Bulletin of the American Physical Society
2008 APS March Meeting
Volume 53, Number 2
Monday–Friday, March 10–14, 2008; New Orleans, Louisiana
Session H37: Optical Properties of Semiconductors (mostly oxides) |
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Sponsoring Units: FIAP Chair: Matthias Schubert, University of Nebraska at Lincoln Room: Morial Convention Center 229 |
Tuesday, March 11, 2008 8:00AM - 8:12AM |
H37.00001: Density-functional theory study of the effects of atomic doping on the band edges of monoclinic WO$_{3}$ Muhammad N. Huda, Yanfa Yan, Su-Huai Wei, Mowafak M. Al-Jassim The effects of impurities in room temperature monoclinic WO$_{3}$ were studied using the local density approximation to density-functional theory. Our main focus is on nitrogen impurity in WO$_{3}$, where both substitutional and interstitial dopings were considered. We have also considered doping with transition-metal atoms and some co-doping approaches in WO$_{3}$. We find that, in general, band gap reduction was a common result due to the formation of impurity bands in the band gap. Also, the changes of band-edge positions, valence-band maxima and conduction-band minima, were found to depend on the electronic properties of the foreign atom and their concentration. Our results, therefore, provide guidance for making WO$_{3}$ a suitable candidate for photo-electrodes for hydrogen generation by water splitting. [Preview Abstract] |
Tuesday, March 11, 2008 8:12AM - 8:24AM |
H37.00002: Materials for Transparent Electronics: Ab initio calculation of wide bandgap semiconductor interfaces Skye Dorsett, Guenter Schneider Materials used in transparent electronics (TE) must be transparent in the visible portion of the electromagnetic spectrum which requires the use of wide bandgap semiconductors as contacts and rectifiers as well as passivation and barrier-shaping layers. Of particular importance are the source and drain contacts of transparent thin-film transistors (TTFT). The contact characteristics at the interface between the channel material (e.g. ZnO, SnO$_2$) and the contact material (commonly Indium Tin Oxide) are determined by the band offset which can be estimated from a heterojunction model based on material properties alone. The development of new materials for TE greatly benefits from estimates of interface properties but for most materials which hold promise for use in TE (e.g. indium gallium zinc oxide) the relevant material parameters such as work function, electron affinity and in particular the charge neutrality level are not known. To close this gap we report ab-initio density functional theory calculations of band offsets for wide bandgap semiconductors which are commonly used or hold promise for use in TTFT. [Preview Abstract] |
Tuesday, March 11, 2008 8:24AM - 8:36AM |
H37.00003: \textit{Ab initio} calculations of the dielectric functions of semiconductors including the electron-hole interactions via LASTO method HyeJung Kim, Yia-Chung Chang We calculate dielectric functions of semiconductors including the electron-hole interactions within the $\it ab$ $\it initio$ framework. The Bethe-Salpeter equation is constructed using a full-potential linear augmented-Slater-type orbital (LASTO) method [1, 2]. Using the LASTO method allows us to compute optical matrix element for large number of k points efficiently. Due to requirements of a dense k-point mesh, we use quasi-minimum residual (QMR) method to solve the equation. The inclusion of the electron-hole interactions both shifts the peak positions and changes peak heights of the imaginary part of the dielectric functions, resulting in better agreement with experimental data than the spectra obtained without including the electron-hole interactions. The calculated dielectric functions are compared to experimental data of ZnSe, CdSe, CdTe, InP, InAs, AlAs and GaN. The self-energy correctoins are described by an empirical tight-binding formula. [1] J. W. Davenport, Phys. Rev. B 29, 2896 (1984) [2] Y.-C. Chang, R.B.James, and J.W.Davenport, Phys. Rev. B 73, 035211 (2006) [Preview Abstract] |
Tuesday, March 11, 2008 8:36AM - 8:48AM |
