Bulletin of the American Physical Society
2009 APS March Meeting
Volume 54, Number 1
Monday–Friday, March 16–20, 2009; Pittsburgh, Pennsylvania
Session X12: Structure and Morphology of Oxide Surfaces and Thin Films |
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Sponsoring Units: DMP DCMP Chair: Shao-Chun Li, Tulane University Room: 308 |
Thursday, March 19, 2009 2:30PM - 2:42PM |
X12.00001: Finding stable $\alpha$-quartz (0001) surface structures via simulations Yun-Wen Chen, Chao Cao, Hai-Ping Cheng Reconstruction of $\alpha$-quartz (0001) surfaces is studied using combined classical molecular dynamics and density functional theory. Five reconstruction patterns are identified, including three (2 $\times$ 1) patterns and two (1 $\times$ 1) patterns. The energetically most stable surface structure is found to be a (2 $\times$ 1) reconstruction pattern, and several patterns can co-exist in a large scale surface. A combination of structures can explain the experimentally observed (2 $\times$ 2) diffraction pattern. [Preview Abstract] |
Thursday, March 19, 2009 2:42PM - 2:54PM |
X12.00002: Investigation of stability issues of TCO barrier layers for CIGS devices during damp heat and dry heat exposures Rajalakshmi Sundaramoorthy, Ingrid Repins, David Albin, John Pern, Xiaonan Li, Tim Gessert, Thomas Gennett The reliability of In$_{2}$O$_{3}$:SnO$_{2}$ (ITO) and In$_{2}$O$_{3}$:ZnO (IZO) as barrier layers for CuInGaSe$_{2}$ (CIGS) solar cells has been investigated. NREL's high-efficiency CIGS devices are prepared using a three-stage process for the CIGS layer, and insulating ZnO and ZnO:Al as the (bi-layer) transparent conducting oxide (TCO) buffer and conducting layers, respectively. These CIGS devices are processed to explore the effectiveness of barrier layers of ITO and IZO sputtered at room temperature and at various temperatures. Devices are exposed to damp heat at 85$^{o}$C and 85{\%} relative humidity (RH) and dry heat conditions (85$^{o}$C/$\sim $0{\%} RH). Some cells are also tested under 1-sun illumination and open-circuit voltage bias. Optical, electrical, structural, and imaging analyses are used to characterize the samples periodically before and after the exposures. Surface depth profiling and relative concentration of the elements present are analyzed using XPS. Results of these stability studies will be discussed. This abstract is subject to government rights. [Preview Abstract] |
Thursday, March 19, 2009 2:54PM - 3:06PM |
X12.00003: Scanning Tunneling Microscopy Study of a Vicinal Anatase TiO$_{2}$ Surface Shao-Chun Li, Olga Dulub, Ulrike Diebold Titanium dioxide finds versatile applications in various technical fields including gas sensing, coatings, pigments, heterogeneous catalysis, photocatalytic degradation of pollutants, and solar cells. TiO$_{2}$ is found in three main crystallographic phases: rutile, anatase and brookite. Rutile is the thermodynamically most stable form and is considered a model system for basic research. However, anatase TiO$_{2}$ is often considered to be catalytically more active than rutile for reasons not yet completely understood. In this work, using scanning tunneling microscopy (STM) and low energy electron diffraction (LEED), the structure of the anatase TiO$_{2}(\mbox{5}$ $\overline{14}$) surface, $\sim $10\r{ } vicinal to the -- lowest energy -- (101) plane, has been studied. The surface was found to facet into a structure composed of ridges with a uniform width of 5 lattice units. Based on atomically-resolved STM and electron counting rules, it is proposed that the sides of the ridges are parallel to ($\mbox{1}$ $\overline{10}$) and (112) planes. These sides might be reconstructed to stabilize the microfaceted structure. Vapor-deposited gold shows pronounced clustering between the ridges, indicating a one-dimensional template effect of the vicinal surface, which supports denser and more uniformly sized Au clusters, as compared to the flat (101) surface. [Preview Abstract] |
Thursday, March 19, 2009 3:06PM - 3:18PM |
