Bulletin of the American Physical Society
APS March Meeting 2013
Volume 58, Number 1
Monday–Friday, March 18–22, 2013; Baltimore, Maryland
Session Y40: Surfaces, Interfaces, and Thin Films: Oxides |
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Sponsoring Units: DCMP Chair: Michael Pierce, Rochester Institute of Technology Room: 349 |
Friday, March 22, 2013 8:00AM - 8:12AM |
Y40.00001: Metal Oxide Growth and Characterization on CVD Graphene Akitomo Matsubayashi, Joseph Abel, Dhiraj Prasad Sinha, Ji Ung Lee, VIncent LaBella Thin metal oxide layers deposited on graphene can be utilized as dielectric barriers between metals and graphene to help isolate a metal contact from the graphene channel. This is important for graphene based spintronic devices as dielectric layers between the ferromagnetic electrode and graphene have been shown to increase the spin relaxation time measured utilizing non-local detection and spin precession measurements. However, simply depositing metal oxide layers such as aluminum oxide on graphene results in non-uniform film lowering the quality of the interface barrier. We will present a systematic study of aluminum oxide layers grown on CVD (chemical vapor deposition) graphene under ultra-high vacuum conditions with and without titanium seed layers. The aluminum oxide layers with the titanium seed layers showed reduced surface roughness. The chemical and structural composition determined by XPS (X-ray photoelectron spectroscopy) will be also presented that shows full oxidation of the aluminum and partial oxidation of the titanium. Our previous work which demonstrated that introducing HfO2 barrier layer in the epitaxial graphene devices on SiC wafer improves the measured lifetime and spin injection efficiency will be briefly presented as well. [Preview Abstract] |
Friday, March 22, 2013 8:12AM - 8:24AM |
Y40.00002: Atomically Resolved Surface of Laser-MBE Grown SrRuO$_ 3$ Thin Films A. Tselev, P. Ganesh, A.P. Baddorf, S.V. Kalinin Surface of SrRuO$_3$ (SRO) thin films is of high interest since SRO layers are used as bottom electrodes in oxide heterostructures demanding sharp interfaces. Here we studied SRO films in-situ using STM with atomic resolution. Films were grown on undoped, SrTiO$_3$ substrates by laser-MBE. Depending on preparation conditions, the film surfaces exhibited varying reconstructions. Films deposited at 650$^{\circ}$C and annealed at deposition conditions for 15 min. revealed surfaces with double-row 1D-structures along $<110>_{pc}$ of SRO. Atoms in the 1D-structures are packed in square or zigzag arrangements. The surface in-between the structures appeared poorly ordered. Similar patterns were observed on surfaces of films deposited at 700$^{\circ}$C without anneal. In turn, deposition at 700$^{\circ}$C with post-anneal resulted in well-ordered surfaces covered by double-rowed structures with square atomic arrangement. Ab initio DFT calculations show a high local DOS from oxygen adatoms with zigzag and square patterns contributing to STM images. Oxygen atoms have high adsorption energies and will be present at our growth conditions. Surface O-adatoms show AFM coupling to the film, with possible ramifications to understand interfacial bonding/magnetism between SRO and oxide-insulators. [Preview Abstract] |
Friday, March 22, 2013 8:24AM - 8:36AM |
Y40.00003: Transport phenomena in SrVO$_3$ thin films Man Gu, Stuart Wolf, Jiwei Lu Bulk SrVO$_{3}$ (SVO) with a 3$d^{1}$ electronic configuration has been found to exhibit metallic and Pauli paramagnetic behavior. We have obtained epitaxial SVO films grown on various substrates (STO, SLAO, LSAT and LAO) using a pulsed electron-beam deposition (PED) technique. The film transport properties were found to be strongly dependent on the substrate. A 40 nm SVO film deposited on an STO substrate exhibited metallic behavior with the electrical resistivity following a T$^{2}$ law that corresponds to a Fermi liquid system, the resistance ratio R(300K)/R(2K) was $\sim$ 1.66. Hall measurements showed that the mobility increased slightly as the temperature was decreased. A small positive out-of-plane magnetoresistance was observed, it was only 0.045{\%} at 5 K and 7 Tesla. SVO films with the same thickness grown on SLAO, LSAT and LAO showed semiconducting behavior, the different transport properties in the SVO films could be attributed to the compressive film strain or the different film-substrate interfaces. [Preview Abstract] |
