Bulletin of the American Physical Society
2007 APS March Meeting
Volume 52, Number 1
Monday–Friday, March 5–9, 2007; Denver, Colorado
Session Y40: Semiconductor Spectroscopy |
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Sponsoring Units: FIAP Chair: Hans-Peter Wagner, University of Cincinnati Room: Colorado Convention Center 503 |
Friday, March 9, 2007 11:15AM - 11:27AM |
Y40.00001: Stress-Induced Shifts of the Photoluminescence and Raman Peaks in Al$_{x}$Ga$_{1-x}$As Observed as a Function of Al Composition Grady White, Albert Paul, Kris Bertness In Al$_{x}$Ga$_{1-x}$As, both photoluminescence (PL) and Raman peak positions are strongly sensitive to x, a fact that has made PL a primary tool for monitoring composition in the optoelectronics industry. However, the peak positions also depend upon stress. Because layered thin film systems inherently experience residual stresses, use of PL or Raman measurements without compensation for existing stresses limits the accuracy of composition determination. We present results of both PL and Raman measurements of Al$_{x}$Ga$_{1-x}$As films as a function of biaxial tensile stress for 0 $\leq $ x $\leq $ 0.9, comparing the sensitivity of the stress-induced shifts with the composition-induced shifts and, thereby, providing an estimate of the uncertainties associated with composition determination. We also discuss an upper limit to the composition sensitivity of the phonon deformation potentials of Al$_{x}$Ga$_{1-x}$As. [Preview Abstract] |
Friday, March 9, 2007 11:27AM - 11:39AM |
Y40.00002: Ballistic carrier injection induced electroluminescence of InAs quantum dots in a hot-electron metal-base transistor Wei Yi, Venkatesh Narayanamurti, Joshua Zide, Seth Bank, Arthur Gossard Utilizing hot electrons ballistically injected by a tunnel junction over the Schottky barrier into a semiconductor collector, ballistic electron emission microscopy (http://www.deas.harvard.edu/venky/pdffiles/29.pdf) BEEM characterizes carrier filtration through buried interfaces with nanometer spatial resolution. Under forward collector bias, interband light emission may occur in a heterostructure collector by injection of minority carriers with sub-bandgap kinetic energies. Such a concept, ballistic electron emission luminescence, is tested using a hot-electron metal-base transistor, the solid-state prototype of BEEM. The heterostructure collector with embedded InAs quantum dots (QDs) is grown on p-GaAs substrate. Either majority carriers (holes) or minority carriers (electrons) are injected into the collector under different collector bias. Light emission from InAs QDs, InAs wetting layer, and bulk GaAs are observed in concert with minority carrier injection. [Preview Abstract] |
Friday, March 9, 2007 11:39AM - 11:51AM |
Y40.00003: Time-resolved x-ray diffraction studies of the thermal properties of AlGaAs Y.M. Sheu, S.H. Lee, D.M. Fritz, J.K. Wahlstrand, M. Reason, R.S. Goldman, D.A. Reis We report on studies of thermal properties of epitaxial AlGaAs on GaAs using an ultrafast laser-excitation and x-ray probing technique. Femtosecond laser pulses were used to excite the material at the buried interface. Heat is generated by electron-hole recombination in the GaAs substrate and diffuses into the AlGaAs film. Following the initial heating the film and substrate cool as heat diffuses into the bulk. Time resolved X-ray diffraction (TRXD) is sensitive to precise~ lattice movement and can resolve the time evolution of the material as it reaches thermal equilibrium. We show that TRXD has the potential to study thermal properties such as the thermal conductivity and thermal boundary resistance in hermoelectric materials. [Preview Abstract] |
Friday, March 9, 2007 11:51AM - 12:03PM |
