observation leads to a tunable Rashba-like spin splitting, which comes from the built-in asymmetry of the AlGaN/GaN interface. This fact, together with the anticipated high curie temperature under Mn doping and the long spin relaxation times, makes GaN an interesting material for spintronics. HL 13.7 Tue 12:30 BEY 118 Detection of few phosphorus donors in silicon — •H. Huebl 1 , D. R. McCamey 2,3 , M. Lunz 1 , W. Hutchison 2,4 , J. C. McCallum 2,5 , A. R. Hamilton 3 , R. G. Clark 2,3 , and M. S. Brandt 1 — 1 Walter Schottky Institut, Germany — 2 Australian Research Council Centre of Excellence for Quantum Computer Technology — 3 School of Physics, The University of New South Wales, Sydney, Australia — 4 School of Physical, Environmental and Mathematical, University College, The University of New South Wales ADFA,Canberra, Australia — 5 School of Physics, University of Melbourne, Australia One of the concepts for scalable solid-state based quantum computing is Kane’s proposal based on phosphorus donors in silicon. To estimate the sensitivity which is reached with magnetic resonance techniques in the detection of donor spins, we have measured electrically detected magnetic resonance (EDMR) on devices containing a few phosphorus donors only. In the devices studied phosphorus with a concentration of 2× 10 17 cm −3 is implanted at 15 keV into intrinsic silicon in an area of 100 × 100 nm 2 defined by electron beam lithography. The leads contacting this island are also obtained by implantation with P, however to a concentration above the Mott transition which does not lead to an EDMR signal. At 5 K and under illumination, a resonant change of the conductivity ∆σ/σ ≈ 10 −5 is observed for an island containing 85±10 atoms. From the signal-to-noise ratio, a sensitivity of about 600 P/ √ number of field scans can be deduced for the samples investigated so far. This experiments demonstrate the possibilities for investigating the properties of a few of donors and indicates that single spin resolution should be achievable. HL 13.8 Tue 12:45 BEY 118 Pure Spin Currents by Spin Dependent Electron Phonon Interaction — •Stephan Giglberger 1 , S.D. Ganichev 1 , S.N. Danilov 1 , V.V. Belkov 2 , E.L. Ivchenko 2 , S.A. Tarasenko 2 , D. Weiss 1 , W. Prettl 1 , W. Janzsch 3 , F. Schaffler 3 , and D. Gruber 3 — 1 Institut für Experimentelle und Angewandte Physik, Universität Regensburg, 93040 Regensburg — 2 A.F. Ioffe Physico-Technical Institute, 194021 St. Petersburg, Russland — 3 Institut für Halbleiterund Festkörperphysik, Johannes-Kepler-Universität, Linz, Österreich It is shown that in gyrotropic quantum wells Drude absorption of terahertz radiation leads to a pure spin current. This current is caused by spin dependent electron phonon interaction wich can be described by klinear terms in their matrix element. Due to these k-linear terms Drude absorption leads to an asymmetric allocation of charge carriers within each subband and hence to a spinpolarised electric current. Due to the fact that direction of this current is antipodal for both spin subbbands the resulting electric net current will be zero, but there will be two spin currents. Here both the theory and the experimental verification of such spin currents will be presented. In experiment the pure spin current was converted to an electric current by applying a small magnetic field parallel to the plane which leads to Zeeman spin splitting and destroys the equilibrium of the opposing spin currents. Experiments were carried out on (001)-oriented asymmetric SiGe quantum wells under excitation with linearly polarised radiation of 140 µm wavelength. HL 13.9 Tue 13:00 BEY 118 Magnetic anisotropy and antiferromagnetic exchange of Co impurities in ZnO — •Roland Hayn 1 , Pascal Sati 1 , Anatole Stepanov 1 , Roman Kuzian 2 , Thomas Chanier 1 , Steffen Schäfer 1 , Sonia Régnier 1 , and Christian Morhain 3 — 1 Laboratoire Materiaux et Microelectronique de Provence, 13397, Marseille Cedex 20, France — 2 Institute for Material Science, Krzhizhanovskogo 3, 03180 Kiev, Ukraine — 3 Centre de Recherche sur l’Hetero-Epitaxie et ses Applications-CNRS, 06560, Valbonne Sophia-Antipolis, France We report on the magnetic properties of (Zn,Co)O epitaxial thin films with low Co concentration. Magnetic and EPR measurements, combined with crystal field theory, reveal that isolated Co impurities possess a strong single ion anisotropy, which would lead to an easy plane ferromagnetic state when a hypothetical Co-Co interaction would be considered. However, the magnetization measurements show the presence of shortrange antiferromagnetic exchange interactions between nearest-neighbor magnetic ions which is supported by LSDA+U calculations.
