Bulletin of the American Physical Society
2006 APS March Meeting
Monday–Friday, March 13–17, 2006; Baltimore, MD
Session K46: Spectroscopy of Semiconductors |
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Sponsoring Units: FIAP Chair: Giti Khodaparast, Virginia Polytechnic Institute and State University Room: Baltimore Convention Center 349 |
Tuesday, March 14, 2006 2:30PM - 2:42PM |
K46.00001: Microwave cyclotron resonance of two-dimensional holes in GaAs/AlGaAs quantum wells on (100) substrates Han Zhu, K. Lai, D. C. Tsui, N. P. Ong, M. Manfra, L. Pfeiffer, K. West Cyclotron resonance at microwave frequencies is used to measure the band mass ($m_b$) of two-dimensional holes (2DH’s) in the GaAs/Al$_x$Ga$_{1-x}$As quantum wells grown on (100) GaAs substrates [1]. The measured $m_b$ shows strong dependences on both the 2DH density ($p$) and the well width ($W$). For a fixed $W$, in the density range (0.4$\times10^{11}$ to 1.1$\times10^{11}$cm$^{-2}$) studied here, $m_b$ increases with $p$, consistent with previous studies of the 2DH’s on the (311)A surface [2]. However, the density dependence is significantly weaker on the (100) surface than that on the (311)A surface for the same well width of 30nm. For a fixed $p$ = 1.1$\times10^{11}$cm$^{-2}$, $m_b$ increases from 0.22$m_e$ at $W$ = 10nm to 0.54 $m_e$ at $W$ = 20nm, and stays around 0.51$m_e$ for $W$ up to 1000nm. With the transport measurement at 0.3K in the dark, the DC scattering time $\tau_{DC}$ deduced for $p$ = 1.1$\times10^{11}$cm$^{-2}$ shows a maximum of 0.6ns at $W$ = 20nm. [1] M. J. Manfra $et$ $al.$, Appl. Phys. Lett. 86, 16 (2005). [2] W. Pan $et$ $al.$, Appl. Phys. Lett. 83, 3519 (2003). [Preview Abstract] |
Tuesday, March 14, 2006 2:42PM - 2:54PM |
K46.00002: Magnetoplasmons and cyclotron resonance in a two-dimensional electron gas Gerard Martinez, C. Faugeras, Yu. A. Bychkov, A. Riedel, R. Hey, K.J. Friedland Cyclotron resonancen (CR) transitions in quasi two-dimensional electron gas (2DEG) are known to involve magnetoplasmons (MP) dispersion. It has been recently [1] possible to derive in the frame of the MP picture, assuming the Hartree-Fock approximation, the magneto conductivity response of such systems for any value of the filling factor $\nu $ and including non-parabolicity effects. As compared to the one-electron picture, the MP theory predicts specific behavior of the response concerning both the oscillator strength as well as the energy variation of the CR transitions as a function of $\nu $. These predictions are confronted to absolute magneto-transmission results obtained on high mobility 2DEG embedded in GaAs quantum wells. It is shown that the MP theory reproduces quite well the experimental findings for 2n+1$<\nu <$ 2n+2 whereas clear discrepancies appear for 2n$<\nu <$ 2n+1. The possible origin of these discrepancies are discussed. [1] Yu. A. Bychkov and G. Martinez, Phys. Rev. B\textbf{72}, 195328 (2005) [Preview Abstract] |
Tuesday, March 14, 2006 2:54PM - 3:06PM |
K46.00003: Infrared Magneto-Optical Probe of Landau Levels of Graphite in the Extreme Quantum Limit Dimitri Basov, Zhiqiang Li, Willie Padilla, Sasa Dordevic, Kenneth Burch, Yong-Jie Wang We present a systematic investigation of the ab plane magneto- reflectance R($% \omega $,H) of highly oriented pyrolytic graphite (HOPG) in magnetic fields up to 18T, with magnetic field parallel to the c axis of the sample. A linear magnetic field dependence of the Landau level energies is observed in contrast to the square root field dependence predicted by Dirac fermion description of the quasiparticles in HOPG. We show that the conventional graphite band model can quantitatively account for the totality of the magneto-optical data. We also find that the resonance linewidth of the lowest inter-Landau-level transition shows an unconventional linear field dependence. These results provide new insights into the magneto- transport of HOPG. [Preview Abstract] |
Tuesday, March 14, 2006 3:06PM - 3:18PM |
