Bulletin of the American Physical Society
APS March Meeting 2013
Volume 58, Number 1
Monday–Friday, March 18–22, 2013; Baltimore, Maryland
Session Y23: Semiconductors: Thermodynamic & Optical Properties I |
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Sponsoring Units: FIAP Chair: Andre Sushkov, University of Maryland Room: 325 |
Friday, March 22, 2013 8:00AM - 8:12AM |
Y23.00001: Temperature Dependence of Band Gaps in Semiconductors: Electron-Phonon Interaction J.S. Bhosale, A.K. Ramdas, A. Burger, A. Mu\~{n}oz, A.H. Romero, M. Cardona, R. Lauck, R.K. Kremer A theoretical investigation with \textit{ab initio} techniques of the electron-phonon interaction of semiconductors with chalcopyrite structure and its comparison with modulated reflectivity experiments yield a striking difference between those with (AgGaS$_2$) and without (ZnSnAs$_2$) $d$ electrons in their valence bands. The former exhibit a non-monotonic temperature dependence of the band gaps whose origin is not yet fully understood. The analysis of this temperature dependence with the Bose-Einstein oscillator model\footnote{G\"{o}bel \textit{et. al.} Phys. Rev. B 57, 15183 (1998).} involving two oscillator terms having weights of opposite signs, provides an excellent agreement with the experimental data and correlates well with the characteristic peaks in the phonon density of states associated with the acoustical phonon modes. This work underscores the need for theoretical understanding of the electron-phonon interaction involving $d$ electrons, particularly in \textit{ab initio} investigations. [Preview Abstract] |
Friday, March 22, 2013 8:12AM - 8:24AM |
Y23.00002: ABSTRACT WITHDRAWN |
Friday, March 22, 2013 8:24AM - 8:36AM |
Y23.00003: The Franz-Keldysh effect revisited: Electroabsorption in GaAs including interband coupling and excitonic effects Federico Duque-Gomez, J.E. Sipe We show numerical results for the linear optical absorption of bulk GaAs in the presence of a homogeneous dc electric field. Our approach, based on gauge-invariant nonequilibrium Green functions \footnote{T. Kita and H. Yamashita, J. Phys. Soc. Jpn \textbf{77}, 024711 (2008).}, is suitable for including many-body effects and using realistic band models. We calculate the time evolution of the interband polarization driven by an optical pulse and derive the absorption coefficient from it. The interband effects of the dc field are captured in a matrix transform in the band indices, which satisfies a differential equation solved efficiently in a separate numerical calculation. For the present calculation we have used a 14-band $\mathbf{k} \cdot \mathbf{p}$ model and treated excitonic effects at a Hartree-Fock level. Previous calculations in the independent particle approximation have shown interesting effects of the band structure and the importance of the interband coupling.\footnote{J. K. Wahlstrand and J. E. Sipe, Phys. Rev. B \textbf{82}, 075206 (2010).} We describe the effect of including the Coulomb interaction, which is especially relevant in low temperature and low field experiments. \footnote{A. Jaeger and G. Weiser, Phys. Rev. B \textbf{58}, 10674 (1998).} [Preview Abstract] |
Friday, March 22, 2013 8:36AM - 8:48AM |
Y23.00004: Photoreflectance and Strain Relaxation Studies of Semipolar InGaN Grace Metcalfe, Nathaniel Woodward, Hongen Shen, Michael Wraback, Po Shan Hsu, James Speck Recently, there has been a surge of interest in semipolar nitride material for quantum well devices to reduce or eliminate the quantum confined stark effect due to the strong internal polarization. Studies on the effect of the strain relaxation in semipolar nitrides are critical to the successful development and operation of long wavelength devices such as LEDs and LDs. In general, the wavefunctions associated with the A, B, and C exciton transitions in wurtzite material are mixed for crystal orientations other than c-plane. Therefore, the polarization and energy of these exciton interband transitions within wurtzite nitrides also depend on the strain and crystal orientation. In this paper, we present the effects of partial strain relaxation on the optical properties of a thickness series of semipolar (11-22) and (20-21) InGaN compressively strained to GaN using polarization-dependent photoreflectance (PR) measurements. We observe that the absolute energy of the exciton transition parallel to the c-axis is greater than that perpendicular to the c-axis, and the energy separation between them increases with strain relaxation. Our PR data compares well with strain relaxation measurements taken using X-ray diffraction, as well as with our calculations. [Preview Abstract] |