X12.00004: $\beta -$Si$_{3}$N$_{4}$/CeO$_{2-x }$Interface Investigated via Atomic Resolution Z-contrast Imaging, Electron Energy-Loss Spectroscopy and First-Principles Methods Weronika Walkosz, Robert F. Klie, Serdar Ogut, Biljana Mikijelj, Stephen J. Pennycook, Juan C. Idrobo The addition of rare-earth oxides, typically forming intergranular glassy films in ceramics, has long been known to markedly affect toughness and creep resistance of Si$_{3}$N$_{4}$. The present work investigates the bonding characteristics of cerium at the interface between Si$_{3}$N$_{4 }$grains and the secondary ceria phases with aberration-corrected scanning transmission electron microscopy techniques. The obtained Z-contrast images and the electron energy-loss spectra taken at the interface of Si$_{3}$N$_{4}$/CeO$_{2-x}$ suggest that the arrangement of Ce at the interface depends on the thickness of the intergranular film, the electronic structure of the rare earth element, as well as the termination of Si$_{3}$N$_{4}$. Possible reasons for these observed structural and electronic variations at the interface, and their agreement with the theoretical predictions of two stoichiometric terminations of Si$_{3}$N$_{4}(10\mathop 1\limits^- 0)$ surface will be briefly discussed. [Preview Abstract] |
Thursday, March 19, 2009 3:18PM - 3:30PM |
X12.00005: Observation of a ($\surd $3x$\surd $3)R30\r{ } Reconstruction on O-Polar ZnO Surfaces Seth King, Somendra Parihar, Kallol Pradhan, H. Trevor Johnson-Steigelman, Paul Lyman Low energy electron diffraction revealed a previously unreported ($\surd $3x$\surd $3)R30\r{ } reconstruction on clean, O-polar ZnO surfaces after\textit{ in-situ} preparation under conditions with an extremely low H background or after \textit{ex-situ }tube-furnace annealing [1]. It has been proposed that unreconstructed, H-free, O-polar ZnO surfaces cannot be produced [2]. As the sample is prepared from the as-received surface, to a clean (1x1), and finally to the clean ($\surd $3x$\surd $3)R30\r{ } reconstruction, x-ray photoelectron spectroscopy shows decreasing intensity of the hydroxyl shoulder (located to the high-binding-energy side of the O1$s$ peak). This reconstruction is stable against H$_{2}$, N$_{2}$, and air, although its formation is suppressed when preparation occurs under an intentional H$_{2}$ background. A structural investigation of the reconstruction with LEED-IV is under way. \underline {\textbf{References:}} [1] S.T. King \textit{et al.}, Surf. Sci. \textbf{604}, L131 (2008) [2] C. W\"{o}ll, Prog. Surf. Sci. \textbf{82}, 55 (2007) [Preview Abstract] |
Thursday, March 19, 2009 3:30PM - 3:42PM |
X12.00006: Density functional study of GaN/ZnO alloy structure and surface properties Li Li, Xiao Shen, Philip B. Allen Recent experiments\footnote{K. Maeda, K. Teramura, D. Lu, T. Takata, N. Saito,Y. Inoue, and K. Domen, Nature 440, 295 (2006).} show that GaN/ZnO alloy is a promising host for photo-catalytic water splitting. The band-gap of this alloy is narrowed down to the visible regime, allowing photo-absorption over a large portion of the solar spectrum. Muckerman et\footnote{L. L. Jensen, J. T. Muckerman, and M. D. Newton, J. Phys. Chem. C 112 3439 (2008).} made a first-principles studies of the structural and electronic properties of this alloy. We present further work on this alloy and on the surfaces which may host the water-splitting redox reactions. We look at structural properties such as bond lengths, and the stability of different configurations for various mixing ratios. We examine the nature of the HOMO and LUMO states, aiming for insight about the active sites of the water splitting catalytic chemistry. [Preview Abstract] |
Thursday, March 19, 2009 3:42PM - 3:54PM |
X12.00007: Study of oxygen octahedra rotations in Ruddlesden-Popper phases of perovskite materials JeeHye Lee, Tomas Arias We present the most systematic theoretical study of Ruddlesden- Popper(RP) phases in perovskites ($Sr_{n+1}Ti_{n}O_{3n+1}$ and others) to date. Consistent with experimental results, we find that the formation of stacking faults is energetically favored for certain rotational reconstructions of the bulk material. In particular, we enumerate the set of possible rotational configurations of oxygen octahedra next to the stacking fault and then reduce this set to a subset of distinct configurations by considering the symmetries of the RP structure. For this reduced set, we calculate the formation energy of stacking faults, and study the interaction between separated faults. We find that the planar faults attract each other, and that the interaction energy scales as the inverse distance between the faults. [Preview Abstract] |
