Bulletin of the American Physical Society
2006 APS March Meeting
Monday–Friday, March 13–17, 2006; Baltimore, MD
Session N41: Insulating and Dielectric Oxides |
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Sponsoring Units: DCMP Chair: Seong Gon Kim, Mississippi State University Room: Baltimore Convention Center 344 |
Wednesday, March 15, 2006 8:00AM - 8:12AM |
N41.00001: Oxygen-deficient defects and hydrogen in irradiated Si/SiO2 systems Renee Van Ginhoven, Harold Hjalmarson Performance of Si/SiO2 devices is degraded by ionizing radiation through the production of interface traps and buildup of trapped charge in the oxide. This process is connected to the presence of hydrogen in the oxide. Exposure to ionizing radiation stimulates release of mobile hydrogen, which can migrate through the oxide to the Si/SiO2 interface and depassivate H-terminated Si-dangling bonds. The resulting interface trap states act as charge recombination centers. Our calculations focus on the interaction of hydrogen with oxygen deficient centers in the oxide. In the bulk oxide, these defects can release hydrogen from Si-H groups, or crack H2 molecules. These active sites may also act as border trap recombination centers when near the interface. Our presentation will describe molecular scale mechanisms for radiation-induced generation of free hydrogen using density functional theory applied to fully periodic models. The oxide is represented by both crystalline and amorphous configurations. Sandia is a multiprogram laboratory operated by the Sandia Corporation, a Lockheed Martin Company, for the United States Department of Energy under contract DE-AC04-94AL85000. [Preview Abstract] |
Wednesday, March 15, 2006 8:12AM - 8:24AM |
N41.00002: Photo-catalytic studies of transition metal doped titanium dioxide thin films processed by metalorganic decomposition (MOD) method P. Talagala, X. Marko, K.R. Padmanabhan, R. Naik, D. Rodak, Y.T. Cheng We have synthesized pure and transition element (Fe, Co and V) doped Titanium oxide thin films of thickness $\sim $ 350 nm on sapphire, Si, and stainless steel substrates by Metalorganic Decomposition (MOD) method. The films were subsequently annealed at appropriate temperatures ( 500-750C) to obtain either anatase or the rutile phase of TiO$_{2}$. Analysis of the composition of the films were performed by energy dispersive X-ray(EDAX) and Rutherford backscattering spectrometry(RBS). Ion channeling was used to identify possible epitaxial growth of the films on sapphire. Both XRD and Raman spectra of the films exhibit that the films annealed at 550C are of anatase phase, while those annealed at 700C seem to prefer a rutile structure. The water contact angle measurements of the films before and after photoactivation, demonstrate a significant reduction in the contact angle for the anatase phase. However, the variation in contact angle was observed for films exposed to UV ($<$10$^{o}$-30$^{o})$ and dark (25$^{o}$-50$^{o})$. Films doped with Fe show a trend towards lower contact angle than those doped with Co. Results with films doped with V will also be included. [Preview Abstract] |
Wednesday, March 15, 2006 8:24AM - 8:36AM |
N41.00003: Proton Conduction and Microstructure of Lanthanum Phosphates Gabriel Harley, Lindsey Karpowich, Lutgard De Jonghe Lanthanum phosphate (LaPO4) has been recently studied as a potential proton conducting ceramic electrolyte for fuel cells. The complexity of the La2O3 - P2O5 system, particularly in the 1000 - 1600K range, leads to the difficulty in forming phase pure LaPO4 by traditional synthesis methods. Complex microstructures involving amorphous and crystalline phases at grain boundaries have been detected by high resolution transmission electron microscopy, and have been shown to affect proton conductivity by several orders of magnitude. A simple model is used to predict conductivity of the material based on grain size and grain-boundary phases, and experimental results match up well with the model. Conductivities as high as 3.6E-5 S/cm at 773 K were found for undoped LaPO4. [Preview Abstract] |
Wednesday, March 15, 2006 8:36AM - 8:48AM |