Friday, March 22, 2013 8:36AM - 8:48AM |
Y40.00004: High quality, hybrid-MBE growth of SrVO$_{3}$ thin films Jarrett Moyer, Craig Eaton, Roman Engel-Herbert Vanadium-based transition metal oxides are an intriguing class of materials to study due to the metal-to-insulator (MIT) transitions that arise in many of the binary oxides (i.e. VO$_{2}$, V$_{2}$O$_{3}$, V$_{2}$O$_{5})$. The perovskite SrVO$_{3}$ is metallic in bulk; however, it is possible to induce an MIT by modulating the bandwidth through strain or dimensional confinement. A mandatory requirement for controlling the electronic phase transition properties in material systems with strong correlation is the growth of high quality, stoichiometric thin films. This is demonstrated here with the growth of SrVO$_{3}$ on LSAT (001) substrates using a hybrid-MBE technique, where the Sr is evaporated from an effusion cell and the V is provided through the metal-organic precursor vanadium oxo-tri-isopropoxide (VTIP). The structural properties of films with varying VTIP:Sr ratios are characterized by RHEED, XRD, AFM and TEM. These measurements demonstrate that SrVO$_{3}$ can be grown with excellent structural quality, atomically flat surfaces and rocking curves of the same width as the substrate, accomplishing a necessary first step in controlling the MIT in SrVO$_{3}$. [Preview Abstract] |
Friday, March 22, 2013 8:48AM - 9:00AM |
Y40.00005: Optimizing Pt/TiO$_{2}$ templates for textured PZT growth and MEMS devices Daniel Potrepka, Glenn Fox, Luz Sanchez, Ronald Polcawich Crystallographic texture of lead zirconate titanate (PZT) thin films strongly influences piezoelectric properties used in MEMS applications. Textured growth can be achieved by relying on crystal growth habit and can also be initiated by the use of a seed-layer heteroepitaxial template. Template choice and the process used to form it determine structural quality, ultimately influencing performance and reliability of MEMS PZT devices such as switches, filters, and actuators. This study focuses on how 111-textured PZT is generated by a combination of crystal habit and templating mechanisms that occur in the PZT/bottom-electrode stack. The sequence begins with 0001-textured Ti deposited on thermally grown SiO$_{2}$ on a Si wafer. The Ti is converted to 100-textured TiO$_{2}$ (rutile) through thermal oxidation. Then 111-textured Pt can be grown to act as a template for 111-textured PZT. Ti and Pt are deposited by DC magnetron sputtering. TiO$_{2}$ and Pt film textures and structure were optimized by variation of sputtering deposition times, temperatures and power levels, and post-deposition anneal conditions. The relationship between Ti, TiO$_{2}$, and Pt texture and their impact on PZT growth will be presented. [Preview Abstract] |
Friday, March 22, 2013 9:00AM - 9:12AM |
Y40.00006: First-principles calculations of water-based surfactant-assisted growth of polar CaO(111) oxide film Xin Tan, Peter Zapol Despite many attempts to grow rocksalt (111) oxide surfaces, the growth of an atomically flat polar oxide film with an arbitrary thickness still remains challenging because of surface roughening during the growth process, such as faceting into neutral \textbraceleft 100\textbraceright\ surface planes. This seemingly unavoidable behavior leads to a grainy morphology and diminished functionality. Here, we present a first-principles investigation of the surfactant-assisted growth of polar CaO(111) film in the presence of a water-based surfactant, both from thermodynamic and kinetic points of view. We show that water molecules not only supply a surfactant by depositing hydrogen on the surface throughout the growth process, but also supply oxygen atoms as an elemental constituent in the film growth, i.e. water oxygen atoms are easily inserted into the top surface layer of the growing film. We suggest that adding water surfactants to conventional synthesis techniques leads to the continuous presence of hydrogen atoms in the surface region during the growth process, which efficiently quenches polarity and dynamically stabilizes the growth of the polar surface, and thus facilitates the growth of defect-free CaO(111) films with arbitrary thickness. [Preview Abstract] |
