Bulletin of the American Physical Society
2006 APS March Meeting
Monday–Friday, March 13–17, 2006; Baltimore, MD
Session N46: Semiconductor Devices / Semiconductors General |
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Sponsoring Units: FIAP Chair: Andrei Sergeev, University at Buffalo Room: Baltimore Convention Center 349 |
Wednesday, March 15, 2006 8:00AM - 8:12AM |
N46.00001: Dielectric and Photovoltaic Physics in Thin-Film Crystalline Sulfides Rodney McKee, Fred Walker Solar energy utilization has been the hope and sought-for solution to local energy needs at least since the late 1800's. In today's terms, solar energy is one of the few renewable energy sources with the potential to have a major impact on domestic energy independence. There is a rich, but incomplete scientific literature on the underpinning photovoltaic physics of solar cell development. This literature does however, clearly identify a pervasive, unsolved physics problem -- \textit{deep level electronic states in wide band gap semiconductors quench the electro-optic behavior of solar cells: either p-type or n-type doping is inhibited both of which are required for the basic function of a semiconducting p-n junction solar cell. }We will report on our approach towards solving this problem via layer-sequenced stabilization of thin-film photovoltaics that enable symmetric p or n-type doping. We will bring interface phase physics to the synthesis process for sulfur-based chalcogenides to show that the valence and conduction band energy levels as well as defect formation energies in these systems can be systematically modified in wide bandgap photovolatics. [Preview Abstract] |
Wednesday, March 15, 2006 8:12AM - 8:24AM |
N46.00002: Metal-semiconductor-metal junctions with silver sulphide barrier layers I. Chaitanya Lekshmi, Yasmin Afsar, Jagadeesh S. Moodera Atomic level electrical switching requires innovative methods of charge transport, wherein the device can be switched between ``on'' and ``off'' states at ambient temperatures by applying reasonably small voltages. Recently, the mixed conducting property of silver sulphide was utilized in making a quantized conductance atomic switch[1] which satisfies these requirements. We present the fabrication of metal-semiconductor-metal junctions where a Ag$_{2}$S layer is sandwiched between two metal electrodes. Current-voltage measurement shows diode characteristics for these junctions at large thickness (100 {\AA}) of Ag$_{2}$S. At lower thicknesses, the nature of transport changes over to a nonlinear tunnel junction like behaviour up to an applied external voltage of 1.5 V. The growth, morphology and transport properties of Ag$_{2}$S layers depend critically on the deposition conditions. Using the tunnel junction, we investigate the effects of parameters such as growth and thickness of semiconducting layers, choice of metal electrodes and the metal-semiconductor interface on the charge transport across the junction. [1] K. Terabe et. al, Nature \textbf{433}, 47 (2005) [Preview Abstract] |
Wednesday, March 15, 2006 8:24AM - 8:36AM |
N46.00003: Characterization of hydrogenation processes for c-Si photovoltaics S. Kleekajai, M. Stavola, F. Jiang A commonly used method to introduce H into Si solar cells to passivate bulk defects is by the post-deposition annealing of an H-rich SiN$_{x}$ surface layer that also acts as an antireflection coating.$^{1}$ It previously had been impossible to characterize the small concentration of H that is introduced by this method. Our work on the properties of the transition-metal-H complexes in Si has led us to develop a novel method to characterize the introduction of H into Si.$^{2}$ We have used IR spectroscopy coupled with transition-metal impurities introduced into Si-test samples to act as traps for H. The transition-metal-H complexes can then be detected with high sensitivity to determine the concentration and penetration depth of H in the samples. This model system has been used to obtain insight into what solar-cell processing strategies lead to the best passivation of defects in the Si bulk. We thank V. Yelundur and A. Rohatgi for an enjoyable and fruitful collaboration.. This work is supported by NSF Grant DMR 0403641 and NREL grant AAT-1-31605-04. \newline 1. A. G. Aberle, Sol. Energy Mater. Sol. Cells \textbf{65}, 239 (2001). \newline 2. F. Jiang \textit{et al.}, Appl. Phys. Lett. \textbf{83}, 931 (2003). [Preview Abstract] |
Wednesday, March 15, 2006 8:36AM - 8:48AM |