Semiconductor Physics Tuesday HL 14 II-VI semiconductors I Time: Tuesday 11:00–13:15 Room: POT 151 HL 14.1 Tue 11:00 POT 151 Magnetic field studies of bound exciton complexes in Lithium doped ZnO — •Rob McKenna 1,2 , Markus R. Wagner 1 , Axel Hoffmann 1 , Joachim Sann 3 , Stefan Lautenschläger 3 , and Bruno K. Meyer 3 — 1 Intitute for Solid State Physics, Technical University Berlin — 2 University of Technology Sydney — 3 I. Physics Institute, Justus Liebig University Giessen Lithium located at the Zn site in the ZnO matrix could be a valid acceptor for p-conduction. Our magneto-PL and -PT investigation of a Lithium doped ZnO film on a ZnO substrate found I6 to I8, as well as I0 and I1 bound exciton complexes, with the smallest FWHM of 80 µm. Zeeman-splitting of peaks up to 580 µeV at 5 T, produced electron gvalues in good agreement with previous publications. An additional splitting of peaks in magnetic fields stronger than 3 T, was also clearly apparent. A non-zero an-isotropic hole effective g-value for B⊥c is discussed as the possible origin of the observed splitting. The neutral or ionised nature of the I0 and I1 bound exciton complexes in the magnetic field was also further clarified and the appearance of a new peak on the lower energy side of the I1 peak, indicating zero-field-splitting, was also investigated. In addition to this, the forbidden exciton could be observed. Temperature dependent measurements were employed to investigate the donor or acceptor character of the bound exciton complexes and angular and polarisation dependent measurements clarify whether the involved holes originate from a valence band with Γ7 or Γ9 symmetry. HL 14.2 Tue 11:15 POT 151 Photoluminescence properties of MgxZn1−xO thin films grown by pulsed laser deposition — •Susanne Heitsch, Gregor Zimmermann, Holger Hochmuth, Daniel Spemann, Gabriele Benndorf, Heidemarie Schmidt, Michael Lorenz, and Marius Grundmann — Universität Leipzig, Institut für Experimentelle Physik II, Linnéstr. 5, D-04103 Leipzig, Germany MgxZn1−xO thin films (0 ≤ x ≤ 0.18) have been grown on a-plane sapphire substrates with or without ZnO buffer layers by pulsed laser deposition. The Mg content in the films was controlled by using different MgZnO targets and by applying different oxygen partial pressures p(O2) during the deposition, respectively. Higher oxygen partial pressures caused a lower Mg content in the thin films. The surface roughness measured by AFM shows no dependence on x, but on p(O2). A minimum could be found for samples grown at p(O2) ∼ 1 × 10 −3 mbar. Samples with equal x grown at lower oxygen partial pressures show a broader photoluminescence (PL) emission than samples grown at higher pressures. With increasing x the PL maximum shifts approximately linearly to higher energies and the emission exhibits a broadening not only due to alloy broadening. The blueshift of the PL peak position on x is found to be larger at room temperature than at 2 K. Deposition of the MgxZn1−xO thin films on ZnO buffer layers improves the surface quality as well as the half width of the emission. HL 14.3 Tue 11:30 POT 151 Excitonic-recombination dynamics of individual ZnO nanowires — •Lars Wischmeier, Tobias Voss, Ilja Rückmann, and Jürgen Gutowski — Institute of Solid State Physics, University of Bremen, P.O. Box 330440, D-28334 Bremen Zincoxide (ZnO) nanowires show near band-edge photoluminescence (PL) in the UV spectral region (Egap = 3.37 eV at room temperature) and have therefore recently attracted many research activities because they are considered to be promising building blocks for nanometerscale optoelectronic devices. Here the optical properties of individual nanowires with diameters < 200 nm prepared from an as-grown ensemble are analyzed. The PL of ZnO is composed of various near band-edge emissions which are accompanied by phonon-assisted recombinations. These different emissions of an individual nanowire are measured time-resolved by a combination of a micro-photoluminescence setup and the time-correlated single-photon counting technique. The temporal development of the PL is studied as a function of intensity, temperature, and size of the wires. From the experimental results the decay times are determined. The results are analyzed by use of rate equations to model the excitonic recombination processes. Additionally the results performed on an individual nanowire are compared to the results performed