K46.00004: Ultrahigh spatial and spectral resolution photoluminescence experiments on excitons bound to the isoelectronic nitrogen impurities in GaAs. Denis Karaiskaj, Angelo Mascarenhas Optical spectroscopy with diffraction limited resolution allows the measurement of luminescence from single impurity centers. Selectively studying individual centers makes it possible to unveil their otherwise concealed polarization anisotropy, identify their particular configuration, map their spatial distribution, and demonstrate the presence of diversity of local environments [1]. Experiments on several single nitrogen impurity pairs in GaAs allowed us to gain valuable insight into their orientation, distribution, and local environment. The nanoscale photoluminescence experiments have been combined with ultrahigh spectral resolution studies in order to better understand the optical transitions of excitons bound to nitrogen pairs in GaAs. The high spectral resolution studies have revealed a new excitonic transition originating most likely from excitons bound to two or more nitrogen impurity atoms, unprecedented since the discovery of isoelectronic impurity bound excitons in the sixties [2]. The temperature dependence demonstrates that all involved transitions originate from the same center, while the polarization dependent studies give insight into the orientation of the different nitrogen atoms in the cluster. [1] S. Francoeur, J. F. Klem, and A. Mascarenhas, Phys. Rev. Lett. \textbf{93}, 067403 (2004). [2] D. G. Thomas and J. J. Hopfield, Phys. Rev. \textbf{150}, 680 (1966). [Preview Abstract] |
Tuesday, March 14, 2006 3:18PM - 3:30PM |
K46.00005: Nuclear spin-lattice relaxation in n-GaAs close to the metal-insulator transition W.G. Moulton, Jun Lu, M.J.R. Hoch, P.L. Kuhns Dynamic nuclear polarization is of considerable interest in semiconductors particularly in GaAs. Nuclear spin-lattice relaxation interactions are important in the polarization process. The coupling of electron and nuclear spins in n-GaAs close to the metal-insulator (MI) transition changes significantly as the dopant concentration n increases through the MI critical concentration n$_{C}$=1.2x10$^{16}$ cm$^{-3}$. The changes correspond to the evolution of localized donor states into itinerant states close to the bottom of the conduction band. Measurements of the $^{71}$Ga relaxation rates $^{71}W$ made as a function of magnetic field (1 -- 13 T) and temperature (1.5 -- 300 K) for n-GaAs samples with n = 5.9x10$^{15}$, 7x10$^{16}$ and 2x10$^{18}$ cm$^{-3}$ show marked changes in the relaxation behavior with n. Korringa-like relaxation is found in the metallic samples for $T<$30 K and power law dependence at lower n. For $T>$ 30 K phonon-induced nuclear quadrupolar relaxation is dominant. Knight shift measurements made on the 2x10$^{18 }$cm$^{-3}$ sample using magic-angle spinning, confirm a small value for the hyperfine coupling constant and permit comparison of the Korringa product with predictions. At lower n, local moments play an important role in relaxation producing striking changes in the $H$ dependence of $^{71}W$. A model will be presented. [Preview Abstract] |
Tuesday, March 14, 2006 3:30PM - 3:42PM |
K46.00006: Low-temperature scanning tunneling spectroscopy of Si(111) 7x7 near the Fermi energy Shengyong Qin, Daejin Eom, Chih-Kang Shih Si(111) 7x7 is probably the most widely studied surface in surface sciences. Scanning tunneling microscopy/spectroscopy (STM/S) have been applied extensively to and have provided very detailed understanding of the atomic and electronic structures of this surface, revealing adatom, dangling bond and backbond states. Most spectroscopic studies however, have focused on the electronic structures far away from the Fermi energy. The metallic nature of this surface, however, makes it interesting to explore the electronic states near Fremi energy. Moreover, theoretical studies of this surface suggested important correlation effects. By using a low temperature STM (about 5K), we reported studies of electronic structures near the Fermi energy. In addition to the real-space distribution of density of states directly revealed by STS, Fourier analysis is used to obtain k-space electronic structures. We confirm that the surface is metallic with appreciable DOS, however with a small gap (about 0.15 eV) below the Fermi energy. Moreover, Fourier space analysis reveals what appears to be a Fermi surface in the extended zone scheme, although further investigation is needed to confirm this observation. [Preview Abstract] |
Tuesday, March 14, 2006 3:42PM - 3:54PM |
