Bulletin of the American Physical Society
APS March Meeting 2010
Volume 55, Number 2
Monday–Friday, March 15–19, 2010; Portland, Oregon
Session L25: Optical and Electronic Properties of Semiconductors |
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Sponsoring Units: DCMP Chair: Alex Hamilton, University of New South Wales Room: D135 |
Tuesday, March 16, 2010 2:30PM - 2:42PM |
L25.00001: Ellipsometric studies of GaAs$_{1-x}$Bi$_{x}$ (0.00 $\le \quad x \quad \le $ 0.13) alloys S.G. Choi, D.E. Aspnes, A.J. Ptak, R. France, A.G. Norman, D.H. Levi A series of GaAs$_{1-x}$Bi$_{x}$ (0.00 $\le \quad x \quad \le $ 0.13) alloy thin films has been grown by molecular beam epitaxy on GaAs(001). Structural properties were characterized by high-resolution x-ray diffraction and cross-sectional transmission electron microscopy. Above-bandgap optical properties were determined by spectroscopic ellipsometry at room temperature. In order to minimize artifacts from surface overlayers in the measured spectra, \textit{in-situ} wet chemical etching procedures were employed. Major optical structures observed are associated with the $E_{1}$, $E_{1}+\Delta _{1}$, $E$', and $E_{2}$ interband-transition critical-points (CPs). An additional broad optical structure was also found at around 2 eV for high Bi-concentration samples ($x \quad \ge $ 0.04). The CP parameters and their composition-dependences were obtained from the spectral analysis done in reciprocal-space. This abstract is subject to government rights. [Preview Abstract] |
Tuesday, March 16, 2010 2:42PM - 2:54PM |
L25.00002: Measurement of the THz Hall conductivity in a GaAs high mobility two dimensional electron gas A. V. Stier, C. T. Ellis, J. Cerne, B. D. McCombe We investigate the THz Hall conductivity in a high mobility two dimensional electron gas. Motivated by predictions of novel step-like features in the optical Hall conductivity ($\sigma _{xy})$ by Morimoto et.al. (Phys. Rev. Lett. 2009), we measure the THz $\sigma _{xy}$ as a function of top gate voltage and magnetic field (up to 10T) at 2K using polarization modulation techniques (Grayson, Phys. Rev. Lett. 2002). Morimoto et.al. found that localization effects open energy gaps between extended states, producing plateaus in the IR $\sigma _{xy}$ and that should be observable in Faraday effect measurements. We sensitively measure the Faraday rotation using a rotating linear polarizer and the Faraday ellipticity/ circular dichroism using a rotating wave plate in combination with strong THz radiation in the 3-10 meV range from a far-infrared molecular gas laser. Unlike the infrared longitudinal conductivity ($\sigma _{xx})$, which probes the sum of the optical responses for left and right circularly polarized light, the infrared Hall conductivity is proportional to the difference and therefore is sensitive to small changes in symmetry. [Preview Abstract] |
Tuesday, March 16, 2010 2:54PM - 3:06PM |
L25.00003: Terahertz-pulse-induced hot-electron transport in photoexcited GaAs Fuhai Su, Francois Blanchard, Gargi Sharma, Luca Razzari, Ayesheshim Ayesheshim, Tyler Cocker, Lyubov Titova, Tsuneyuki Ozaki, Jean-Claude Kieffer, Roberto Morandotti, Matthew Reid, Frank A. Hegmann We observe self-induced absorption bleaching of intense terahertz (THz) pulses in photoexcited GaAs using optical-pump/THz-probe techniques. The sample is excited using 800nm, 50fs laser pulses and probed collinearly using few-cycle THz pulses with energies up to 0.6$\mu $J. The relative transmission of the THz probe pulse through the photoexcited GaAs increase at high probe fields. A simple dynamic electron transport model demonstrates that the nonlinear response arises from a decrease in electron conductivity due to intervalley electron scattering induced by the THz electric field over sub-picosecond time scales as well as an increase in the intravalley scattering rate. [Preview Abstract] |
Tuesday, March 16, 2010 3:06PM - 3:18PM |
L25.00004: Anisotropic carrier transport induced by high-field terahertz pulses in $n-$doped InGaAs F. Blanchard, F.H. Su, L. Razzari, G. Sharma, R. Morandotti, T. Ozaki, M. Reid, F.A. Hegmann We use ultrafast THz-pump/THz-probe techniques to study hot electron dynamics at high THz pump fields in $n-$doped InGaAs films. The transmission of the THz probe pulse increases at high THz pump fields for probe polarizations both parallel and perpendicular to the pump field. However, a significantly larger ultrafast modulation in the transmission of the THz probe pulse is observed in the parallel polarization case. We attribute this anisotropic behavior in THz probe pulse transmission to nonparabolicity in the conduction band that results in an anisotropic effective mass for carriers excited high in the band by the THz pump field. [Preview Abstract] |