H37.00004: Structural and optical properties of a transparent conductor oxide: Nb:In$_{2}$O$_{3}$ O. Lozano, P.V. Chinta, P.V. Wadekar, L.H. Chu, H.W. Seo, Q.Y. Chen, X.M. Wang, D. Wijesundera, L.W. Tu, N.J. Ho, W.K. Chu Thin films of niobium-doped indium oxide, Nb:In2O3, have been deposited on YSZ(001) and MgO(111) substrates by magnetron sputtering at 450\r{ }C. The transparent semiconducting films obtained on YSZ(001) were epitaxial, but when deposited under the same condition on MgO(111), the film qualities worsen upon Nb doping. The structural and optical properties in relation to the Nb content and the general growth conditions were studied by Rutherford backscattering, ion channeling, optical absorption spectroscopy, x-ray diffraction, and atomic force microscopy. The magneto-transport behaviors will also be discussed. [Preview Abstract] |
Tuesday, March 11, 2008 8:48AM - 9:00AM |
H37.00005: Nature of the bandgap in In$_{2}$O$_{3}$ revealed by first-principles calculations and X-ray spectroscopy Aron Walsh, Juarez L.F. Da Silva, Su-Huai Wei, Christoph K\"orber, Andreas Klein, L.F.J. Piper, Alex DeMasi, K.E. Smith, G. Panaccione, P. Torelli, D.J. Payne, A. Bourlange, R.G. Egdell The origin of weak absorption 1 eV below the onset of strong optical absorption in In2O3 has previously been attributed to the presence of an indirect fundamental bandgap or surface band bending. We demonstrate conclusively that this is not the case. Through the application of bulk and surface sensitive X-ray spectroscopic techniques, we reveal that the valence band edge is found much closer to the bottom of the conduction band than expected on the basis of the widely quoted bandgap of 3.75 eV. First-principles theory shows that the upper valence bands of In2O3 exhibit small dispersion and the conduction band minimum is positioned at $\Gamma $; however, direct optical transitions give minimal dipole intensity until 0.8 eV below the valence band maximum. Our results set an upper limit on the fundamental bandgap of 2.9eV. [Preview Abstract] |
Tuesday, March 11, 2008 9:00AM - 9:12AM |
H37.00006: Raman scattering properties of $\mathrm{SnO}_x$ Ralf Meyer, Cedrik Meier, Axel Lorke Oxidic semiconductors like $\mathrm{ZnO}_x$ and $\mathrm{SnO}_x$ have recently attracted a lot of attention as possible optical materials for novel technological applications. Results from Raman scattering experiments at $\mathrm{SnO}_{1.5}$ nanoparticles are presented which show strong differences compared to the Raman spectra of bulk $\mathrm{SnO}_2$. In order to understand these differences, ab-initio calculations of the Raman scattering properties of bulk $\mathrm{SnO}_2$ and $\mathrm{SnO}_{1.5}$ have been performed. Raman spectra derived from these calculations compare qualitatively well with the experimental findings. From this, it is concluded that the differences in the experiments are an effect of the bulk materials. An analysis of the nature of the calculated Raman active vibrational modes makes it possible to draw further conclusions on the reasons behind the differences between the stochiometric $\mathrm{SnO}_2$ and the understochimetric $\mathrm{SnO}_{1.5}$. [Preview Abstract] |
Tuesday, March 11, 2008 9:12AM - 9:24AM |
H37.00007: Studies of electronic relaxation and coherent control in sensitized semiconductor surfaces. Victor Batista, Luis Rego This talk addresses the feasibility of using sequences of unitary pulses for coherent-control of quantum dynamical phenomena, including superexchange hole tunneling in sensitized TiO$_{2}$ surfaces and control of tunneling and decoherence in archetype model systems. The proposed dynamical decoupling scenario is based on the repetitive application of unitary pulses, affecting the interference phenomena between wave-packet components. The pulses affect the overall relaxation dynamics without collapsing the coherent-quantum evolution of the system. It is shown that both bound-to-bound state tunneling and bound-to-continuum tunneling processes can be inhibited and eventually halted by sufficiently frequent pulse fields that exchange energy with the system but do not collapse the unitary evolution or affect the potential energy tunneling-barriers. The reported results are therefore particularly relevant to the understanding of coherent optical manipulation of electronic excitations in semiconductor devices where performance is limited by quantum tunneling and decoherence. [Preview Abstract] |
Tuesday, March 11, 2008 9:24AM - 9:36AM |