Y40.00004: Polariton Condensation in CdTe Microcavities: the phase diagram. Marzena Szymanska, Jacek Kasprzak, Jonathan Keeling, Francesca Marchetti, Regis Andre, Peter Littlewood, Daniel Le Si Dang The first realisation of a polariton condensate has been very recently achieved in a CdTe microcavity [Kasprzak et al., Nature 443, 409 (2006)]. The direct comparisons with theoretical calculations reveal crucial information about the nature of the condensed phase for such composite light-matter particles. In particular, we compare the phase boundaries obtained experimentally, for different values of detuning and cryostat temperature, with those evaluated theoretically using a model which takes into account features of microcavity polaritons such as their reduced dimensionality, their internal composite structure, the disorder in the quantum wells, the polariton-polariton interaction, and their finite lifetime. [Preview Abstract] |
Friday, March 9, 2007 12:03PM - 12:15PM |
Y40.00005: The parametric oscillation threshold of semiconductor microcavities Michiel Wouters, Iacopo Carusotto Since its first experimental observation a few years ago, triply resonant optical parametric oscillation (OPO) in continuously pumped semiconductor microcavities in the strong coupling regime has attracted a lot of attention not only in view of applications to integrated nonlinear optics devices, but also from the point of view of fundamental physics. A most important property is in both cases the behaviour of the OPO around the threshold: because of the third-order nature of the excitonic nonlinearity, a much richer variety of behaviours can be observed as a consequence of the interplay of parametric oscillation and optical bistability effects. In the present talk, we classify the different behaviours in terms of the continuous or discontinuous switching to the OPO state, akin to respectively a second or a first order phase transition, and we characterize the properties of the quantum fluctuations as the threshold is approached as a function of the pump, signal and idler frequencies. [Preview Abstract] |
Friday, March 9, 2007 12:15PM - 12:27PM |
Y40.00006: Single-photon nonlinearity of a semiconductor quantum dot in a cavity Frank Bello, Daniele Sanvitto, Fabrice Laussy, Paolo Guimaraes, David Whittaker, Maurice Skolnick, A. Tahraoui, P.W. Fry, M. Hopkinson A single atom in a cavity is the model system of cavity quantum electrodynamics (CQED).In the weak coupling regime, where losses exceed the interaction energy between atoms and the cavity mode, irreversible decay of the excitations occurs. Strong coupling, which corresponds to the reversible exchange of energy between the atom and the mode opens up a much wider range of CQED phenomena. We present evidence depicting non-linear effects for a quantum dot embedded within a semiconducting micropillar cavity. Emission spectra show transitions from the strong to weak coupling regime between the n=2 and n=1 photon states. Transforming from a system with weak coupling and low pumping to one with higher pumping, lessens the conditions needed for strong coupling due to the increased number of photons inside the cavity. Good agreement with theoretical calculations using the Jaynes-Cummings ladder model is also shown, along with corresponding transition rates. [Preview Abstract] |
Friday, March 9, 2007 12:27PM - 12:39PM |
Y40.00007: Phase coherent photorefractive effect in ZnMgSe/ZnSe quantum wells using ultrashort light pulses Pradeep Bajracharya, Amin Kabir, Hans - Peter Wagner We report on an efficient exciton resonant phase coherent photorefractive (PCP) effect in ZnMgSe/ZnSe single quantum wells (QWs) using ultrashort light pulses that do not overlap in time. The diffraction efficiency as well as the electron grating dynamics that is responsible the PCP effect is studied in a four-wave-mixing (FWM) configuration using 90 and 30 fs pulses. Experiments with significantly reduced repetition rate of subsequent pulse pairs reveal an electron grating lifetime of greater 10 $\mu $s at 55 K. For spectrally broad 30 fs pulses when both excitons and electron-hole-pairs are excited in the QW the PCP signal is strongly reduced. The PCP signal reappears at higher pulse energies when ZnMgSe barrier excitons are additionally excited. The observation of PCP at incident light intensities as low as 100$\mu $Wcm$^{-2}$ has potential for optical coherence imaging. This work is supported by the National Science Foundation (DMR 0305076). [Preview Abstract] |