N46.00004: Optical switching and structural properties of Ge2Sb2Te5 and Ge2Sb2Te7 films T. W. Herring, P. C. Taylor, C. E. Inglefield, M. J. Nelson, D. A. Baker, M. A. Paesler Ge2Sb2Te5$_{ }$and Ge2Sb2Te7 are materials important in phase change memory applications, but the structures of both the amorphous and crystalline phases are not well known. Large areas of optically switched material are needed in order to probe the structure. Films of amorphous Ge2Sb2Te5$_{ }$varying in thickness between 20 nm and 100 nm were crystallized by exposure to a focused beam of 532 nm laser light with a power density of approximately 50 kW/cm$^{2}$. Rastering of the crystallized spots produces areas of several square millimeters suitable for experiments to probe the structure of the films. The switching causes little change in surface topography as measured by atomic force microscopy. Ablation of the films occurs if the power density is too high. The structure of optically crystallized films studied by EXAFS will be discussed. Films of Ge2Sb2Te7 will also be discussed. [Preview Abstract] |
Wednesday, March 15, 2006 8:48AM - 9:00AM |
N46.00005: Interface Dielectric Function in ZnO/Ag Structures for Applications as Back-Reflectors in Thin Film Solar Cells Deepak Sainju, Nikolas Podraza, Jian Li, Robert Collins, Maarij Syed Sequential deposition of optically-opaque Ag followed by the transparent conductor ZnO, both by magnetron sputtering on substrates such as stainless steel, is a key process for efficient optical back-reflectors (BRs) of thin film solar cells. The roughness scale investigated in our work is an order of magnitude smaller than that studied previously. We have first analyzed Ag deposition by real time spectroscopic ellipsometry (RTSE) over the energy range from 1.0 eV to 6.5 eV in order to establish the final roughness thickness on the Ag just prior to \textit{in situ} deposition of ZnO. Values from 10 to 50 {\AA} are obtained, for a relatively narrow range of substrate temperature (20-90\r{ }C). We employ the same RTSE probe to analyze the interface and bulk optical properties of ZnO and thus deduce a complete optical model of the BR. Our model for the dielectric function of the interface layer helps explain the losses in the BR structure. It includes contributions from free electrons associated with the Ag component, and bound electrons associated with a metal particle plasmon resonance near 2.7 eV and with interband transitions from Ag and ZnO. The effect of the interface layer on reflectance of BR structures is evaluated. [Preview Abstract] |
Wednesday, March 15, 2006 9:00AM - 9:12AM |
N46.00006: High-performance ZnO/ZnMgO FET using a hetero-MIS stricture Shigehiko Sasa, Masashi Ozaki, Kazuto Koike, Mitsuaki Yano, Masataka Inoue We propose a new structure of ZnO/ZnMgO field-effect transistors (FETs) for simplifying the fabrication process as well as for the improvement of the FET characteristics. Recently, we developed a ZnO/ZnMgO heterostructure FET (HFET) by utilizing a two-dimensional electron gas channel layer formed in the selectively-doped single quantum well structure.$^{1)}$ In the HFET fabrication process, the ohmic contact formation is crucial because of the difficulty in removing the top ZnMgO barrier layer with the underlying ZnO channel remained. The use of a very thin (1-2 nm) ZnMgO top barrier layer enables the formations of both good ohmic contacts without the ZnMgO etching and the gate electrode�D We used metal-insulator semiconductor (MIS) gate structure with the use of a 50-nm-thick Al$_{2}$O$_{3}$ gate insulator. The thin ZnMgO barrier acts as a setback layer for the channel electrons from the ZnMgO/Al$_{2}$O$_{3}$ interface. The 1-$\mu $m-gate device showed a complete FET operation with a transconductance of as high as 28 mS/mm and the effective mobility of 62 cm$^{2}$/Vs. 1) K. Koike et al., Appl. Phys. Lett. \textbf{87}, 112106 (2005). [Preview Abstract] |
Wednesday, March 15, 2006 9:12AM - 9:24AM |
N46.00007: First-principles calculations of mobilities in novel MOSFETs Matthew Evans, Sokrates Pantelides Nanoscale metal-oxide-semiconductor field-effect transistors (MOSFETs) incorporating novel materials demonstrate unusual electron transport behavior. Straining the silicon lattice results in significant increases in electron and hole mobility. However, mobility calculations using standard approximations have difficulty explaining this increase. MOSFETs using novel gate dielectrics (e.g. hafnium oxide) have mobilities that are much lower than MOSFETs using silicon dioxide as the dielectric. ``Interface quality'' has been invoked as a likely cause of this difference, but few attempts have been made to tie the mobility decrease to scattering mechanisms associated with the novel dielectric structure. In this talk, we report results of mobility calculations in MOSFETs with a strained-Si channel and with alternate gate dielectrics. The calculations employed a recently developed first-principles method based on atomic-scale interface models.[1] Changes in the local environment of atomic-scale interface roughness defects are shown to potentially account for the increase in mobility under strain. Interstitial Hf defects near the silicon-oxide interface can act as traps and are shown to impact the mobility in MOSFETs with hafnium oxide gate dielectrics. [1] M. H. Evans, X.-G. Zhang, J. D. Joannopoulos, and S. T. Pantelides, Phys. Rev. Lett., v. 95, p. 106802 (2005). [Preview Abstract] |