N41.00004: Comparison of the electronic structures of four crystalline phases of FePO$_4$ Ping Tang, N. A. W. Holzwarth LiFePO$_4$ in the olivine structure is a promising cathode material for Li-ion batteries. During normal battery operation, an olivine form of FePO$_4$ is formed. Experimental evidence suggests that the olivine phase is meta-stable relative to a quartz-like trigonal phase. In addition, several other meta-stable phases have been reported\footnote{M. S. Whittingham, Chem. Rev. {\bf{104}}, 4271 (2004); Y. Song and co-workers, Inorg. Chem. {\bf{41}}, 5778 (2002); H. N. Ng and C. Calvo, Can. J. Chem. {\bf{53}}, 2064 (1975); J. P. Attfield and co-workers, J. Solid State Chem. {\bf{57}}, 357 (1985); A. S. Andersson and co-workers, Solid State Ionics {\bf{130}}, 41 (2000)} including a monoclinic and a high pressure CrVO$_4$ structure. We have carried first-principles electronic structure calculations on all of these phases in order to investigate their relative stability and other properties using both LAPW and PWPAW methods.\footnote{http://www.wien2k.at/; http://pwpaw.wfu.edu/ } We find that the LSDA approximation systematically underestimates the lattice constants and the calculated stability ordering of the phases is inconsistent with experiment. In contrast the GGA approximation models the phase stability in closer agreement with experiment, although the lattice constants are systematically over-estimated. [Preview Abstract] |
Wednesday, March 15, 2006 8:48AM - 9:00AM |
N41.00005: First-principles study of phase stability and phase transition in Ga$_{2}$O$_{3}$ Jianjun Dong, Bin Xu Gallium oxide (Ga$_{2}$O$_{3})$ is a promising opto- and/or electronic wide-band-gap semiconductor. For example, it has been considered as a gate dielectric oxide for MOS device based on GaN or GaAs. In addition to the ground-state monoclinic $\beta $-phase, a rhombohedral $\alpha $-phase metastably exists at ambient conditions. The conditions of stability of either phase are not well understood. In this talk, we will present our recent \textit{ab initio} calculation results of thermodynamic properties of the $\alpha $ and $\beta $ phase. We have computed Gibbs free energies of the two phases based on the total energy density functional theory (DFT) and the statistical quasi-harmonic approximation (QHA), calculated the thermal equations of states, and estimated the equilibrium phase boundary. We have further calculated the pressure dependence of Raman and IR frequencies in the two phases. Our results will be compared with some recent experimental data. [Preview Abstract] |
Wednesday, March 15, 2006 9:00AM - 9:12AM |
N41.00006: Phase transition of Ta-modified Pb(Sc$_{0.5}$Nb$_{0.5}$)O$_{3}$ nanoceramics Margarita Correa, Ram Choudhary, Ram Katiyar Ferroelectric relaxors are promising candidates for multilayer
ceramic
capacitors. We have synthesized nanocrystalline
Pb(Sc$_{0.5}$Nb$_{(1-x)/2}$Ta$_{x/2})$O$_{3}$, (0.1$ |
Wednesday, March 15, 2006 9:12AM - 9:24AM |
N41.00007: Substrate-induced strain effects on the transport properties of pulsed laser deposited Nb doped SrTiO$_{3}$ films. Wegdan Ramadan, Satish Ogale, Sankar Dhar , Shixiong Zhang, Darshan Kundaliya, Issei Satoh, Thirumalai Venkatesan Thin films of Nb doped SrTiO$_{3 }$(NSTO) are grown via pulsed laser deposition (PLD) on LaAlO$_{3}$ (LAO, 001), MgAl$_{2}$O$_{4}$ (MAO, 001), SrTiO$_{3}$ (STO, 001), and Y-stabilized ZrO$_{2 }$(YSZ, 001) substrates. The film growth is examined under various growth conditions. The dependence of film properties on the film-substrate lattice mismatch, film thickness, and substrate temperature is investigated. The electrical transport in NSTO films is shown to exhibit a strong sensitivity to strain, which is suggested to arise from the dependence of carrier mobility on bond distortions/stretching and related changes in phonon modes. [Preview Abstract] |
Wednesday, March 15, 2006 9:24AM - 9:36AM |
N41.00008: Blue-light emission at room temperature from Ar$^{+}$-irradiated SrTiO$_{3}$ Daisuke Kan, Takahito Terashima, Ryoko Kanda, Atsunobu Masuno, Atsushi Ishizumi, Yoshihiko Kanemitsu, Yuichi Shimakawa, Mikio Takano SrTiO$_{3}$ is a key material for fabricating oxide-based electronic devices. We found that Ar$^{+}$-irradiated, metallic SrTiO$_{3}$ crystals emit 430-nm blue-light at room temperature. Oxygen-deficient metallic SrTiO$_{3}$ thin films also show the blue-light emission. Reciprocal mapping using synchrotron x-ray radiation at SPring-8 reveals a slight elongation of the lattice parameter along the out-of-plane direction both for these samples. We, therefore, suggest that the Ar$^{+}$-irradiation introduces oxygen deficiency in the crystal surface, and that the deficiencies generate conduction carriers which wait ready for the recombination with photo-exited holes, and play an important role in the emission. It is emphasized that the emitting region could be patterned into any size and shape by combining conventional photolithography and Ar$^{+}$-milling. These new features of SrTiO$_{3}$ will open up new possibilities for the oxide-based electronic devices. [Preview Abstract] |
