Bulletin of the American Physical Society
APS March Meeting 2014
Volume 59, Number 1
Monday–Friday, March 3–7, 2014; Denver, Colorado
Session J45: Semiconductor Electronic Structure: Thermodynamic & Optical Properties |
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Sponsoring Units: FIAP Room: Mile High Ballroom 4D |
Tuesday, March 4, 2014 2:30PM - 2:42PM |
J45.00001: Optical properties of AlGaAsSb thin films lattice-matched to InP(100) Sukgeun Choi, Glen Hillier, Jessica Adams AlGaAsSb quaternary compounds lattice-matched to InP are of interest for applications in InP-based high-efficiency multi-junction solar cells and surface normal opto-electronic devices operating in the wavelength range from 1.3 to 1.55 $\mu $m. Knowledge of optical properties of constituent layers in photonic and photovoltaic devices plays an important role in designing the device structure and modeling the device performance. However, only a limited number of theoretical and experimental studies have been done on AlGaAsSb lattice-matched to either GaSb or InAs, and no systematic optical study is available for AlGaAsSb lattice-matched to InP. Here we apply spectroscopic ellipsometry to study the optical properties of AlGaAsSb thin films grown by MOVPE on InP substrates. The fundamental optical functions such as dielectric function, refractive index, reflectivity, and absorption coefficient are determined by modeling the data. Energies for several critical points and their compositional dependence are obtained from standard lineshape analysis. The results from our study can be used (1) to improve our understanding of the electronic structure of AlGaAsSb and related compounds and (2) to provide optical information for the design of InP-based multi-junction solar cell structures. [Preview Abstract] |
Tuesday, March 4, 2014 2:42PM - 2:54PM |
J45.00002: Precise Determination of the Direct-Indirect Band Gap Energy Crossover In Al$_{\mathrm{x}}$Ga$_{\mathrm{1-x}}$As Brian Fluegel, Daniel Beaton, Kirstin Alberi, Angelo Mascarenhas Al$_{\mathrm{x}}$Ga$_{\mathrm{1-x}}$As is a technologically important semiconductor material system for optoelectronic applications due to its type I band alignment with GaAs under nearly lattice-matched conditions. Heterostructure design often relies on exactly controlling the relative positions of the $\Gamma $ and X conduction band edges, yet despite over three decades of research on this alloy, the precise energy and composition of the direct-indirect band gap crossover is still not well resolved. We report the results of our most recent investigation of Al$_{\mathrm{x}}$Ga$_{\mathrm{1-x}}$As (0.28 \textless $x$\textless 0.42) epitaxial films, in which the observation of concurrent photoluminescence (PL) emission peaks from the direct and indirect band gaps combined with time-resolved PL information yields a precise determination of the direct-indirect band gap crossover energy and composition. [Preview Abstract] |
Tuesday, March 4, 2014 2:54PM - 3:06PM |
J45.00003: Anisotropic optical properties of Fe/GaAs nanolayers from first principles Sebastian Putz, Martin Gmitra, Jaroslav Fabian We investigate the anisotropy of the optical properties of few-monolayer Fe films on GaAs from first principles calculations. Both intrinsic and magnetization-induced anisotropy are covered by studying the systems in the absence or presence of external magnetic fields. We use the linearized augmented plane wave (LAPW) method, as implemented in the WIEN2k density functional theory code, to show that the $C_{2v}$ symmetric anisotropy of the spin-orbit coupling fields at the Fe/GaAs interface manifests itself in an analogous anisotropy of the optical properties of the system, such as its optical conductivity and its reflectivity. We find that the optical properties vary significantly when the direction of the external magnetic field is changed. This suggests that the anisotropic spin-orbit coupling fields in experimentally relevant Fe/GaAs slabs can be studied by purely optical means. [Preview Abstract] |
Tuesday, March 4, 2014 3:06PM - 3:18PM |