Friday, March 22, 2013 9:12AM - 9:24AM |
Y40.00007: The Electronic Structure of Nonpolar Surfaces in Insulating Metal Oxides Danylo Zherebetskyy, Lin-Wang Wang Understanding the electronic and geometric structures of metal oxide surfaces has a key interest in many technological areas. A randomly chosen crystal surface has a high probability of being polar, unstable and containing in-gap states due to surface dangling bonds. As a result, the surface should be stabilized by passivation or reconstruction. However, do the nonpolar surfaces of ionic crystals of insulating metal oxides need the passivation or reconstruction similar to covalent crystals? We address this question by analyzing the nonpolar surfaces and their electronic structure for the common crystal structures of metal oxides. The study using periodic DFT calculations is performed for following representatives: Cu2O, ZnO, Al2O3, TiO2, V2O5, WO3, CaTiO3, Mg2SiO4. It has been shown that the nonpolar surface can be constructed out of dipole-free, charge-neutral and stoichiometric unit cells for each crystal. We demonstrate that all constructed and relaxed nonpolar surfaces of the metal oxides show a clear band gap. It should be emphasized that the constructed surfaces are neither reconstructed nor passivated. Additionally, we show a correlation between the electronic structure of the relaxed surfaces and Ewald energies calculated for the surface ions. [Preview Abstract] |
Friday, March 22, 2013 9:24AM - 9:36AM |
Y40.00008: In-situ study of Nb oxide and hydride for SRF cavity applications using aberration-corrected STEM and electron energy loss spectroscopy Runzhe Tao, Robert Klie, YoonJun Kim, David Seidman, Lance Cooley, Alexander Romanenko We present an atomic-resolution study of the effects that a 48 hour bake at 120 $^{\circ}$C in vacuum has on the high-field properties of Nb-based SRF cavities. This bake results a significant increase in the high-field quality factor Q, reversely, 800 $^{\circ}$C bake for 2 hour reduces the H$_{\mathrm{c3}}$/H$_{\mathrm{c2}}$-ratio of cavities. Several mechanisms have been proposed, including an increased NbO$_{\mathrm{x}}$ surface layer thickness and the precipitation of NbH$_{\mathrm{y}}$. Using a combination of atomic-resolution Z-contrast imaging and electron energy-loss spectroscopy with in-situ heating and cooling experiments, we examine the atomic and electronic structures of Nb and related oxides/hydrides near the cavity surface. We quantify the oxygen diffusion on surface during bake by measuring the local Nb valence using EEL spectra. Also, we demonstrate that hydrogen atoms incorporated into the Nb crystal, forming $\beta $-NbH precipitates, can be directly visualized using annular bright field imaging in our aberration-corrected JEOL ARM-200CF. Finally, the effects of the 800 $^{\circ}$C baking process on the local hydrogen and other impurity will be examined by in-situ heating and cooling experiments. Our results will be combined with atom-probe tomography to develop a 3-D impurity and phase profile of Nb near the SRF cavity surface. [Preview Abstract] |
Friday, March 22, 2013 9:36AM - 9:48AM |