Friday, March 9, 2007 12:39PM - 12:51PM |
Y40.00008: In-plane optical anisotropy in self-assembled Ge quantum dots induced by interfacial chemical bonds Chih-Ming Wei, Tzung-Te Chen, Yang-Fang Chen In-plane optical anisotropy has been observed in self-assembled Ge quantum dots (QDs). It is found that the photoluminescence (PL) spectrum polarized along $[110]$ exhibits different features compared to that corresponding to $[1\overline 1 0]$. Besides, the polarized PL spectrum is able to reveal the detailed fine structure much more pronounced than that in unpolarized spectrum. It is shown that the observed optical anisotropy is a result of the inherent property of the type-II band alignment of Ge QDs embedded in Si matrix. The light emission arises from the recombination of electrons and holes across the interface, and it thus reflects the anisotropic nature of the interfacial chemical bonds. The predicted results according to our proposed mechanism have been successfully tested for SiGe/Si multiple quantum wells as well as superlattices. [Preview Abstract] |
Friday, March 9, 2007 12:51PM - 1:03PM |
Y40.00009: Superradiance Of Quantum Dots Michael Scheibner, Thomas Schmidt, Lukas Worschech, Alfred Forchel, Gerd Bacher, Thorsten Passow, Detlef Hommel Quantum dots (QDs) may be considered to form a coupled quantum system if they interact with a common radiation field. By its nature the range of this coupling mechanism is on the order of the radiation wavelength and its signature is a modified radiation rate. Here we analyze the decay time of the photoluminescence emitted from a single layer of self assembled CdSe/ZnSe QDs [1]. We find that the decay time depends on the number of QDs. For example, under (quasi-) resonant excitation conditions the decay time increases as QDs are removed from the sample by etching mesas. This indicates that in the as grown sample QDs radiate cooperatively. The range of this interaction is shown to be at least 150 nm. [1] Scheibner, et al. `Superradiance of Quantum Dots', submitted for publication [Preview Abstract] |
Friday, March 9, 2007 1:03PM - 1:15PM |
Y40.00010: Excitonic effects in optical absorption spectra of CdTe. Kalum Palandage, Gayanath Fernando, Rampi Ramprasad We have used a first principles, quasiparticle, self-consistent GW and similar approximations to predict the electronic structure of various nanocrystals with high accuracy. In this preliminary stage, we applied the method to a selection of different classes of materials including alkali metals, Transition metals and semiconductors. It was observed that the self-consistency improves the agreement with experiment. Our goal is to analyze dynamical signatures of excitons and multi-excitons in CdTe nanocrystals using several self-consistent approximations. [Preview Abstract] |
Friday, March 9, 2007 1:15PM - 1:27PM |
Y40.00011: Photoluminescence properties of CdTe/CdSe core-shell type-II quantum dots Chun-Hsiung Wang, Tzung-Te Chen, Kee-Wee Chen, Yang-Fang Chen We report investigations on the optical properties of type-II CdTe/CdSe core-shell quantum dots. By varying the core size, we provide an elegant way to verify that the detected emission signal indeed arises from type-II band alignment. The photoluminescence (PL) peak energy increases with a third root of the excitation power. Both of the PL peak energy and linewidth exhibit unique temperature dependence. All these observations can be rationalized by the band bending effect resulting from the spatially separated photo-excited carriers in a type-II band alignment. [Preview Abstract] |
Friday, March 9, 2007 1:27PM - 1:39PM |