Friday, March 22, 2013 8:48AM - 9:00AM |
Y23.00005: X-ray induced optical transparency and x-ray/optical photon interactions in GaAs Stephen Durbin, Tim Graber, Rob Henning An intense x-ray synchrotron pulse transforms a thin crystal of GaAs from being opaque to transparency in picoseconds for probe photon energies near the band gap energy. X-ray absorption and subsequent de-excitation processes pump a high density of electrons from the valence band into the conduction band, causing Pauli blocking of the band gap photons and hence their transmission through the bulk of the specimen. Although the GaAs photocarrier lifetime is less than 300 ps, the transmission decay time constant was as large as 2000 ps when the laser intensity was increased, an effect that can be partially understood in terms of photobleaching and the depth of x-ray absorption. Finally, the excess transmission of band gap photons due to high laser intensity could be suppressed by the onset of the x-ray pulse, evidence for x-ray quenching of laser hole burning. These effects are manifestations of x-ray/optical photon interactions mediated by their conduction band excitations in GaAs. [Preview Abstract] |
Friday, March 22, 2013 9:00AM - 9:12AM |
Y23.00006: Exciton absorption of entangled photons in semiconductor quantum wells Ferney Rodriguez, David Guzman, Luis Salazar, Luis Quiroga The dependence of the excitonic two-photon absorption on the quantum correlations (entanglement) of exciting biphotons by a semiconductor quantum well is studied. We show that entangled photon absorption can display very unusual features depending on space-time-polarization biphoton parameters and absorber density of states for both bound exciton states as well as for unbound electron-hole pairs. We report on the connection between biphoton entanglement, as quantified by the Schmidt number, and absorption by a semiconductor quantum well. Comparison between frequency-anti-correlated, unentangled and frequency-correlated biphoton absorption is addressed. We found that exciton oscillator strengths are highly increased when photons arrive almost simultaneously in an entangled state. Two-photon-absorption becomes a highly sensitive probe of photon quantum correlations when narrow semiconductor quantum wells are used as two-photon absorbers. [Preview Abstract] |
Friday, March 22, 2013 9:12AM - 9:24AM |
Y23.00007: Localized-delocalized transitions in GaAsN Kirstin Alberi, Brian Fluegel, Scott Crooker, Daniel Beaton, Aaron Ptak, Angelo Mascarenhas Dilute nitride semiconductors are promising materials for high efficiency multijunction solar cells and light emitting diodes, yet they exhibit an unusual evolution of their optical and electronic properties as they transition from an impurity-doped semiconductor into an alloy upon the addition of N. For example, a significant change in the photoluminescence spectrum of GaAsN is accompanied by a rapid increase in the broadening parameters of the E$_{\mathrm{0}}$ and E$_{\mathrm{1}}$ critical point transitions in electromodulated reflectance spectra as the N concentration is increased from 0.12{\%} N to 0.32{\%} N. We demonstrate that these changes result from the percolation of localized N cluster states bound below the conduction band into fully extended superclusters and the emergence of a mobility edge. Furthermore, photoluminescence studies show that we are able to reverse this localized to delocalized transition through the application of high magnetic fields to 57 tesla. These experimental results provide new insight into the percolation behavior of isoelectronic cluster states in semiconductor alloys. [Preview Abstract] |
Friday, March 22, 2013 9:24AM - 9:36AM |
Y23.00008: A Tunable Terahertz Detector Based On Self Assembled Plasmonic Structure on a GaAs 2DEG Chejin Bae, Deepu George, Rohit Singh, Andrea Markelz To improve detector sensitivity, tunability and remove polarization dependence, we develop the gated grid plasmonic structure on 2DEG by using nanosphere self-assembly lithography. The measured transmission clearly is not following Drude response, but rather has three sharp resonances corresponding fundamental, 3rd, and 5th harmonics of plasmon resonance respectively. Measurements at 80K show a large transmission change of 25{\%}. We also confirmed a magneto plasmon dispersion of this device. In this paper we will discuss the radiative damping effect which affects enhanced absorption at the higher harmonics mode relative to fundamental [1] and inductive grid resonance of this self-assembled plasmonic structure by demonstrating an angular dependence of transmission due to 2D plasmon[2]. [1] V. Popov et al., J. Appl. Phys. \textbf{94}, 3556 (2003) [2] T. W. Ebbesen et al., Nature, \textbf{391}, 667 (1998) [Preview Abstract] |