Tuesday, March 16, 2010 3:18PM - 3:30PM |
L25.00005: Single nitrogen dyad magnetoluminescence in GaAs C. Ouellet-Plamondon, S. Marcet, J.F. Klem, S. Francoeur We report a study on the excitonic states of single nitrogen dyads in GaAs by magneto-photoluminescence. For light emitted along the [001] direction, a dyad of $C_{2v}$ symmetry oriented along [110] shows four excitonic optical transitions that are linearly polarized along and perpendicular to the dyad. As expected, a magnetic field does not induce additional splitting, but all transitions are subject to a Zeeman and a diamagnetic shift. The energy and transition probabilities of all excitonic states were modeled using a Hamiltonian fully accounting for the $C_{2v}$ symmetry. We determine the electron $g$-factor as well as the isotropic and anisotropic interaction factor for the hole. The diamagnetic shift allows us to calculate an electron radius of 16.2 \AA, confirming the strong localization of the electron at the dyad. Finally, some dyads observed emitted at an energy smaller than others, exhibited no exchange interaction splitting and were fully degenerate at zero field. This emission is tentatively assigned to a bound trion state. [Preview Abstract] |
Tuesday, March 16, 2010 3:30PM - 3:42PM |
L25.00006: Optical and Electrical Characterization of Bulk-Grown Ternary In$_{x}$Ga$_{1-x}$As Yung Kee Yeo, Austin Bergstrom, Robert Hengehold, Jean Wei, Shekhar Guha, Leonel Gonzalez, Geeta Rajagopalan Crystal growth technology breakthroughs have led to the growth of good quality melt-grown bulk In$_{x}$Ga$_{1-x}$As crystals. These crystals are promising candidates for electro-optical applications in the infra red. The optical and electrical properties of bulk In$_{x}$Ga$_{1-x}$As have been investigated as a function of temperature and indium mole fraction. Photoluminescence (PL) measurements show several band edge luminescence peaks including band-to-band, free-to-bound, and donor-acceptor pair peaks. Temperature dependent bandgaps were estimated from the PL results. The carrier concentrations and mobilities were determined by the Hall-effect measurements. The bandgaps estimated from the Hall-effect and optical transmission measurements were compared with those obtained from the PL results. [Preview Abstract] |
Tuesday, March 16, 2010 3:42PM - 3:54PM |
L25.00007: Bandgap dependence in the multiphoton absorption coefficient of semiconductors Dongmin Seo, Justin Gregory, Leonard Feldman, Norman Tolk, Philip Cohen Reports of two and three photon absorption coefficients in common semiconductors show a remarkably large lack of agreement and lack of scaling, indicating the need for more controlled experiments. We report nonlinear photon absorption in single-crystal Si, GaAs, and Ge carried out under identical conditions using an ultrafast high-power mid-IR laser. Wavelength- and bandgap-dependent multiphoton absorption coefficients were extracted and compared to current literature values, as well as the simpler scaling predictions for different bandgap materials. Our experimental data support the theoretical scaling law for the bandgap-dependent multiphoton absorption coefficients. Importantly, our coefficients are approximately 2 orders of magnitude smaller than current literature values. [Preview Abstract] |
Tuesday, March 16, 2010 3:54PM - 4:06PM |
L25.00008: The band gap of the chalcopyrite and spalerite phases of epitaxial ZnSnP$_{2}$ Philippe St-Jean, George Seryogin, Sebastien Francoeur Using contactless electroreflectance, we accurately determined the band gap of the two known phases of epitaxial ZnSnP$_{2}$. Induced by small changes in Sn/Zn flux ratio during epitaxy, the order-disordered transition between the chalcopyrite and sphalerite phases considerably affects the band gap energy. The chalcopyrite ordered phase, unambiguously identified from x-ray diffraction of sphalerite forbidden reflection peaks, exhibits a band gap of 1.67 eV at 293K. A small splitting of this transition is observed at 80K, indicating the presence of a crystal field produced by a small tetragonal distortion and a deviation from the ideal c/a=2 ratio. The band gap of the disordered sphalerite phase is 1.38 eV, somewhat higher than the value of 1.32 eV reported from bulk crystals, indicating either a remaining trace of ordering or a slight deviation from stoichiometry. Tailoring the electronic and optical properties of ZnSnP$_{2}$ at a fixed composition through an ordered- disordered transition has interesting technological applications. For example, it is an interesting material for multijunction solar cells because of its non-toxicity, abundance and low-cost. [Preview Abstract] |