Y40.00012: The Virtual Scanning Tunneling Microscope: A Novel Probe Technique for Imaging Two-Dimensional Electron Systems Adam Sciambi, Kathryn Todd, David Goldhaber-Gordon, Seth Bank, Arthur Gossard We propose a novel probe technique, the virtual scanning tunneling microscope (VSTM), which could provide both spatial and spectroscopic information about two-dimensional electron systems (2DESs) in semiconductor heterostructures. The VSTM's innovation is the addition of a second 'probe' 2DES separated by a low barrier from the sample 2DES below. Simulations show that a positively-biased tip held above the sample surface can diminish the interlayer barrier and induce tunable tunneling between the two 2DESs. If the tip is scanned, the tunneling region will follow below, acting as a virtual tip while screening the true tip from the sample 2DES. This probe technique is motivated by interesting local 2DES physics that can only be studied indirectly because of the depth of 2DESs; we describe a range of predicted spatially-organized phases of 2D electrons, and transport properties of ErAs self-assembled quantum dots, which could be accessed with this new probe. We also present preliminary experimental results from a GaAs/AlGaAs bilayer 2DES sample, supporting the results of the simulation. [Preview Abstract] |
Friday, March 9, 2007 1:39PM - 1:51PM |
Y40.00013: A non-contact system for spatial mapping of carrier lifetimes Jamiyanaa Dashdorj, Reuben Collins, Steven Johnston, Bhushan Sopori A novel approach to spatially resolved, non-contact, carrier lifetime mapping using resonance-coupled photoconductivity decay (RCPCD) was developed. Here, a pulsed laser excites a sample and the resulting transient is detected using a resonantly coupled antenna. The key to the present system is a patch antenna, which operates at 425 MHz and was designed using the EM simulation software, HFSS. By providing a uniform, high sensitivity detection area, the antenna only requires tuning to be performed once, after initial placement of the sample. Using this antenna, a fully computerized system has been developed for scanning samples, acquiring transients, and automatically extracting lifetimes. The present resolution is 0.25 mm, although there is no real limit on this. The new system was successfully applied to the evaluation of polycrystalline silicon wafers. In addition, effects of surface passivation on lifetime was determined. This work was supported by NREL Award {\#} KXEA-3-33607-17. [Preview Abstract] |
Friday, March 9, 2007 1:51PM - 2:03PM |
Y40.00014: Origin of a Localized Vibrational Mode in a GaSb Substrate With a MBE-grown ZnTe Epilayer A. K. Ramdas, Hyunjung Kim, E. Tarhan, G. Chen, M. Dean Sciacca, R. L. Gunshor In the infrared spectrum of a MBE-grown ZnTe epilayer grown on GaSb, a localized vibrational mode (LVM) is observed with a remarkable fine structure. On the basis of the Zn and Te deposited on the GaSb substrate during the MBE growth of ZnTe, it is deduced that $^{64}$Zn, replacing Sb substitutionally as an anti-site impurity, is responsible for the LVM. The fine structure can then be interpreted in terms of the infrared active modes of a XY$_{4}$ quasimolecule, X$\equiv$$^{64}$Zn and Y$\equiv$$^{69}$Ga and $^{71}$Ga occupying the nearest neighbor sites, reflecting all the possible combinations and permutations as well as their natural isotopic abundance. [Preview Abstract] |
Friday, March 9, 2007 2:03PM - 2:15PM |
Y40.00015: Valley-splitting in strained Silicon quantum wells on a miscut substrate using tight-binding model. Neerav Kharche, Marta Prada, Timothy Boykin, Gerhard Klimeck The splitting of the two-fold degeneracy of conduction-band valleys in bi-axially strained SiGe/Si/SiGe quantum wells (QWs) grown on (001) and $2^o$ miscut substrates is computed as a function of magnetic field using semi-empirical nearest-neighbour sp$^3$d$^5$s$^*$ tight-binding model in NEMO-3D. Unlike flat QWs, miscut QWs show two degenerate valleys centred at $(k_x,k_y)=(\pm k_x^0,0)$. Interaction between these two valleys due to confinement in the lateral (perpendicular to growth) dimension, leads to valley splitting (VS) in miscut quantum wells. VS in miscut QWs is suppressed by at least two orders of magnitude as compared to flat QWs. Numerical calculations with perfect step ordering underestimate experimentally observed VS. Simulation of experimental non-idealities such as step-roughness and alloy-disorder raise computed VS to the experimentally observed values. [Preview Abstract] |
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