Friday, March 22, 2013 9:36AM - 9:48AM |
Y23.00009: Electron-hole sound: Observation of coherent acoustic plasmons in photoexcited GaAs Prashant Padmanabhan, Steve Young, Meredith Henstridge, Sishir Bhowmick, Pallab Bhattacharya, Roberto Merlin Three-dimensional multi-component plasmas involving species with very different masses are expected to show a new branch of charge density fluctuations with a frequency dispersion that is linear with respect to the wave vector [1]. Not to be confused with similarly named modes of metallic surfaces [2], these bulk excitations are known as \textit{acoustic plasmons}. In the past, they have been identified in some gas plasmas [3] and, notably, also in electron-hole plasmas in GaAs via spontaneous Raman scattering [4]. Here, we present the first observation of \textit{coherent} acoustic plasmons in photoexcited GaAs. We utilize an ultrafast double pump-probe scheme to probe, in the time domain, the oscillations in the sample reflectivity associated with these modes. Results agree well with theoretical calculations based on the random phase approximation. The data also suggests that the coherent acoustic oscillation is driven by the interaction with modes resulting from the coupling between the longitudinal-optical-phonons and the conventional optical plasmons of the electrons. [1] J. Appel and A. W. Overhauser, Phys. Rev. B 26, 507 (1982). [2] B. Diaconescu, et al., Nature 448, 57 (2007). [3] A. Y. Wong, R. W. Motley, and N. D'Angelo, Phys. Rev. 133, A436 (1964). [4] A. Pinczuk, J. Shah, and P. A. Wolff, Phys. Rev. Lett. 47, 1487 (1981). [Preview Abstract] |
Friday, March 22, 2013 9:48AM - 10:00AM |
Y23.00010: Polarization and Interface Effects on THz Emission from c-plane InGaN/GaN Heterostructures Nathaniel Woodward, Chad Gallinat, Ryan Enck, Grace Metcalfe, Hongen Shen, Michael Wraback Nitride semiconductors have strong piezoelectric and spontaneous polarizations, which, when terminated at a heterointerface, create a large internal electric field. This field enables transport-based THz radiation with intensities comparable to that from conventional contactless semiconductor surface emitters such as InAs. We observed THz emission from 200-nm thick c-plane InGaN coherently strained to various doped GaN substrates due to photocarrier acceleration toward the surface in the field resulting from the polarization charge at the InGaN/GaN interface. We compare THz emission from the samples pumped from the substrate side as well as the epilayer side such that diffusive and polarization field-induced transport were in the same and opposite directions, respectively. When pumped from the substrate side, we observed several spectral features that did not appear when pumping the InGaN surface. These features may be attributed to effects from the InGaN/GaN heterointerface. [Preview Abstract] |
Friday, March 22, 2013 10:00AM - 10:12AM |
Y23.00011: Helicity-dependent photocurrent in a (110) GaAs quantum well stack D.C. Schmadel, M.-H. Kim, A.B. Sushkov, G.S. Jenkins, J.D. Koralek, J.E. Moore, J. Orenstein, Yuzo Ohno, Hideo Ohno, H.D. Drew There have been many reports on the circular photogalvanic effect (CPGE) in GaAs quantum wells. A recent theoretical study suggests that the CPGE can be governed by a quantum confinement-induced Berry phase effect that depends only on the quantum-well width and crystal orientation (J.E. Moore, Phys. Rev. Lett. 2010). We have measured the photocurrent in a (110)-oriented GaAs quantum well stack under illumination of circularly polarized THz radiation. We will report measurements of the helicity-driven photocurrent as a function of frequency, polarization, angle of incident, and temperature, and compare with theoretical predictions of the Berry phase contribution. [Preview Abstract] |
Friday, March 22, 2013 10:12AM - 10:24AM |