J45.00004: In situ tuning biexciton antibinding-binding transition and fine structure splitting through hydrostatic pressure in single InGaAs quantum dots Hai Wei, Xuefei Wu, Xiuming Dou, Kun Ding, Ying Yu, Haiqiao Ni, Zhichuan Niu, Yang Ji, Shushen Li, Desheng Jiang, Guang-can Guo, Lixin He, Baoquan Sun We demonstrate that the exciton and biexciton emission energies as well as exciton fine structure splitting (FSS) in single (In,Ga)As/GaAs quantum dots (QDs) can be efficiently tuned using hydrostatic pressure \textit{in situ} in an optical cryostat at up to 4.4 GPa. The maximum exciton emission energy shift is up to 380 meV, and the FSS is up to 150 $\mu$eV. We successfully produce a biexciton antibinding-binding transition in QDs, which is the key experimental condition that generates color- and polarization-indistinguishable photon pairs from the cascade of biexciton emissions and that generates entangled photons via a time-reordering scheme. We also perform the atomistic pseudopotential calculations on realistic (In,Ga)As/GaAs QDs to understand the physical mechanism underlying the hydrostatic pressure-induced effects. [Preview Abstract] |
Tuesday, March 4, 2014 3:18PM - 3:30PM |
J45.00005: X-ray and optical pulse interactions via electron trapping in GaAs Stephen Durbin, Shih-Chieh Liu, Anthony DiChiara, Robert Henning A highly excited state of GaAs is created by the absorption of an extremely intense focused 80 ps pulse of hard x-rays at the Advanced Photon Source synchrotron. This state is probed by 2 ps laser pulses with photon energies near the semiconducting band gap, which has previously revealed x-ray induced optical transparency. Two unexpected results are found: x-ray induced luminescence is dramatically enhanced when a high intensity laser pulse precedes the x-ray pulse, and the decay of the induced transparency becomes much slower when the intensity of the subsequent probe laser is increased. Both results require that energy be stored in GaAs by the first pulse, and then released by the second pulse. We describe how this can be explained by electron trapping centers in GaAs with trapping lifetimes of a few nanoseconds. We compare these results with lifetime measurements of other excitations produced by ultrafast optical absorption. We also show how minor improvements in focusing will lead to single-pulse x-ray induced temperature jumps of thousands of Kelvin, allowing new x-ray excited dense matter states to be explored. [Preview Abstract] |
Tuesday, March 4, 2014 3:30PM - 3:42PM |
J45.00006: Individually Contacted Electron-Hole Bilayers of InAs/GaSb Ruiyuan Liu, Lingjie Du, Rui-Rui Du, Gerald Sullivan Electron-hole bilayers made of InAs/GaSb semiconductors are promising quantum structures in realizing novel condensed phases of excitons. Using low temperature transport we have measured a InAs/GaSb composite quantum well with a AlGaSb tunneling barrier between the layers, and have been able to adjust the Fermi energy of the electron or hole layers independently by double gates. In order to study the interactions between the two layers, we processed devices with a flip-chip technique, where gates were placed on both sides of the wafer within a micrometer distance from respective layers. Additional gates placed on top of the contact leads to facilitate independent contacts to the individual layer. We will present preliminary data for standard and flip-chip devices measured by low temperature transport. The work in Rice is supported by a grant of DOE-BES. [Preview Abstract] |
Tuesday, March 4, 2014 3:42PM - 3:54PM |
J45.00007: Mechanism of excitonic dephasing in layered InSe crystals D. Karaiskaj, P. Dey, J. Paul, N. Glikin, Z. Kovalyuk, Z. Kudrynskyi, A. Romero The dephasing and lifetime of excitons in InSe layered crystals has been carefully measured using three pulse four-wave mixing and two-dimensional Fourier transform (2DFT) spectroscopy. We obtain a complete and detailed picture of the mechanism of excitonic dephasing in this layered material. The temperature dependence provides a detailed description of the phonon-exciton interactions and the zero Kelvin limit of the homogeneous linewidth. The excitation density dependence reveals strong excitation induced dephasing due to exciton-exciton scattering. [Preview Abstract] |
Tuesday, March 4, 2014 3:54PM - 4:06PM |
J45.00008: Exploring layered GaSe crystals with 2DFT spectroscopy P. Dey, J. Paul, D. Karaiskaj, Z. Kovalyuk, Z. Kudrynskyi The dephasing and lifetime of excitons and biexcitonic effects in GaSe layered crystals have been carefully measured using three pulse four-wave mixing and two-dimensional Fourier transform (2DFT) spectroscopy. Strong biexciton signatures are observed in the 2DFT spectra. Excitation density and temperature dependence 2DFT spectra provide new insights into the many-body interactions in this material. [Preview Abstract] |
Tuesday, March 4, 2014 4:06PM - 4:18PM |