Wednesday, March 15, 2006 9:24AM - 9:36AM |
N46.00008: Negative Bias Temperature Instability (NBTI) recovers fully with bake at 325 C or above and the device is equivalent to new. Anastasios Katsetos Negative Bias Temperature Instability (NBTI) is one of the major degradation mechanisms of PMOSFET devices. When the p-channel Field Effect Transistor (PFET) gate is biased negatively with respect to the channel, as in CMOS inverter, at elevated temperature the threshold voltage (Vt) decreases (absolute value increases for application temperatures) and the drive current (Ion) decreases. This degrades the device performance and may lead to circuit failure. NBTI is process dependent and has strong dependence on temperature, gate voltage, time, and gate oxide thickness. It also depends on device area and or geometry. NBTI models used in industry are empirical. I have observed, on different technologies, in the last several years that NBTI recovers with bake. The recovery amount depends on the bake temperature, which can be the stress temperature, and happens very fast at any temperature. Full recovery is achieved at temperatures above 325 degrees C. After full bake recovery the device behaves like new with NBTI equal to the NBTI of the original stress. [Preview Abstract] |
Wednesday, March 15, 2006 9:36AM - 9:48AM |
N46.00009: Calculation of the Phonon Lifetime of Photoexcited Bismuth Eamonn Murray, David Prendergast, Tadashi Ogitsu, Stephen Fahy Phonon lifetimes of the zone-center longitudinal optical phonon in bismuth are calculated with respect to the fraction of valence band electrons excited into the conduction bands. Second order density-functional perturbation theory (DFPT), combined with the frozen phonon technique, is used to calculate the third-order anharmonic couplings between phonons. Calculations on the photoexcited system are performed by constraining the occupations of the valence and conduction bands, giving a certain excited electron-hole plasma density. It is found that the calculated decrease in the phonon lifetime with excitation is due both to the reduction of the phonon frequency and an increase in the coupling to other phonons. [Preview Abstract] |
Wednesday, March 15, 2006 9:48AM - 10:00AM |
N46.00010: Defect levels in semiconductors - is the “band gap problem” truly a problem? Peter A. Schultz Quantitative predictions of defect properties in semiconductors using density functional theory (DFT) have been crippled by standard supercell methods, which have incorrect boundary conditions for an isolated defect, and the “band gap problem,” where DFT drastically underestimates the band gap. I present a generalized supercell method with boundary conditions appropriate to point defects, to fix the electrostatic boundary conditions, remove ambiguity in charge reservoir, include bulk polarization effects, and specifically account for defect level dispersion. I compute formation energies for an extensive set of defects in silicon. The resulting defect level spectrum in silicon exhibits no band gap problem. The results agree remarkably well with experiment for those values that are experimentally known, and predict heretofore unobserved electronic transitions important for the electrical response of irradiated semiconductor devices. [Preview Abstract] |
Wednesday, March 15, 2006 10:00AM - 10:12AM |
N46.00011: Structural, optical, and electrochromic properties of V$_{2}$O$_5$ thin films by Metalorganic Decomposition M.B. Sahana, G. Lawes, K. R. Padmanabhan, R. Naik, V.M Naik V$_{2}$O$_{5}$ a n-type semiconductor has been widely used in variety of technological applications such as solid state battery cathodes, solar cell windows, and electrochromic devices as it allows easy intercalation/deintercalation of different ions due its open layered structure. Recently the attention has been focused on the development of thin films of V$_{2}$O$_{5}$ as a cathode material in microbatteries owing to the miniaturization of electronic devices. We report the preparation of V$_{2}$O$_{5}$ thin films by cost effective easy method of metalorganic decomposition technique using vanadium naphthenate oxide precursor. The solution is spin coated on glass and ITO coated glass substrates. The resulting films on annealing at 450\r{ }C are comprised of V$_{2}$O$_{5}$ nanoparticles as evidenced from X-ray diffraction and Raman spectra. UV-VIS studies indicate band gap of $\approx $ 2.4 eV. The dependence of electrochromic properties of these films, heat treated at various temperatures, on microstructure and crystallinity will be presented. [Preview Abstract] |