Y23.00012: Direct-Indirect Crossover in Ga$_{\mathrm{x}}$In$_{\mathrm{1-x}}$P Alloys Angelo Mascarenhas, Kirstin Alberi, Brian Fluegel Advances in metamorphic growth of high quality Ga$_{\mathrm{x}}$In$_{\mathrm{1-x}}$P ($x$ \textgreater\ 0.5) on GaAs substrates have improved the practicality of using these alloys in visible light emitting diodes and lasers. The wavelength range over which these materials are efficient light emitters is determined by the direct-indirect crossover energy, yet considerable discrepancies still remain in the literature regarding the precise crossover composition, $x_{C}$. We revisit this topic and present new experimental results that precisely pinpoint the crossover composition without extrapolation of the direct and indirect bandgap trends. Observation of concurrent yet distinct direct and indirect transitions in the 2 K time integrated and time resolved photoluminescence spectra of disordered Ga$_{0.719}$In$_{0.281}$P films places the crossover very near the composition $x_{C} =$ 0.71. This revised value is critical for facilitating realistic engineering of Ga$_{\mathrm{x}}$In$_{\mathrm{1-x}}$P alloys for light emitting and photovoltaic applications. [Preview Abstract] |
Friday, March 22, 2013 10:24AM - 10:36AM |
Y23.00013: Temperature Dependent Dielectric Functions of MBE-grown GaMnAs Thin Films F.C. Peiris, Z.J. Weber, N. Mandel, T. Scully, X. Liu, J.K. Furdyna Spectroscopic ellipsometry was used to measure the dielectric functions of a series of Ga$_{\mathrm{1-x}}$Mn$_{\mathrm{x}}$As samples from 20 K to 300 K. Initially, by modeling the ellipsometric data in the transparent region, the film thickness and the index of refraction of Ga$_{\mathrm{1-x}}$Mn$_{\mathrm{x}}$As alloys were obtained. Extending the analysis into the absorption region, the dielectric function for the entire spectral range between 0.6 eV and 6.5 eV was determined. Monitoring the temperature dependence of the critical points, corresponding to electronic transitions in the Brillouin zone, we deduced the electron-phonon coupling parameters using Bose-Einstein occupation distributions. In comparison to GaAs, we find that the ternary alloy Ga$_{\mathrm{1-x}}$Mn$_{\mathrm{x}}$As shows a slight enhancement in its electron-phonon coupling. [Preview Abstract] |
Friday, March 22, 2013 10:36AM - 10:48AM |
Y23.00014: Enhanced THz emission from stacked $+c$-plane InGaN/GaN heterostructures grown by plasma-assisted molecular beam epitaxy Chad Gallinat, Nathaniel Woodward, Ryan Enck, Grace Metcalfe, Hongen Shen, Michael Wraback We have previously demonstrated THz emission in a single, fully-strained 200 nm InGaN layer grown on GaN. This emission was due to the acceleration of electrons toward the surface in the piezoelectric polarization charge-induced electric field. We observed a reduction in the THz emission from a fully relaxed InGaN layer where the piezoelectric polarization was removed. In order to increase the InGaN layer thickness to maximize the absorption of the excitation pulse, we introduced GaN spacers to limit strain relaxation. We observed an increase in THz emission strength from samples with three stacks of coherently strained 100 nm/10 nm InGaN/GaN layers over the emission from single layer structures. We explored the balance of In alloy content, InGaN layer thickness and InGaN layer strain to maximize the piezoelectric polarization for enhanced THz emission. [Preview Abstract] |
Friday, March 22, 2013 10:48AM - 11:00AM |
Y23.00015: Optical spectroscopies of materials from orbital-dependent approximations Ismaila Dabo, Andrea Ferretti, Matteo Cococcioni, Nicola Marzari Electronic-structure calculations based upon density-functional theory (DFT) have been fruitful in diverse areas of materials science. Despite their exceptional success and widespread use, a range of spectroscopic properties fall beyond the scope of existing DFT approximations. Failures of DFT calculations in describing electronic and optical phenomena take root in the lack of piecewise linearity of approximate functionals. This known deficiency reverberates negatively on the spectroscopic description of systems involving fractionally occupied or spatially delocalized electronic states, such as donor-acceptor organic heterojunctions and heavy-metal organometallic complexes. In this talk, I will present a class of orbital-dependent density-functional theory (OD-DFT) methods that are derived from a multidensity formulation of the electronic-structure problem and that restore the piecewise linearity of the total energy via Koopmans' theorem. Such OD-DFT electronic-structure approximations are apt at describing full orbital spectra within a few tenths of an electron-volt relative to experimental photoemission spectroscopies and with the additional benefit of providing appreciably improved total energies for molecular systems with fractional occupations. [Preview Abstract] |
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