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Semiconductor Physics Tuesday effect which redshifts the energy of the observed phonons. The PL emission wavelength (between 600nm and 1000nm) shifts towards lower wavelengths with decreasing particle sizes. Investigations of the PL intensity as a function of the temperature reveal an interesting behaviour. The PL intensity has a maximum at about T=80K and decreases for higher as well as lower temperatures. To get a better understanding about the origin of these effects, micro- HL 20 Spin controlled transport II photoluminescence is employed. First results showing sharp emission lines (FWHM ≈ 1meV) originating from excitonic and biexcitonic recombination are presented. As a result of the strong Coulomb interaction in the particles the spectra show a remarkable high exciton to biexciton energy splitting of 32meV. Time: Tuesday 15:15–16:30 Room: BEY 118 HL 20.1 Tue 15:15 BEY 118 Vertical cavity surface emitting lasers for amplification of spin information at room temperature — •Stephan Hövel 1 , Nils C. Gerhardt 1 , Martin Hofmann 1 , Junling Yang 2 , Dirk Reuter 2 , and Andreas D. Wieck 2 — 1 AG Optoelektronische Bauelemente und Werkstoffe, Ruhr-Universität Bochum, IC2/152, 44780 Bochum — 2 Angewandte Festkörperphysik, Ruhr-Universität Bochum, NB 03, 44780 Bochum Spin injection into semiconductor light emitting diodes at room temperature usually results in effects much too small for applications. This is due to low injection efficiencies and, in particular, to strong spin relaxation in the semiconductor. We show experimentally and theoretically that spin information can be amplified using the nonlinearity of vertical cavity surface emitting lasers at threshold. By polarized optical excitation of a GaInAs VCSEL we can achieve an output polarization which is higher than that of the excitation [1]. For further improvement, we analyse the spin relaxation in different semiconductor materials to determine the optimal active material for a spin-VCSEL. Finally, we discuss electrical injection schemes for room temperature and low external magnetic fields [2].We thank the DFG for providing support in the SFB 491. [1] S. Hövel, N. Gerhardt, M. Hofmann, J. Yang, D. Reuter and A. Wieck, Electronics Letters 41, 251 (2005) [2] N. C. Gerhardt, S. Hövel, C. Brenner, M. R. Hofmann, F.-Y. Lo, D. Reuter, A. D. Wieck, E. Schuster W. Keune and K. Westerholt, Appl. Phys. Lett. 87, 032502 (2005) HL 20.2 Tue 15:30 BEY 118 Spin Dynamics During Transport Via Dynamic Quantum Dots — •James AH Stotz, Rudolph Hey, Paulo V Santos, and Klaus H Ploog — Paul-Drude-Institut für Festkörperelektronik, Hausvogteiplatz 5-7, 10117 Berlin, Germany Quantum information processing using electron spins in semiconductor structures requires the coherent transport and manipulation of spin polarized carriers. Previous studies have typically focussed on either the transport of spins with little control of their microscopic movement or the use of quantum dots to manipulate spins locally without microscopic transport. Recent work [1] using the unique system of dynamic quantum dots (DQDs) shows that electron spins can be transported over long distances and manipulated while retaining their microscopic confinement. The DQDs are produced by the superposition of piezoelectric potentials from surface acoustic waves propagating along orthogonal directions on a GaAs/(Al,Ga)As quantum well sample. While it is clear that the confinement potential of the DQDs reduces D’yakonov-Perel’ spin dephasing during transport, the effects of the strain and magnetic fields on spin dephasing are much more complicated. We will discuss the underlying mechanisms behind the ability to transport spins over long distances including the impact of confinement on spin coherence. In addition, the strong dephasing of the spin coherence in an external magnetic field will be addressed, and the influence of the acoustic strain field on the transport will be introduced. [1] J.A.H. Stotz et al., Nature Materials 4, 585-588 (2005) [2] Financial support from the BMBF Nanoquit project is appreciated. HL 20.3 Tue 15:45 BEY 118 Entanglement distillation by adiabatic passage in coupled quantum dots — •jaroslav fabian 1 and ulrich hohenester 2 — 1 Institute for Theoretical Physics, University Regensburg — 2 Institute of Physics, University of Graz Adiabatic passage of two correlated electrons in three coupled quantum dots is shown to provide a robust and controlled way of distilling, transporting and detecting spin entanglement, as well as of measuring the rate of spin disentanglement. Employing tunable interdot coupling the scheme creates, from an unentangled two-electron state, a superposition of spatially separated singlet and triplet states. A single measurement of a dot population (charge) collapses the wave function to either of these states, realizing entanglement to charge conversion. The scheme is robust, with the efficiency close to 100\%, for a large range of realistic spectral parameters. HL 20.4 Tue 16:00 BEY 118 Conductance Quantization in Quantum Point Contacts with Spin-Orbit Interaction — •Sebastian von Oehsen, Guido Meier, Toru Matsuyama, and Ulrich Merkt — Institut für Angewandte Physik und Zentrum für Mikrostrukturforschung, Universität Hamburg, Germany InAs has a strong and tunable spin-orbit interaction [1] and is thus an interesting material for spintronic devices. We present measurements on quantum point-contacts on InAs/InGaAs heterostructures. Hallbars are predefined on the samples by wet etching. To achieve lateral electrostatic confinement we use either split gates on a SiO2 layer or side gates. With the latter technique peripheral charging effects can be avoided because of the absence of an isolator. The measurements are performed at 250 mK and in magnetic fields up to 5 Tesla. Quantization steps in the conductance are measured. The transition from electrostatic to magnetic confinement is examined and compared with recent theoretical results [2]. [1] Ch. Schierholz, T. Matsuyama, U. Merkt, and G. Meier. Phys. Rev. B 70, 233311 (2004) [2] S. Debald and B. Kramer, Phys. Rev. B 71, 115322 (2005) HL 20.5 Tue 16:15 BEY 118 Current Assisted Magnetization Switching in (Ga,Mn)As Nanodevices — •K. Pappert, C. Gould, C. Rüster, R. Giraud, T. Borzenko, G. M. Schott, K. Brunner, G. Schmidt, and L. W. Molenkamp — Physikalisches Institut (EP3), Universität Würzburg, Am Hubland, D-97074 Würzburg, Germany Current induced magnetization switching of metallic nanometer-sized magnets has attracted much attention over the past years. It is viewed as an attractive alternative technique for magnetic information storage. However, the current densities presently needed highly exceed the limits tolerated by today’s integrated circuits. Ferromagnetic semiconductors on the other hand are anticipated to react to much smaller current densities. Yamanouchi et al.[1] observed current assisted magnetization switching in a (Ga,Mn)As Hall bar close to its Curie temperature using the magnetooptical Kerr and Hall effect. Here we present current assisted switching of the island of a (Ga,Mn)As double constriction device at 4.2 K. We adapt a read-out scheme demonstrated by Rüster et al.[2]. They used the resistance of domain walls pinned by nanoconstrictions to determine the magnetic configuration in a similar structure. Combining current assisted switching and the pinned domain wall resistance read-out in a single device constitutes a significant step forward towards a spintronic storage device, which may use domain walls to realize information storage, transport, manipulation and readout. [1] M. Yamanouchi et al., Nature 428, 539 (2004). [2] C. Rüster et al., Phys. Rev. Lett. 91, 216602 (2003).
Semiconductor Physics Tuesday HL 21 Transport properties I Time: Tuesday 16:30–19:30 Room: BEY 118 HL 21.1 Tue 16:30 BEY 118 DENSITY-OF-STATES IN MICROCRYSTALLINE SILICON FROM THERMALLY-STIMULATED CONDUCTIVITY — •Nacera Souffi, Rudolf Brüggemann, and Gottfried H. Bauer — Institut für Physik, Carl von Ossietzky Universität Oldenburg, D-26111 Oldenburg The technique of thermally stimulated currents has been applied to extract the density-of-states profile in microcrystalline silicon. Exploiting the experimental parameter space a consistent density-of-states profile emerges with an exponential conduction band tail and a broader deeper distribution. Calibrating the absolute density of states profile from other techniques like modulated photoconductivity, steady-state photocarrier grating technique and intensity-dependent photoconductivity allows a determination of the capture coefficient of the probed localised states. HL 21.2 Tue 16:45 BEY 118 Characterization of Ti / TiO 2 / Pt - Schottky diodes — •Philipp Zabel 1 , Sven Bönisch 1 , Germà García-Belmonte 2 , Juan Bisquert 2 , and Thomas Dittrich 1 — 1 Hahn-Meitner-Institut, Glienicker Str. 100, 14109 Berlin, Germany — 2 Departament de Ciències Experimentals, Universitat Jaume I, E-12080 Castelló, Spain UV photodiodes (sglux tw30sx, TiO 2 made by sol-gel processing) were used as a model system to study Ti / TiO 2 / Pt - Schottky diodes by temperature dependent current-voltage, impedance spectroscopy and transient photocurrent measurements. The equivalent circuit has been taken into account for the analysis. The Schottky barrier is about 1.3 eV which is equal to the work function difference between Ti and Pt. At lower potentials, the current is limited by the barrier while at higher potentials, control by space charge limited currents sets on. The temperature dependent dielectric constant of the TiO 2 layer was obtained. Photocurrent transients were excited with short UV laser pulses. The current at 1 µs and the integral of the photocurrent transients depend linearly on the laser intensity until an electron density of about 10ˆ{12} cmˆ{-2} is reached in the TiO 2 layer (thickness 160 nm). At higher intensities, the photocurrent goes to saturation due to recombination. The electron drift mobility depends only weakly on the illumination intensity and temperature. HL 21.3 Tue 17:00 BEY 118 Surface photovoltage for characterization of porous semiconductors — •Thomas Dittrich 1 , Iván Mora-Seró 2 , Germà García-Belmonte 2 , and Juan Bisquert 2 — 1 Hahn-Meitner- Institut, Glienicker Str. 100, 14109 Berlin, Germany — 2 Departament de Ciències Experimentals, Universitat Jaume I, E-12080 Castelló, Spain The surface photovoltage method is a locally sensitive technique with respect to even extremely short charge separation lengths. In conventional semiconductors, the charge separation length is given mainly by the thickness of the space charge region. The situation may change for