H37.00008: Electron-phonon interaction and charge carrier mass enhancement in electron doped alkali earth titanate semiconductors Dook van Mechelen, Dirk van der Marel, Claudio Grimaldi, Peter Armitage, Alexey Kuzmenko, Hans Hagemann, Nicolas Reyren, Rolf Lortz, Igor Mazin We have studied the electron-phonon coupling in electron doped SrTiO3 for which the carrier concentration ranges from a dilute gas of polarons to a polaron liquid. Here we report a comprehensive THz, infrared and optical study together with DC conductivity, Hall effect and specific heat measurements. Our THz spectra at 7 K show the presence of a very narrow ($<$ 2 meV) Drude peak, the spectral weight of which shows approximately a factor of three enhancement of the band mass for all carrier concentrations. The missing spectral weight is regained in a broad `mid-infrared' band which originates from electron-phonon coupling. Analysis of the results yields an electron-phonon coupling parameter of an intermediate strength, a $\sim$4. Specific heat measurements below 4 K show the mass enhancement to be about eight times the band mass for all carrier concentrations. The ostensible discrepancy with the optical mass is interpreted together with the temperature dependence of the Hall constant, the optical spectral weight and the dc scattering rate within the framework of a polaron liquid. [Preview Abstract] |
Tuesday, March 11, 2008 9:36AM - 9:48AM |
H37.00009: Non-resonant inelastic x-ray scattering spectra of lithiated titanium oxides for battery applications Kenneth Nagle, Mali Balasubramanian, Christopher Johnson, Gerald Seidler, Ilias Belharouak Although lithium-ion batteries now see widespread use, there remain considerable questions concerning the basic solid state chemistry of both electrodes. Improved understanding of the local electronic structure, particularly the mechanism of charge transfer upon insertion and removal of lithium, could lead to innovation in battery design and improved performance. We present non-resonant inelastic x-ray scattering (NRIXS) spectra from 2p initial states in titanium; these spectra are among the first recorded for such states in a transition metal. These spectra were obtained using the lower energy resolution inelastic x-ray scattering (LERIX) spectrometer, which is capable of making simultaneous measurements at nineteen values of momentum transfer. We demonstrate the ability to obtain soft x-ray absorption-like information using a bulk-sensitive, hard x-ray technique. In addition, at high momentum transfer NRIXS provides information about non-dipole transitions that are inaccessible by soft x-ray spectroscopic methods. [Preview Abstract] |
Tuesday, March 11, 2008 9:48AM - 10:00AM |
H37.00010: New green phosphor Ba$_{3}$Si$_{6}$O$_{12}$N$_{2}$:Eu for white LED: crystal structure and optical properties Masayoshi Mikami, Kyota Uheda, Satoshi Shimooka, Hiroyuki Imura, Naoto Kijima A new oxynitride, Ba$_{3}$Si$_{6}$O$_{12}$N$_{2}$, has been synthesized. The crystal structure has been successfully determined by close collaboration between experiment and first-principles band calculation based on density functional theory. This compound doped with Eu exhibits intense green photoluminescence with high color purity under near-ultraviolet to blue light excitation. It has much less thermal quenching than other green phosphor (Ba,Sr,Eu)$_{2}$SiO$_{4}$. Hence (Ba,Eu)$_{3}$Si$_{6}$O$_{12}$N$_{2}$ appears promising green phosphor for white LED backlight for display. The atomic/electronic structure is discussed in comparison with a similar oxynitride Ba$_{3}$Si$_{6}$O$_{9}$N$_{4}$, which could not become efficient phosphor by doping Eu due to strong thermal quenching at room temperature. The optical properties of these compounds have been interpreted from theoretical and crystallographic viewpoint. [Preview Abstract] |
Tuesday, March 11, 2008 10:00AM - 10:12AM |
H37.00011: Optical properties of group-II oxides -- excitons and absorption in MgO, ZnO and CdO Andr\'e Schleife, Claudia R\"odl, Frank Fuchs, Friedhelm Bechstedt ZnO is a material that has been very attractive for researchers for many decades by now. However, recently also alloys and heterostructures with other group-II oxides are becoming more and more interesting. Together with MgO or CdO the tuning of electronic and optical properties becomes possible with potential applications for optoelectronic devices in the blue or UV spectral region. For the three materials we study the influence of excitonic effects on the dielectric function in the region of interband transitions and on the electron-hole binding near the absorption edge by solving the Bethe-Salpeter equation. As starting point we compute the electronic structure using a GGA+$U$ approach. The $U$ is chosen to widely reproduce more sophisticated HSE03+$GW$ calculations. We combine two efficient approaches to calculate the spectrum and bound excitonic states, using a large number of ${\bf k}$-points in combination with hybrid ${\bf k}$-point meshes to ensure convergence. We find good agreement of our challenging \emph{ab initio} calculations with experimental absorption spectra as well as values for the binding energies. [Preview Abstract] |