Wednesday, March 15, 2006 9:36AM - 9:48AM |
N41.00009: Measurement of the Electric Field Gradient at $^{181}$\textit{Ta} in \textit{ZrSiO}$_{4 }$and \textit{HfSiO}$_{4}$ using Perturbed Angular Correlation Spectroscopy Herbert Jaeger, Sean McBride Perturbed angular correlation spectroscopy (PAC) is a nuclear technique often used to probe the hyperfine interaction of a nuclear moment with extra-nuclear fields. For example the electric field gradient (EFG) at a $^{181}$Ta probe nucleus in zircon (ZrSiO$_{4})$ depends on the arrangement of the Zr, Si, and O-atoms and is very sensitive to structural rearrangements. Our PAC experiments with zircon show that a very subtle rearrangement of Si-atoms within the unit cell leads to a change in the temperature dependence of the EFG. We are currently performing a series of PAC experiments on the isostructural hafnon (HfSiO$_{4})$. Preliminary results show no evidence of a similar structural rearrangement. In addition to the EFG, we also measure the anisotropy of the $\gamma \gamma $-cascade emitted during the decay of a $^{181}$Ta nucleus. The measured anisotropy depends somewhat on the geometry of the sample and detector arrangement. However, with a given nucleus and a fixed geometry one would not expect a substantial change in the anisotropy during a series of measurements, say as a function of temperature. Yet our PAC spectra of zircon show a consistent decrease of the anisotropy in the temperature range between 650 and 800\r{ }C. Preliminary PAC spectra of hafnon show no change of the anisotropy. Reasons for this apparent loss in anisotropy will be discussed. [Preview Abstract] |
Wednesday, March 15, 2006 9:48AM - 10:00AM |
N41.00010: Structure, Energetics, and Clustering Interactions for Cu on TiO$_{2}$ (110) at Various Coverages Scott J. Thompson, Steven P. Lewis TiO$_{2}$ (110) is one of the prototypical metal-oxide surface systems, studied extensively and under a variety of different conditions by experimentalists and theorists alike. Recent experiments have enhanced our understanding of the structure of the stochiometric surface, and our Density Functional Theory calculations show excellent quantitative agreement with these latest results. In addition, strong interactions between metal catalysts and their supporting oxide substrates give rise to enhanced catalytic properties, and we are exploring this phenomenon for the prototypical system of Cu on the (110) surface of rutile TiO$_{2}$. In this talk, we will present our theoretical results for the surface structure and elaborate on the agreement with the latest experimental findings as well as the differences from previous theoretical work on this important system. We will also discuss predictions of the structure and energetics for Cu on TiO$_{2}$ (110) at various coverages and on both stoichiometric and reduced surfaces, where specific focus will be upon clustering interactions and the formation of Cu islands, which has been observed experimentally. [Preview Abstract] |
Wednesday, March 15, 2006 10:00AM - 10:12AM |
N41.00011: {\it Ab initio} molecular dynamics of a proton in amorphous SiO$_2$ illustrating the hopping mechanism Julien Godet, Alfredo Pasquarello The scaling of metal-oxide-semiconductor devices to smaller dimensions is a major issue in current silicon technology. In order to understand the role of hydrogen at silicon-oxide interfaces, we here investigate charged states of hydrogen in amorphous SiO$_2$ ($a$-SiO$_2$) using first-principles calculations (DFT-GGA). We first show that the formation energies of H$^0$, H$^+$ and H$^-$ in $a$-SiO$_2$ are essentially equivalent to those in $\alpha$-quartz. In particular, the H$^+$ and H$^-$ species are always more stable than their neutral counterpart. Then, we focus on the basic diffusion mechanism of the proton in $a$-SiO$_2$. Our molecular dynamics simulations show that the proton hops between O atoms. The hopping does not occur between first O neighbors connected through the network, but takes place across rings when the O--O distance is about 2.3 \AA. The hopping process is favored by the thermal vibrations of the O atoms. [Preview Abstract] |
Wednesday, March 15, 2006 10:12AM - 10:24AM |