Wednesday, March 15, 2006 10:12AM - 10:24AM |
N46.00012: Structure of III-Sb(001) Surfaces Under Extreme Sb-rich Conditions Jeffery Houze, Sungho Kim, Seong-Gon Kim, Steven C. Erwin We use density functional theory to study the structure of III-Sb(001) (III = Al or Ga) growth surfaces. Various reconstruction models are considered to construct the surface stability diagram under different III-Sb growth conditions. We found that AlSb surface stability diagram identifies experimentally observed surface reconstructions quite well. For GaSb, however, all $(n\times 5)$-like reconstructions proposed to date have too high surface formation energies compared to the ones with wrong periodicities and thus cannot adequately model the structures observed experimentally under extreme Sb-rich growth conditions. Our results indicate that the existing reconstruction models for GaSb(001) surface require revisiting and demonstrate the need for a better reconstruction model. [Preview Abstract] |
Wednesday, March 15, 2006 10:24AM - 10:36AM |
N46.00013: A density functional study of the effect of pressure on GeTe Leonard Kleinman, B.R. Sahu, Adrian Ciucivara We compare local density approximation (LDA) and generalized gradient approximation (GGA) calculations of GeTe as a function of applied pressure. The LDA yields a poor result for the zero pressure trigonal angle but good to excellent results for the zero pressure lattice constants, energy gap, and relative positions of the two sublattices. More importantly, it yields results within the wide range of experimental values for the critical pressure at which the ferroelectric trigonal to rock-salt transition takes place. We also calculate the zero pressure polarization. [Preview Abstract] |
Wednesday, March 15, 2006 10:36AM - 10:48AM |
N46.00014: Pressure-Raman study of optical phonon anharmonicity and metastable phase in $^{68}$Zn$^{76}$Se R.E. Tallman, B.A. Weinstein, R. Lauck, M. Cardona The effects of hydrostatic pressure on the one- and two-phonon Raman spectra of isotopic purity $^{68}$Zn$^{76}$Se are studied to 15GPa at 300K. With increasing pressure the TO-TA(X,K) difference-mode shifts rapidly to higher energy, moving above the 2TA overtone band at 5.8GPa with no significant 4 -phonon mixing. Above 10GPa, the one-phonon TO($\Gamma )$ peak broadens rapidly, reaching $\sim $ 60cm$^{-1}$ FWHM and overlapping both TO-TA(X,K) and LO($\Gamma )$. After the sample undergoes the forward and reverse high-pressure transitions, the sphalerite-structure Raman features return (including the strongly broadened TO($\Gamma )$ peak) and a new sharp line attributed to a metastable ZnSe phase appears. The TO($\Gamma )$ broadening in ZnSe is much stronger than that due to pressure-tuning of the anharmonic decay TO($\Gamma )=>$ TA+LA(X,W,K) in GaP and ZnS.[1] Our results suggest that the resonant anharmonic interactions in ZnSe may be strongly enhanced by spatial confinement and disorder in the domains of nucleating phases. [1] J. Serrano et. al., Phys. Rev. B\underline {69}, 014301(2004). [Preview Abstract] |
Wednesday, March 15, 2006 10:48AM - 11:00AM |
N46.00015: Raman scattering in Zn$_{1-x}$Fe$_{x}$Te, a van Vleck diluted magnetic semiconductor X. Lu, S. Tsoi, I. Miotkowski, S. Rodriguez, A.K. Ramdas, H. Alawadhi, T.M. Pekarek Zn$_{1-x}$Fe$_{x}$Te, a zinc blende II-VI diluted magnetic semiconductor(DMSs), exhibits van Vleck paramagnetism, thanks to the electronic level structure of Fe$^{2+}$ with T$_{d}$ site symmetry, Subjected to crystal field and spin-orbit coupling, the lowest level of its ground state multiplet has a $\Gamma _{1}$ non-magnetic level, with a $\Gamma _{4}$ magnetic level just above it. This level ordering leads to its van Vleck paramagnetism. The Raman spectra of this DMS display the $\Gamma _{1}\to \Gamma _{4 }$electronic transition($\Gamma ^{\ast })$ whose Zeeman splittings are interpreted in terms of symmetry considerations and numerical calculations. The magnetic field and the temperature dependence of the spin-flip Raman line of the donor-bound electron in Zn$_{1-x}$Fe$_{x}$Te exhibit characteristics typical of van Vleck paramagnetism and, in combination with magnetization measurements, yield the s-d exchange constant $\alpha $N$_{0}$=236.9$\pm $9 meV. The Raman spectra also show $\Gamma ^{\ast }$ in combination with LO phonons which exhibit an intermediate mode behavior. [Preview Abstract] |
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