Tuesday, March 11, 2008 10:12AM - 10:24AM |
H37.00012: Mechanistic model for the influence of Ag thickness on the electrical and optical properties of ZnO/Ag/ZnO nanoscale multilayers Terry Alford, Hauk Han, N. Theodore In this study, we have engineered TCO structures with greatly improved electrical conductivity by introduction of a thin layer of Ag. Results show that carrier concentration, mobility, and conductivity increase with Ag thickness. The electrical conductivity decreased from 1 $\Omega $-cm to 8$\times $10$^{-5} \quad \Omega $-cm for a 14 nm Ag middle layer, by increasing carrier concentration and mobility. AFM and TEM results indicate that low Ag thickness results in Ag island formation. The optical transmittance of the composite structure decreases when compared to a single ZnO layer of comparable thickness. However, we demonstrate that an optimum Ag thickness exists (12 nm) to fulfill the conductivity and transmittance requirements for optoelectronic devices. Moreover the optical band-gap of ZnO/Ag/ZnO composite multilayer films decreases with increasing the Ag thickness and also increases with carrier concentration. Based on these results, we propose a mechanistic model for the influence of Ag thickness on the electrical and optical properties of this system. [Preview Abstract] |
Tuesday, March 11, 2008 10:24AM - 10:36AM |
H37.00013: Soft X-Ray Spectroscopic studies of Intrinsic Quantum Well States, Shallow Core Level Hybridization, and Valence Band Structure in CdO and InN L.F.J. Piper, L. Colakerol, A. DeMasi, T.D. Moustakas, K.E. Smith, J. Zuniga-P\'erez, V. Munoz-Sanjos\'e, Alexei Fedorov, T. Veal, C. McConville InN and CdO are post-transition metal compounds that display significant metal-ligand shallow core level hybridization [1], and have recently been discovered to posses intrinsic quantum well states in electron accumulation layers near their surfaces [2]. We report here new synchrotron-based soft x-ray spectroscopic measurements of the electronic structure of CdO and InN single crystal thin films. Resonant x-ray emission spectroscopy has been employed to study the detailed valence band and shallow core level electronic structure, while high resolution angle-resolved photoemission spectroscopy was used to measure quantized electron subbands at the near-surface of both InN and CdO. [1]. L .F. J. Piper \textit{et al}., Phys. Rev. B (2007) \textit{in press; }[2]. L. Colakerol \textit{et al}., Phys. Rev. Lett. 97, 237601, (2006) [Preview Abstract] |
Tuesday, March 11, 2008 10:36AM - 10:48AM |
H37.00014: Probing defects in ZnO nanostructures by Photoluminescence and Positron Annihilation Spectroscopy Manoranjan Ghosh, A.K. Raychaudhuri, S.K. Chaudhuri, Dipankar Das We have investigated defect related emission in the blue green region (2.2 eV -- 2.5 eV) of ZnO nanostructures having spherical (5 nm-15 nm) as well as those with hexagonal platelet and rod like morphologies (20nm-100 nm), synthesized by solvo-thermal route. This emission show anomalous size dependence. Emission energy near 2.2 eV, shifts to higher energy (2.5 eV) for increase in size beyond 20nm when shape of the nanostructures changes. This change in photoluminescence has a close correlation with the size (and shape) induced change in the positron trapping rate which is directly proportional to the defect concentration. The trapping rates show non-monotonous dependence on size. It increases initially as the size increases (5nm-15nm) and then decreases as the size increases beyond 20nm. While increase of the trapping rate on size reduction is expected due to accumulation of more defects at the surface, the initial dependence of the trapping rate on the size (below 20nm) is anomalous. The data are explained by the presence of defects like Zn vacancy and confinement due to size reduction. [Preview Abstract] |
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