Tuesday, March 16, 2010 4:06PM - 4:18PM |
L25.00009: Electronic Structure Studies of Amorphous Hydrogenated Boron Carbide M. Sky Driver, Joseph Sandstrom, Teak Boyko, Alexander Moewes, Anthony Caruso Boron carbide is a technologically relevant material with importance in voltaic transduction. However, the local physical, chemical and electronic structure of low temperature deposited thin films of amorphous boron carbide is far from understood, hindering its progress in application. X-ray absorption and emission spectroscopies (XAS/XES) were applied to thin films of B$_{4}$C and B$_{5}$C:H$_{x}$ to study the near Fermi edge structure; the films were prepared by RF magnetron sputtering and plasma enhanced chemical vapor deposition (PECVD) and were thermally treated after deposition from 400 to 800 C. XES spectra indicate a physical structure transition from amorphous to nanocrystalline at 700 C, a much lower temperature than expected from traditional physical vapor deposition or flash annealing temperatures reported. These structural differences are of significant interest to transport measurements and will be discussed as a correlation. Further, x-ray and ultraviolet photoemission were also collected as a compliment to XES/XAS and will be discussed in the context of understanding the local intra vs. intermolecular electronic structure of these boron-rich molecular based solids. [Preview Abstract] |
Tuesday, March 16, 2010 4:18PM - 4:30PM |
L25.00010: Solution-biased measurements and simulation of extraordinary electroconductance in Ti-GaAs hybrid structures W.-J. Chang, H.-J. Suk, R. Bashir, A.K.M. Newaz, S.A. Solin This study addresses the design, fabrication, characterization and simulation of a new type of sensor using the extraordinary electroconductance (EEC) effect [1] in a microscopic metal-semiconductor hybrid (MSH) structure at room temperature for real time detection of an external electric field. Three electron beam lithography steps were used to fabricate a 5 micron by 5 micron MSH device that contains a Ti/Au shunt forming a Schottky junction with a GaAs epitaxial layer. When the external electric field was applied by biasing the phosphate buffered saline solution on top of the nitride insulating layer deposited on the sensor, MSH structures showed higher sensitivity than bare semiconductor structures that do not have the metal shunt. A maximum EEC effect of 13.7 percent was observed from the MSH structure for an applied external electric field of -46.6 V/cm. It should be possible to use scaled-down nanoscopic EEC sensor arrays as a novel device for real time imaging of the surface charge distribution of a single cell. [Preview Abstract] |
Tuesday, March 16, 2010 4:30PM - 4:42PM |
L25.00011: Relative stability, electronic, optical, magnetic, piezoelectric properties (Cr, Sc) N and (Mn, Sc) N alloys and CrN/ScN superlattices Ahmad Alsaad Pure MnN, CrN binaries and their (Cr, Sc) N and (Mn, Sc) N alloys are investigated using the \textit{ab initio} local density and generalized gradient approximation. These methods are found to predict the electronic structures, optical behavior, piezoelectric response, magnetic behavior, and relative stabilities of MnN, CrN and their Cr$_{x}$Sc$_{1-x}$N and Mn$_{x}$Sc$_{1-x}$N alloys. The magnetic configurations of MnN and CrN are studied and the results suggest that magnetic states of these compounds in zinc-blende and rocksalt phases might be more complicated than previously found by several theoretical and experimental studies. The structural stabilities of (Cr, Sc) N and (Mn, Sc) N systems were checked for the entire compositional range and crossovers from fourfold coordinated phases to sixfold coordinated phases were found at high Cr and Mn concentrations. High Curie temperature ferromagnetism in both systems was found to be possible under carefully chosen conditions. Characterization of nitride superlattices was revisited and several properties of superlattices were discussed. In particular, optical and electronic properties of CrN/ScN superlattices were studied in details. [Preview Abstract] |