on the as-grown nanowire ensemble. HL 14.4 Tue 11:45 POT 151 Optical and microelectrical characterization of ZnO single crystals implanted with group V elements — •Matthias Brandt 1 , Holger von Wenckstern 1 , Gabriele Benndorf 1 , Jörg Lenzner 1 , Heidemarie Schmidt 1 , Michael Lorenz 1 , Marius Grundmann 1 , Gabriel Braunstein 2 , and Gerhard Brauer 3 — 1 Universität Leipzig, Institut für Experimentelle Physik II, Leipzig, Germany — 2 University of Central Florida, Department of Physics, Orlando, Florida, USA — 3 Institut für Ionenstrahlphysik und Materialforschung, FZ Rossendorf, Dresden, Germany We have implanted ZnO single crystals produced by pressurized melt growth with nitrogen, phosphorous and arsenic ions, as well as with argon. The samples have been annealed at temperatures ranging from 300 to 1000 ◦ C in an oxygen atmosphere for 45 minutes or in air for 60 minutes. They were characterized optically by cathodoluminescence and photoluminescence measurements and electrically by scanning capacitance microscopy and scanning surface potential microscopy. Results have been compared to the properties of as-grown samples. We find a strong dependence on a) the implanted species and b) the annealing temperature. An intense donor-acceptor pair transition was observed in luminescence of N-implanted crystals annealed at 700 ◦ C only. Strong indication for a ptype conductivity surface layer was found for P-implanted ZnO annealed at 700 ◦ C. HL 14.5 Tue 12:00 POT 151 Deep defects generated in n-conducting ZnO:TM thin films — •Heidemarie Schmidt, Mariana Diaconu, Holger Hochmuth, Michael Lorenz, Holgerr von Wenckstern, Gisela Biehne, Daniel Spemann, and Marius Grundmann — Universität Leipzig, Fakultät für Physik und Geowissenschaften, Institut für Experimentelle Physik II, Linnéstrasse 5, D-04103 Leipzig, Germany The ferromagnetism in highly transparent and intrinsically n-type conducting zinc oxide doped with 3d transition metals (TM), is predicted to be defect mediated. We investigate the generation of deep defects in nconducting 1 µm thick ZnO:TM films (TM=Co, Mn, Ti) with a nominal TM content of 0.02, 0.20 and 2.00 at% grown by pulsed laser deposition on a-plane sapphire substrates using deep level transient spectroscopy. We find that a defect level is generated, independent of the TM content, located 0.31 eV and 0.27 eV below the conduction band minimum of ZnO:Mn and ZnO:Ti, respectively. Different defect levels are generated in dependence on the Co content in ZnO:Co. The undoped ZnO reference sample reveals the well-known E1, E3 and Eα1 [1] defect level. This work shows that an optimization of defect-related ferromagnetism in nconducting ZnO:TM thin films will only be possible if the preparation sensitive formation of deep defects is controlled in the same time. [1] F. D. Auret et al., Appl. Phys. Lett. 80 (2002) 1340 and F. D. Auret et al., phys. stat. sol. (c) 1 (2004) 674. HL 14.6 Tue 12:15 POT 151 Resonantly and Non-Resonantly Excited Bound Excitons in ZnO Epilayers — •Frank Bertram, Sören Giemsch, Jürgen Christen, Armin Dadgar, and Alois Krost — Institute of Experimental Physics, Otto-von-Guericke-University Magdeburg, Germany The photoluminescence (PL) spectrum of ZnO exhibits a rich excitonic structure in the near bandgap region at 4K. A 8 µm thick ZnO epi-layer MOVPE grown on a GaN / sapphire template was investigated. The non-resonant PL spectrum is dominated by the impurity bound exciton I8 and exhibits further individual BE lines (I1, I2, I6, I9), the TES-lines of I8 and I9 as well as the LO phonon replica exclusively from I8. Under resonant excitation, i.e. Eexcitation= E(I8) the TES-I8 line and the I8-LO line are much more intense and reveal a smaller line width. Under excitation at the spectral position of I9 the optical features associated with I8 completely disappear, while TES-I9 increases and I9-LO shows up. Resonantly excited time-resolved PL yields a mono-exponential decay with lifetimes τ(I8)=270 ps and τ(I9)=280ps, respectively, - distinctively different from the lifetimes obtained for non-resonant excitation. Measuring the decay lifetime as a function of tuned laser photon energy, both, I8 and I9 reveals a clear resonance, i.e. drop in time constant (< 200 ps), for Elaser-EBE=5 meV. This corresponds to the A-B-valence band splitting and indicates strong scattering into the B-valence band.
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