K46.00007: Electrostatic potential screened by a two-dimensional electron system: A real-space observation by scanning tunneling spectroscopy Masanori Ono, Yoshihiro Nisigata, Takahiro Nishio, Toyoaki Eguchi, Yukio Hasegawa Scanning tunneling spectroscopy at 5K was used to investigate the electrostatic potential profile on the Si(111)-$\surd $3$\times \surd $3 Ag surface at sub-nanometer spatial resolution. The potential was measured from an energy-level shift of electronic states on the surface. The potential images obtained reveal that the potential drops around the steps and Ag adsorbates, upon which positive charges are presumably accumulated. The profiles of the reduced potentials are explained with the screening of potential due to the charges by two-dimensional electron gas (2DEG) existing on the surface. The Friedel oscillation, which results from the screening and has a period of the half Fermi wavelength of the 2DEG, was also observed in the potential images as well. [Preview Abstract] |
Tuesday, March 14, 2006 3:54PM - 4:06PM |
K46.00008: Phonon Confinement and Laser Heating Effects on Germanium Nanowires. Romaneh Jalilian, Hari Chandrasekhar, Mahendra Sunkara, Gamini Sumanasekera We explore the combined effects of phonon confinement, local heating and stress on the Raman spectra of Germanium nanowires with varying diameters. Germanium nanowires were synthesized using spontaneous nucleation and basal growth from gallium droplets. The asymmetric broadening and downshifting of the first order Raman band is studied as a function of average diameter, local temperature of the nanowires. This basic phenomenological model with a modified confinement function incorporated with thermal effects and stress are in agreement with our experimental results. Fano-resonance effect was excluded from this study. [Preview Abstract] |
Tuesday, March 14, 2006 4:06PM - 4:18PM |
K46.00009: Optical and Electronic Properties of Ge-Sb-Te films Heng Li, P. C. Taylor Amorphous and crystalline films in the system Ge-Sb-Te are of interest because of their use in reversible phase change optical storage media or electrical switches. These applications utilize differences in optical or electrical properties between the crystalline and amorphous phases of the same material. The most commonly employed composition is Ge$_{2}$Sb$_{2}$Te$_{5}$, which lies along the pseudobinary tie line GeTe-Sb$_{2}$Te$_{3}$. We present data on amorphous films of GeTe, Sb$_{2}$Te$_{3}$, Ge$_{2}$Sb$_{2}$Te$_{5}$, and Ge$_{2}$Sb$_{2}$Te$_{7}$ grown by rf sputtering and examine the effects of growth rate and oxygen impurities on the optical and electronic properties. For Ge2Sb2Te5, the optical gap decreases with increasing growth rate, and the inverse slopes of the exponential band tails (Urbach tails) extending into the gap also decrease with increasing growth rate (i.e., the band tails become sharper at slower growth rate). The increase of the optical gap for oxygen concentrations greater than about 10$^{21}$ cm$^{-3}$ is almost certainly due to the presence of oxygen at levels approaching alloy compositions. On the other hand, the sharpening of the band tail absorption is probably due to increased diffusion on the surface during growth with decreasing growth rate. [Preview Abstract] |
Tuesday, March 14, 2006 4:18PM - 4:30PM |
K46.00010: Structural, Vibrational, and Electronic Properties of Ternary SiGeSn Alloys Vijay R. D'Costa, Jos\'{e} Men\'{e}ndez, Andrew V.G. Chizmeshya, John Tolle, John Kouvetakis The recent demonstration of ternary SiGeSn alloys represents a significant breakthrough that adds new levels of flexibility to the design of microelectronic and optoelectronic devices based on group-IV materials. We present a systematic experimental and theoretical study of the structural, vibrational, and electronic properties of this material. X-ray reciprocal lattice maps are used to determine the equilibrium lattice constant of the alloys, whose composition is measured independently by Rutherford Backscattering. It is found that the system follows Vegard's law quite closely, in contrast to GeSn alloys, which show evidence for bowing in the compositional dependence of the lattice constant. The Raman spectrum shows the three standard Si-Si, Ge-Si, and Ge-Ge peaks found in GeSi alloys. No evidence for Sn-Ge or Sn-Si modes is seen, but the three observed peaks shift in frequency as a function of the Sn concentration. The energy of the direct optical transitions display significant bowing as a function of the alloy composition. The dominant source of bowing appears to be the simultaneous presence of Si and Sn atoms in the ternary alloy. [Preview Abstract] |