Tuesday, March 16, 2010 4:42PM - 4:54PM |
L25.00012: Photoluminescence anisotropy of excitons in sexithiophene cofacial chiral stacks Benoit Gosselin, Richard Leonelli, Carlos Silva We have investigated exciton dynamics in a thin film of sexithiophene molecules assembled in chiral H-aggregate stacks. The energetic disorder and the strong phonon-electron interactions leads to high localization of the photoexcitations. The initial photoexcited species branches into self-trapped exciton (95\% yield) and charge-transfer excitons (5\% yield). To characterize exciton diffusion and recombination mechanisms, we have carried out photoluminescence anisotropy measurements where the difference between the parallel and perpendicular components of the photoluminescence was monitored as a function of time. We measure an anisotropy of 0,1 after 20 ns which stays constant for 50 ns. Charge-transfer states induce a rise of the anisotropy up to 0,15 after 50 ns. These measurements show that exciton localization is very strong at 14K and higher for the charge-transfer states than the self-trapped excitons. A simple kinetic model based on a system of differential equations with radiative and depolarization time constants can reproduce the experimental data. [Preview Abstract] |
Tuesday, March 16, 2010 4:54PM - 5:06PM |
L25.00013: Studies on the crystal distortion -- birefringence relationship in ZnGeP$_{2}$ Ramazan Atalay, Mustafa Alevli, Max Buegler, Goksel Durkaya, Nikolaus Dietz ZnGeP$_{2}$ as a member of the ternary II-IV-V$_{2}$ compound chalcopyrite family has been extensively studied over the last decades as promising birefringent material for nonlinear optical applications in the near- and mid infrared (IR) wavelength region. The high birefringence in the ZnGeP$_{2}$ material system enables applications such as second-harmonic generation (SHG), optical parametric amplifier (OPO), and sum/difference frequency mixing. The birefringent effect in the ZnGeP$_{2}$ crystal structure is related to the compressive distortion of the chalcopyrite lattice in c-direction. Utilizing unpolarized/polarized Raman spectroscopy (RS) and infrared reflectivity (IR) measurement, we studied the vibrational phonon modes in ZnGeP$_{2}$ crystals and analyzed phonon contributions related to the anisotropic lattice distortions. The effect of birefringence is analyzed from the shift of the $\Gamma_4^{L0}$ Raman mode. Furthermore, the symmetry forbidden Raman (SFR) scattering modes, observed in the ZnGeP$_{2}$ Raman spectra, have been analyzed to understand contributions related to the lattice distortion and contributions related to crystal defects. Our studies show that the non-linear mixing of radiation only occurs along the [001] crystalline plane, where conservation of momentum or so-called phase matching is present. [Preview Abstract] |
Tuesday, March 16, 2010 5:06PM - 5:18PM |
L25.00014: Effect of the In-Plane Magnetic Field on the Recombination Radiation Spectrum and Recombination Kinetics of Spatially-Separated Electron-Hole Layers Oleksandr Rossokhaty, Igor Kukushkin In presented work the radiative recombination time and changes in the recombination radiation spectrum of spatially separated electron-hole layers has been investigated as a function of the magnetic field parallel to the plane of a wide quantum well. Changes in the radiation spectrum of e-h layers has been studied under variation of the in-plane magnetic field and interlayer distance. The observed dependence has been shown to agree with the theoretical conceptions, according to which the line shift is quadratic in the magnetic field and interlayer distance and inversely proportional to the sum of the electron and hole masses. This total mass obtained in the experiment has been found to depend on the electric field that separates the layers and may substantially differ from the expected value. The luminescence intensity has been found to decrease with increasing parallel magnetic field. This dependence is inconsistent with the theoretical predictions relating the decrease in the intensity to a decrease in the population of the energy levels allowed for radiative recombination. It has been shown that the intensity decrease is related to the exponential increase in the radiative recombination time with the magnetic field (due to a decrease in the overlapping of the electron and hole wavefunctions) and to the nonradiative recombination processes. [Preview Abstract] |
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