porous semiconductors. In such systems, a time dependent development of the charge separation length from an initial value, given also by the duration time of the laser pulse, up to a screening length (Debye screening length in homogeneous media) has been observed and described by a diffusion model. The processes of diffusion and screening will be discussed on the example of porous TiO 2 layers sensitized with dye molecules. HL 21.4 Tue 17:15 BEY 118 Quantitative modelling of the temperature and magnetic-field dependence of the resistivity of paramagnetic Ga1−xMnxAs with x up 7% — •C. Michel 1 , S. Ye 1 , V. Rajevac 1 , P.J. Klar 1 , S.D. Baranovskii 1 , P. Thomas 1 , W. Heimbrodt 1 , and B. Goldlücke 2 — 1 Dept. Physics and WZMW, Philipps-University of Marburg, Germany — 2 MPI for Computer Science, Saarbrücken, Germany We measured and modelled quantitatively the magneto-resistance behaviour above the Curie-temperature of several different p-type Ga1−xMnxAs samples with x up to 7%. A network model [1] accounting for alloy disorder and tuning of the band structure due to the strong s, p-d exchange interaction between the spins of the extended band states and the localized Mn 3d spins was employed. The band structure description is based on parabolic hole bands and an acceptor level with a Gaussian broadening. The calculated temperature dependence of the resistance in zero-field as well as the magnitude of the magneto- resistance effects are very sensitive to the choice of model parameters, e.g. the valence band exchange-integral N0β, the width of the Gaussian broadening of the acceptor level, the degree of zero-field disorder etc., allowing one to determine these parameters by fitting the experimental data. The magnitude of the extracted parameters and the trends with x will be discussed and compared with literature values. [1] Phys. Rev. B 69, 165211 (2004). HL 21.5 Tue 17:30 BEY 118 Transistor characteristics of three leaky contacts defined in a two dimensional electron gas — •Daniela Spanheimer, Lukas Worschech, Christian R. Müller, and Alfred Forchel — Technische Physik, Universität Würzburg, Am Hubland, 97074 Würzburg, Germany The capacitance of leaky nanojunctions has recently attracted considerable attention. It has been stated that in mesoscopic system the capacitance not only depends on the geometry, but also on electrochemical entities and the transmission probability between the reservoirs [1]. We have studied the capacitive couplings between three leaky contacts by studying the gain properties of a transistor like junction. For that purpose we have realized two rows of etched holes in a modulation doped GaAs/AlGaAs heterostructure. The two rows define three electron reservoirs, which are leaky coupled to each other. We have determined the current voltage characteristics for several possible variations of the three terminals for different separations between the rows and different hole diameters. For row separations in the order of 1 µm transistor characteristics have been observed even at room temperature, which we discuss in terms of a leaky capacitor model. [1] T. Christen and M. Büttiker, Phys. Rev. Lett. 77, 143 (1996). HL 21.6 Tue 17:45 BEY 118 Full non-equilibrium quantum transport theory of highscattering semiconductor devices — •Tillmann Kubis, Alexandros Trellakis, and Peter Vogl — Walter Schottky Institut, Technische Universität München, Am Coulombwall 3, 85748 Garching We present fully self-consistent non-equilibrium Green’s function (NEGF) calculations for semiconductor heterostructures in a regime where multiple scattering and quantum effects such as interference, carrier confinement and carrier capture must be treated on an equal footing. We have implemented non-local energy- and momentum-dependent scattering self-energies in the self-consistent Born approximation for charged impurities, as well as acoustic and polar optical phonons. Electronelectron interaction is included via the self-consistent Hartree potential. This ab-initio implementation of quantum transport theory allows us to predict carrier dynamics in any kind of semiconductor nanodevice, ranging from simple n-i-n resistors to complex multi-quantum well structures. We present the I-V characteristics, the charge density and potential profile of a 12nm InGaAs quantum well that illustrates carrier capture into multiple bound states/resonances and compare the results with ballistic and quantum drift diffusion models. HL 21.7 Tue 18:00 BEY 118 Source switching in an electron Y-branch switch — •Stefan Lang, David Hartmann, Lukas Worschech, and Alfred Forchel — Technische Physik, Universität Würzburg, Am Hubland, 97074 Würzburg, Germany We have observed gate controlled source switching in an electron Ybranch switch. The Y-branched nanojunctions were realized on the basis of a modulation doped $GaAs/AlGaAs$ heterostructure and are controlled by four side-gates. Gate voltage up-sweeps at one side-gate firstly open the first source channel. Then the second source-branch becomes conductive and above a critical gate voltage the first source is pinchedoff. This leads to a peak in the current-voltage characteristic of the first source-branch. We have modeled the source switching taking into account gating as well as selfgating of the Y-branch nanojunction.
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