Y40.00009: Incorporation of Non-metal Impurities at the Anatase TiO$_2$(001)-(1 $\times$ 4) Surface Jun Hee Lee, Daniel Fernandez Hevia, Annabella Selloni Surface doping of TiO$_2$ is of special interest because the distribution of impurities at or near the surface can have a significant influence on the photocatalytic properties of TiO$_2$. We have used first-principles density functional theory (DFT) calculations to determine the incorporation mechanisms of nitrogen (N) and carbon (C), two widely used $p$-type dopants, at the reconstructed (001) surface of anatase, the TiO$_2$ polymorph most relevant for photocatalysis. Starting from adsorbed impurities, we identify various incorporation pathways and show that the non-exposed oxygen sites just below the surface play a crucial role in accomodating non-metal impurities at the TiO$_2$(001) surface. Based on the obtained results, we propose strategies which could help to increase the doping concentration and the photocatalytic activity at the TiO$_2$ surface by exploiting the morphology of the reconstructed surface [1].\\[4pt] [1] Incorporation of Non-metal Impurities at the Anatase TiO$_2$(001)-(1 $\times$ 4) Surface, \underline {arXiv:1209.1602}. [Preview Abstract] |
Friday, March 22, 2013 9:48AM - 10:00AM |
Y40.00010: TiO$_{2}$ Surface Defects with the Tetrahedral Cationic Coordination Ken Park, Vincent Meunier, Nan Hsin Yu, Ward Plummer Titanium dioxide is one of the most extensively investigated transition metal oxide. It has well-known applications in catalytically converting toxic organic and inorganic materials to benign products, as well as turning solar energy into a chemical. In these processes, it is believed that surface defects with lower coordination and/or stoichiometry play crucial roles. Our study of a (2$\surd$2 $\times \surd $2) R45 reconstructed TiO$_{2}$(001) using scanning tunneling microscopy and density functional theory reveals that the basic building blocks of the reconstruction can be modeled as fully stoichiometric nanocluster defects. As in the bulk-terminated (001) surface, the atoms in the nanocluster are under-coordinated, for example, 4-coordinated Ti, 1-coordinated, and 2-coordinated O atoms. However, the absence of neighboring atoms drives the nanocluster to relax into a structure, which possesses tetrahedrally coordinated Ti atoms. This result will be compared and discussed with the reported nanocluster defects on TiO$_{2}$(110). [Preview Abstract] |
Friday, March 22, 2013 10:00AM - 10:12AM |
Y40.00011: Increase of Structural Phase Transition Temperature with Cr doping in Cr:VO$_{2}$ Thin Films B.L. Brown, Mark Lee, P. Clem, C.D. Nordquist, T.S. Jordan, S.L. Wolfley, D. Leonhardt, J.A. Custer Bulk crystal VO$_{2}$ has a well-known structural phase transition near T$_{c} =$ 68 $^{\circ}$C that separates a low-temperature insulating phase from a high-temperature metallic phase with several orders-of-magnitude resistance contrast between the two phases. We report electrical and optical studies of the effect of Cr doping on the T$_{c}$ in Cr:VO$_{2}$ thin films. Resistivity, Hall effect, and infrared reflectivity all show that Cr doping systematically increases T$_{c}$ from 50 $^{\circ}$C up to $\sim$ 75 $^{\circ}$C at 11{\%} Cr with similar transition width and hysteresis from DC to infrared, but the effect appears to saturate. At the same time, there is a modest decrease in resistance contrast. We will discuss possible effects of both carrier density and scattering changes across T$_{c}$ on the resistance. Sandia National Laboratories is a multi-program laboratory managed and operated by Sandia Corporation, a wholly owned subsidiary of Lockheed Martin Corporation, for the U.S. Department of Energy's National Nuclear Security Administration under contract DE-AC04-94AL85000. [Preview Abstract] |
Friday, March 22, 2013 10:12AM - 10:24AM |
Y40.00012: Non-Destructive Element Specific Density Depth Profiling by Resonant Soft X-ray Reflectometry Sebastian Macke, Adriano Verna, Maurits Haverkort, Abdullah Radi, Ronny Sutarto, Georg Christiani, Gennady Logvenov, Bernhard Keimer, George Sawatzky, Vladimir Hinkov X-ray resonant reflectometry (XRR) is the ideal tool to study the depth resolved and element-specific electronic structure of multilayer films. By changing angle, energy and polarization of the incoming beam complete reflectivity maps can be measured leading in principle to an accurate picture of the depth resolved electronic states of thin films. The standard model used in