Thursday, March 19, 2009 3:54PM - 4:06PM |
X12.00008: Reconsideration of SrTiO3(100)-$\surd $5\textbf{x}$\surd $5-R26.6 surfaces - existence of O-vacancy and Sr adatom Ichiro Shiraki, Kazushi Miki SrTiO$_{3}$(100)-$\surd $5x$\surd $5-R26.6 surfaces were studied by scanning tunneling microscope (STM) in ultra-high vacuum conditions. The results suggest the existence of O-vacancies on the surfaces, which have not been confirmed experimentally so far. In occupied states observations of -1.2 V sample bias voltage, a pair of dangling bonds of Ti atoms neighboring an O vacancy is clearly seen as an oval shape protrusion that can be resolved apart into two protrusions, which is consistent with previous theoretical study by spin polarized LDA+U calculations [1]. Making a comparison between occupied states and unoccupied states (-1.2 V sample bias voltage) in the same scan area, the oval shape protrusions present dark contrast, which is also consistent with the theoretical study. The present proposed surface structural model requires the existence of Sr adatoms as reported by Kubo et al. [2] [1] Z. Fang and K Terakura, Surf. Sci. 470, L75(2000) [2] T. Kubo and H. Nozoye, Phys. Rev. Lett. 86, 1801(2001) [Preview Abstract] |
Thursday, March 19, 2009 4:06PM - 4:18PM |
X12.00009: ABSTRACT WITHDRAWN |
Thursday, March 19, 2009 4:18PM - 4:30PM |
X12.00010: Ab initio modeling of interfacial oxygen defects in ultrathin high-k gate dielectric stacks Blanka Magyari-Kope, Eric Cockayne, Yoshio Nishi As the thickness of the metal-oxide-semiconductor field-effect transistor (MOSFET) is further reduced, and the gate dielectric is under constant voltage, traps due to defects are generated and may form a conductive path. At a critical density of defects this may lead to gate dielectric breakdown. First principles methods based on density functional theory and non-equilibrium Green's function calculations are employed to calculate the tunneling current through ultrathin oxide layers of HfO$_{2}$ and SiO$_{2}$ in a gate stack structure with Pt metal electrode. The microscopic effects of the oxygen defects formation on the electronic transport through the gate oxides are analyzed and discussed. A number of interface models are constructed that include oxygen vacancies at various interfaces and isolated oxygen vacancy defects in the bulk oxide. The band offsets and the tunneling currents are calculated as a function of defects positions in the gate dielectric. [Preview Abstract] |
Thursday, March 19, 2009 4:30PM - 4:42PM |
X12.00011: Tunable Metallicity at the Surface of La$_{5/8}$Ca$_{3/8}$MnO$_{3 }$Thin Films Kenji Fuchigami, Zheng Gai, Thomas Z. Ward, Lifeng Yin, Paul Snijders, Ward Plummer, Jian Shen A series of in-situ STM studies of La$_{5/8}$Ca$_{3/8}$MnO$_{3}$(001) thin film reveals that the surface metallicity can be tuned by extrinsic oxygen doping at the surface. By in-situ annealing with or without oxygen, we can convert the surface back and forth between a ({\AA},,2 $\times $ {\AA},,2)R45 reconstructed surface and a (1 $\times $ 1) surface. Electrical properties of the surfaces are investigated by scanning tunneling spectroscopy (STS). I-V curves clearly show that the oxygen doping renders the surface insulating while the (1 $\times $ 1) surface without the oxygen doping is metallic. Structural models and their correlation to the surface metallicity have been proposed. [Preview Abstract] |
Thursday, March 19, 2009 4:42PM - 4:54PM |
X12.00012: Surface properties of ultrathin ferroelectric films in external electric field Renat Sabirianov, Minoru Otani, Osamu Sugino The electric polarization of free standing ultrathin films of BaTiO$_{3}$ is analyzed using pseudopotential plane wave calculations within effective screening medium method. The polarization loop in asymmetrically terminated (Pb,Ba)TiO$_{3}$ film is biased, providing the existence of polarization without applied electric voltage across the film. We attribute the origin of bias to a creation of a bias field due to difference in surface work functions of TiO$_{2}$ and BaO terminations. This results in the formation of surface polarizations at each termination and inhomogeneous polarization profile across the thickness of the film. We show that the surface develops in-plane component of polarization in paraelectric state, and also in case of the ferroelectric films when the electric field applied perpendicularly to the plane of the film. [Preview Abstract] |