Tuesday, March 14, 2006 4:30PM - 4:42PM |
K46.00011: Two-photon Photoemission Electron Microscopy Imaging of TiO$_{2}$ Gang Xiong, Alan Joly, Wayne Hess, S. Chambers, T. Kaspar, Kenneth Beck Photoemission electron microscopy (PEEM) utilizing two-photon excitation was used to image a novel TiO$_{2}$ thin film sample grown by molecular beam epitaxy. The TiO$_{2}$ thin film consists of small (50-100 nm) TiO$_{2}$ rutile nanocrystallites embedded in an epitaxial anatase TiO$_{2}$ film. Individual rutile crystallites can be identified from the anatase background, in part due to the work function difference and morphology contrast. For both phases the photoelectron signals result from electrons at the surface defect levels in the gap rather than valence band electrons. We compare the two-photon PEEM images excited by p- and s-polarized light. The greater image intensity excited by p-polarized light can be attributed to a stronger refracted wave and is further enhanced by the two-photon effect. How the differences in optical refraction, absorption and photoelectron yield affect the contrast between rutile nanocrystallites and anatase background is discussed. We summarize the contrast mechanisms for PEEM imaging and discuss the role of the complex dielectric constant in PEEM contrast mechanisms. Enhanced photoemissions from localized regions are also discussed. [Preview Abstract] |
Tuesday, March 14, 2006 4:42PM - 4:54PM |
K46.00012: Electron Paramagnetic Resonance Study of Amorphous V$_{2}$O$_{5}$ D.B. Baker, K. Hartman, K.C. Maynes, P.H. Bunton, S. Feller Vitreous V$_{2}$O$_{5}$ is generally non-stoichiometric when cooled from above the melt, leading to the formation of paramagnetic V$^{4+}$ ions. At low temperature ($\sim $120 K), previous workers have observed well-resolved EPR spectra associated with these centers. By contrast, recent results from our laboratory show EPR spectra that remain broad ($\sim $20 mT, peak-to-peak) from ambient down to 120 K, with no apparent hyperfine interactions. EPR spin counting analyses also indicate high concentrations ($\sim $10$^{20}$ spins/cm$^{3})$ of paramagnetic centers, suggesting that linewidths are limited by spin-spin relaxation of the electron spin system. In addition, current results reveal that EPR signal intensities increase more rapidly than (Temperature)$^{-1}$, as indicated by monitoring signal intensities over the temperature range from 120-300 K. In particular, when temperature is decreased by a factor of two (from 240 to 120 K), signal intensity increases by a factor of approximately 10. Such observations indicate the possibility of superparamagnetic or spin glass behavior within the amorphous V$_{2}$O$_{5}$ system. In order to further investigate the magnetic behavior of amorphous V$_{2}$O$_{5}$, several experimental EPR parameters are evaluated over the temperature range from 120-370 K. [Preview Abstract] |
Tuesday, March 14, 2006 4:54PM - 5:06PM |
K46.00013: Analysis of Reflectivity Measurements William Karstens, D. Y. Smith Reflectivity measurements over a limited wavelength range do not uniquely determine a material's optical constants. To achieve the latter, reflectivity must be known over a sufficient range to apply phase dispersion analysis, or reflectivity at several angles of incidence must be measured in order to solve Fresnel's equations. An approximate alternative is to fit reflectivity data with a model for the optical constants that is consistent with the material's known, or at least plausible, electronic and/or phonon structure. We have explored this procedure for analysis of IR and visible reflectivity of crystalline, amorphous, and porous silicon using both theoretical and empirical models. Limitations of this procedure, especially at the extremes of the measured range will be discussed. [Preview Abstract] |
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