J45.00009: Electronic Structure of Ge$_{1-y}$Sn$_{y}$ and Ge$_{1-x-y}$Si$_{x}$Sn$_{y}$ Alloys from Optical and Electro-Optical Measurements James Gallagher, Charutha Senaratne, Chi Xu, Liying Jiang, Doug Bopp, John Kouvetakis, Jose Menendez Optical transitions in Ge$_{1-y}$Sn$_{y}$ and Ge$_{1-x-y}$Si$_{x}$Sn$_{y}$ alloys have been studied in detail using spectroscopic ellipsometry, photocurrent, and photoluminescence experiments on films grown on Si, Ge, and Ge-buffered Si platforms using CVD and gas-source MBE reactions of germanes, silanes, and deuterated stannane. The compositional, $x$ and $y$, dependence of the lowest direct and indirect band gaps, as well as other transitions, are determined through these techniques. This has enabled mapping the direct-indirect gap crossover in composition space to reveal the potential of these alloys for optoelectronic applications via band gap engineering. All the measured transition energies can be described by second-order polynomials as functions of composition whose quadratic coefficients (bowing parameters) show systematic chemical trends. The transferability of these parameters between binary and ternary alloys is studied in detail. Due to a larger negative bowing for the direct than for the indirect gap, the crossover to direct gap behavior occurs for Sn concentrations much lower than predicted from a simple linear interpolation between the corresponding elemental semiconductors. [Preview Abstract] |
Tuesday, March 4, 2014 4:18PM - 4:30PM |
J45.00010: Structure band-gap correlations in semiconductors: Implications for computational band gap prediction Guenter Schneider, David H. Foster Large scale structure prediction for novel materials requires computationally inexpensive lattice relaxation methods, which are typically based on density functional theory (DFT) using a semi-local approximation for the exchange-correlation functional. These methods provide structural parameters accurate to within a few percent, but cannot predict band-gaps. Band-gap calculations, require much more computationally expensive methods such as hybrid functionals or the GW approximation. Such an accuracy-tiered method fails dramatically for Cu3PSe4. When the generalized gradient approximation (GGA) is used to relax the lattice and ions, band-gaps calculated using both the single shot GGA+GW method and the Heyd-Scuseria-Ernzerhof (HSE) hybrid functional method are a full 0.5 eV lower than the band gaps calculated for the unrelaxed, experimental structure. The GW and HSE methods predict accurate band gaps only when used with the correct experimental structure. We show that in Cu3PSe4, the calculated band-gap depends strongly on the P-Se bondlength, which can be explained by the P-Se* anti-bonding character of the lowest conduction band state. We show this effect for different lattice relaxation methods including recently developed meta-GGAs. [Preview Abstract] |
Tuesday, March 4, 2014 4:30PM - 4:42PM |
J45.00011: Carrier lifetimes in group-IV semiconductors Nandan Tandon, L.R. Ram-Mohan We have demonstrated that electron-phonon coupling in semiconductors shows a variation over the Brillouin zone and is not constant as considered traditionally within the long wavelength approximation. In group IV semiconductors, the variation in the electron-phonon coupling can range from $50-400$~meV. We evaluate electron-phonon coupling matrix elements over the entire Brillouin zone and use this information to calculate the electron lifetimes in group IV semiconductors C, Si, and Ge. Our results, within the framework of the single particle approximation, allow us to evaluate the lifetimes for different initial electron momenta. For our calculations, we have used the methodolgy developed by Giustino et. al for evaluating the electron-phonon coupling employing the Wannier-Fourier interpolation (Phys. Rev. B {\bf 76}, 165108 (2007)). [Preview Abstract] |
Tuesday, March 4, 2014 4:42PM - 4:54PM |
J45.00012: All-electron GW quasiparticle band structures of group 14 nitride compounds Iek-Heng Chu, Anton Kozhenikov, Thomas Schulthess, Hai-Ping Cheng We have investigated the group 14 nitrides (M$_{3}$N$_{4})$ in both the spinel phase (with M$=$C, Si, Ge and Sn) and the beta phase (with M$=$Si, Ge and Sn) using density functional theory (DFT) with the local density approximation (LDA). The Kohn-Sham energies of these systems are first calculated within the framework of full-potential LAPW and then corrected using single-shot G$_{0}$W$_{0}$ calculations, which we have implemented in the Exciting-Plus code. Direct bands gap at the $\Gamma $ point are found for all spinel-type nitrides. The calculated band gaps of Si$_{3}$N$_{4}$, Ge$_{3}$N$_{4}$ and Sn$_{3}$N$_{4}$ agree with experiment. We also find that for all systems studied, our GW calculations with and without the plasmon-pole approximation give very similar results, even when the system contains semi-core 3d electrons. These spinel-type nitrides are novel materials for potential optoelectronic applications. [Preview Abstract] |