N41.00012: Unusual compressibility in the negative-thermal-expansion material ZrW$_{2}$O$_{8}$ Albert Migliori, C. Pantea, H. Ledbetter, y. Zhao, T. Kimura, Peter B. Littlewood, J. Van Duijn, G.R. Kowach The negative thermal expansion (NTE) compound ZrW$_{2}$O$_{8}$ has been well-studied because it remains cubic with a nearly constant, isotropic NTE coefficient over a broad temperature range. However, its elastic constants seem just as strange as its volume because NTE makes temperature acts as \textit{positive} pressure, decreasing volume on warming and, unlike most materials, the thermally-compressed solid$_{ }$\textit{softens}. Does ZrW$_{2}$O$_{8}$ also soften when pressure alone is applied? Using pulse-echo ultrasound in a hydrostatic SiC anvil cell, we determine the elastic tensor of monocrystalline ZrW$_{2}$O$_{8 }$near 300 K as a function of pressure. We indeed find an unusual decrease in bulk modulus with pressure. Our results are inconsistent with conventional lattice dynamics, but do show that the thermodynamically-complete constrained-lattice model can relate NTE to elastic softening as increases in either temperature or pressure reduce volume, establishing the predictive power of the model, and making it an important concept in condensed-matter physics. [Preview Abstract] |
Wednesday, March 15, 2006 10:24AM - 10:36AM |
N41.00013: Oxide films imaged on a nanometer-scale by single-electron tunneling force microscopy Clayton C. Williams, Ezra Bussmann, Ning Zheng Recently, we reported a scanning probe technique to manipulate a single electron to and from states in a nonconducting surface by electron tunneling. Each electron is detected by electrostatic force as it tunnels between a scanning probe and the surface. Electrons are manipulated by tuning the probe Fermi level with respect to the states by a dc voltage. This manipulation serves as the mechanism for imaging and for performing electronic spectroscopy of states in dielectric films. The energy distribution of the states is measured by counting the electrons tunneling to the surface at incrementally varied voltages. The spatial distribution of states is imaged on a nanometer-scale by counting each electron tunneling on a 2-D grid. We present spectroscopic and imaging results from silicon dioxide and hafnium oxide. The density and spatial distribution of states is compared for various growth parameters. The measurements reveal evidence for energy relaxation and charge movement in the states. This new nanometer-scale approach provides the means to locate and identify electronic states in nonconducting surfaces, opening for exploration a whole class of materials not accessible to the STM. \newline [1] E. Bussmann, N. Zheng {\&} C. C. Williams, \textit{Appl. Phys. Lett.} \textbf{86}, 163109 (2005). [Preview Abstract] |
Wednesday, March 15, 2006 10:36AM - 10:48AM |
N41.00014: A novel approach to the growth of polycrystalline hydroxyapatite thin films Zhendong Hong, Lan Luan, Se-Bum Paik, Bin Deng, Donald Ellis, John Ketterson, Alexandre Mello, Alexandre Rossi, Joice Terra Hydroxyapatite (HA) thin films on metals have been extensively studied in connection with bioimplants. Conventional sputtering techniques have shown some advantages over the commercially utilized plasma spray method; however, the as-sputtered films are usually amorphous which can cause serious adhesion problems when post-deposition heat treatment is necessitated. We present a novel opposing RF magnetron sputtering approach for the room temperature preparation of HA thin films on various substrates at low power levels. The as-sputtered films are found to be polycrystalline and the preferred orientations of the films vary with the substrate material and orientation. The effects of different sputtering parameters on the physical, chemical and structural properties was also studied. Finally, patterned films fabricated both before and after deposition have been prepared for further \textit{in vitro} cell culture experiments. [Preview Abstract] |
Wednesday, March 15, 2006 10:48AM - 11:00AM |
N41.00015: Finite temperature properties of (Ba,Sr)TiO$_3$ disordered alloys and BaTiO$_3$/SrTiO$_3$ superlattices from first-principles Sergey Lisenkov, Laura Walizer, Laurent Bellaiche We develop and use a first-principles-based scheme to predict properties of (Ba,Sr)TiO$_3$-based systems at finite temperature. This scheme yields a composition-versus-temperature phase diagram of disordered (Ba$_{1-x}$Sr$_{x}$r)TiO$_3$ solid solutions that is in rather good agreement with experimental data. We further use this scheme to reveal and understand the strain-versus-temperature phase diagram of several BaTiO$_3$/SrTiO$_3$ superlattices. A wide variety of dipole patterns, including homogeneous ferroelectric phase and periodic stripe patterns, are predicted to occur depending on the interplay between temperature, strain and superlattice periods. [Preview Abstract] |
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