Thursday, March 19, 2009 4:54PM - 5:06PM |
X12.00013: Growth and Characterization of EuTiO$_{3}$ films on SrTiO$_{3}$ (001) H. Q. Wang, J. D. Ferguson, A. R. Woll, D. A. Muller, J. D. Brock Perovskite oxides display a variety of interesting physical properties, and their heteroepitaxial structures are of significant interest in oxide electronics. EuTiO$_{3 }$and SrTiO$_{3}$ are nearly perfectly lattice matched and have the same valence structure, and are therefore well suited as a model system for the study of perovskite interfaces. One outstanding question about such interfaces concerns the factors that determine and limit atomic and electronic abruptness. In this work, several monolayer thick EuTiO$_{3}$ films are grown on single-crystal SrTiO$_{3}$ (001) substrates using Pulsed Laser Deposition (PLD). The growth mechanisms are probed by the combination of synchrotron based \textit{in situ }small angle x-ray scattering (SAXS) and \textit{in situ} Reflection High Energy Electron Diffraction (RHEED). The atomic-scale interfacial properties are investigated by high resolution Scanning Transmission Electron Microscopy (STEM) and spatially resolved Electron Energy Loss Spectroscopy (EELS). [Preview Abstract] |
Thursday, March 19, 2009 5:06PM - 5:18PM |
X12.00014: Density functional calculations of the structure of near-surface oxygen vacancies and electron localization on CeO$_2$(111) M. Veronica Ganduglia-Pirovano, Juarez L.F. Da Silva, Joachim Sauer The use of ceria as a key component in catalysts and as an electrolyte for solid oxide fuel cells relies on its notorious capability of storing and releasing oxygen. This property results from the facility of both formation and healing of oxygen vacancies in ceria. Several studies have been reported in the last years for reduced CeO$_2$(111), however, one of the most topical issues surrounding oxygen vacancies on CeO$_2 $(111), namely, the relative stability of surface and subsurface defects, is still under intense debate. Using density functional theory with the Heyd-Scuseria-Ernzerhof (HSE06) hybrid functional as well as the DFT+U approach, we find subsurface vacancies with $(2{\times}2)$ periodicity to be energetically more favorable by 0.45 (HSE06), 0.47 (PBE+U), and 0.22 eV (LDA+U). The excess-electrons localize not on Ce ions which are nearest neighbor to the defect as priorly suggested, but instead on those that are next-nearest neighbors. The excess-electron distribution and the preference for subsurface vacancies are explained in terms of defect-induced lattice relaxation effects. [Preview Abstract] |
Thursday, March 19, 2009 5:18PM - 5:30PM |
X12.00015: Interface structure determination of crystalline oxides on silicon using synchrotron x-ray diffraction F.J. Walker, J.W. Reiner, A.M. Kolpak, Y. Segal, Z. Zhang, D. Su, Y. Zhu, M.S. Sawicki, C.C. Broadbridge, S. Ismail-Beigi, C.H. Ahn As electronic devices are reduced in size, well controlled atomically abrupt interfaces become increasingly important. This is especially true for field effect devices where the conducting channel is only a few atoms thick. Here we discuss the experimental determination of the atomic structure of an interface for a model system, crystalline oxides on silicon, which contains the essential elements of field effect devices. For both BaO/Si and SrTiO$_{3}$/Si structures, we use a combination of synchrotron x-ray scattering, Z-contrast transmission electron microscopy and density functional theory to determine the structure. The combination of these approaches has led to a unique detailed model of the interface. We have discovered features of these interfaces that emerge during growth that can be used to understand important elements of the measured electrical properties and microscopically identify sources of fixed charge, interface traps and field-dependent mobility. [Preview Abstract] |
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