Tuesday, March 4, 2014 4:54PM - 5:06PM |
J45.00013: Attosecond time-resolved studies of band-gap excitations in semiconductors Krupa Ramasesha, Martin Schultze, Stephen Leone, Daniel Neumark Attosecond time-resolved spectroscopy is an emerging technique that has proven to be powerful in investigating electron dynamics in atoms, molecules and dielectrics. Attosecond transient absorption spectroscopy is used to follow electron dynamics in semiconductors. In these experiments, a few-cycle visible pulse excites electrons across the band gap of a semiconductor, followed in time by an attosecond extreme ultraviolet pulse to probe changes to the core level transitions, which report on the electronic structure of the material. Using the few-cycle strong field visible pulse to excite the direct band gap of silicon and an attosecond pulse to probe the Si L$_{\mathrm{2,3}}$ edge, we have resolved ultrafast carrier generation in the conduction band as well as band gap renormalization. These experiments have revealed transient shifts and broadening to the L$_{\mathrm{2,3}}$ edge of Si on a few femtosecond timescale, reflecting an instantaneous response of the electronic structure to ultrafast excitation. The studies will be extended to probe electron dynamics in heteroatom semiconductors such as copper oxide and three-band materials such as tellurium-doped zinc oxide. [Preview Abstract] |
Tuesday, March 4, 2014 5:06PM - 5:18PM |
J45.00014: Dipoles in III-V MOSFETs: Scattering and Threshold shifts Raj Jana, Debdeep Jena A scattering mechanism arising from the charge dipoles at the oxide/III-V semiconductor interface with different crystallographic orientations is identified. We quantitatively evaluate the effect of interface charge dipoles with angular distributions taking into account the effect of crystallographic orientations on electron transport in semiconductor channels of III-V field-effect transistors. The time-independent distribution of dipoles leads to two effects relevant to transistor operation. They cause shifts in the threshold voltage of III-V MOSFETs. The dipoles also scatter conducting charges at the III-V/oxide interface due to their long-range Coulomb potential. The dipole-scattering-limited mobility decreases with increasing dipole parameters such as the dipole length and dipole density, and dipole angle. Higher electron mobility is obtained for aligned dipoles relative to the angular-oriented random dipoles at the interface. A smaller threshold voltage shift is generated for angular-oriented dipoles over the aligned dipoles at the interface. The charge dipole scattering mechanism can be applied to ALD/InGaAs MOSFETs and also to ALD/Nanowire FETs. [Preview Abstract] |
Tuesday, March 4, 2014 5:18PM - 5:30PM |
J45.00015: High resolution EELS study of novel semiconductor alloys: Ge$_{1-x-y}$Si$_{x}$Sn$_{y}$ and AlPSi$_{3}$ Liying Jiang, Toshi Aoki, John Kouvetakis Metastable alloys play a fundamental role in modern semiconductor science and technology as a major tool for band gap and strain engineering. When these alloys incorporate highly dissimilar materials, such as Si and Sn in Ge$_{1-x-y}$Si$_{x}$Sn$_{y}$ alloys or III-V pairs in group-IV matrices, as in the new (III-V)$_{x}$(IV)$_{5-2x}$ systems synthesized by our group, the atomic distribution at the sub-nanometer scale is of paramount concern, since even slight deviations from randomness or from predicted ordered structures can have a dramatic impact on the electronic structure. Aberration-corrected microscopes provides the opportunity to generate atom-selective images with unprecedented structural and chemical detail. For this work, we used Electron Energy Loss Spectroscopy (EELS) to map the Sn distribution in Ge$_{1-y}$Sn$_{y}$ and Ge$_{1-x-y}$Si$_{x}$Sn$_{y}$ alloys, as well as the distribution of Al and P atoms in AlPSi$_{3}$ to elucidate local bonding configurations and atom substitutionality. The EELS measurements also provide information on the electronic structure, which is compared with optical results and theoretical calculations. [Preview Abstract] |
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