reflectometry is based on compound layers with a defined thickness, roughness and dielectric tensor. But such a simple model is usually not capable to reproduce a full measured reflectivity map. The main reasons are especially contaminations, additional oxide layers and interdiffusion between layers. However, introducing a layer system based on the element specific atomic density and scattering factors instead of dielectrics tensors allows more degrees of freedom for the system and allows to reproduce the reflectivity maps. Thereby the advanced model is capable to retrieve the element specific density profiles of thin films. The method is introduced by analyzing a simple film of PrNiO3 grown on an [Preview Abstract] |
Friday, March 22, 2013 10:24AM - 10:36AM |
Y40.00013: Investigation of electronic and magnetic properties of wurtzite NiO thin films Brian Busemeyer, Michael Shaughnessy, Liam Damewood, C.Y. Fong We study the electronic and magnetic properties of wurtzite (B4) NiO thin films grown epitaxially on wide gap semiconductors to understand factors affecting their half-metallic properties, in particular, the effect of film thickness, interface geometry, and dangling bonds. One, two, and four consecutive layers of NiO are considered, both buried within bulk ZnO, and as thin surface films on bulk ZnO. We perform GGA+U calculations, with the U value determined via a self consistent linear response approach. We find that the interface generates small s-p hybridized states at the Fermi energy, which can possibly destroy the half metallicity; these states are likely due to effects from strain at the interface. We also find that the interface can influence the Ni d states in markedly different ways, depending on the geometry and the presence of dangling bonds. These factors can determine whether the interface Ni d states resemble those from Ni in bulk wurtzite NiO, or demonstrate semiconductivity, more akin to d states of Zn atoms within bulk ZnO. [Preview Abstract] |
Friday, March 22, 2013 10:36AM - 10:48AM |
Y40.00014: Structure of Oxide Surfaces Rong Yu Surfaces of metal oxides are of crucial importance for a variety of technological applications such as heterogeneous catalysis, thin film growth, gas sensing, and corrosion prevention. Due to the complexities of oxides in crystal structure and electronic structure, however, the surface science of oxides lags far behind that of metals or semiconductors. Conventional surface-science techniques are usually limited to surfaces of single crystals of conductors. Metal oxides are usually good insulators, making them difficult for conventional surface science techniques. On the other hand, the complex atomic structures of oxides results in too many structural parameters to be determined by spectroscopy or diffraction methods. We will show that the surface structure of oxides can be directly imaged and measured at the sub-angstrom scale using aberration-corrected transmission electron microscopy. The atomic positions of oxide surfaces can be measured to an accuracy of picometers, comparable to that obtained by conventional surface science techniques on single crystals.\\[4pt] [1] R. Yu, L.H. Hu, Z.Y. Cheng, Y.D. Li, H.Q. Ye, J. Zhu, Phys. Rev. Lett., 105, 226101 (2010).\\[0pt] [2] M.R. He, R. Yu, J. Zhu, Angew. Chem. Int. Ed., 124, 7864 (2012). [Preview Abstract] |
Friday, March 22, 2013 10:48AM - 11:00AM |
Y40.00015: UV-induced stable photoconductivity in Indium Oxide films Ehab Abdelhamid, Rupam Mukherjee, Debabrata Mishra, Ambesh Dixit, Boris Nadgorny, Gavin Lawes We have investigated the effects of UV radiation on the electrical conductivity of sputter deposited Indium Oxide films for samples annealed under different conditions. The films were annealed in air, hydrogen, argon, nitrogen, and vacuum to modify the microstructure and distribution of point defects. X-ray diffraction shows the formation of polycrystalline single phase films, with the average crystallite size changing under different annealing conditions. We find that the resistance sharply decreases to between 0.1{\%} and 50{\%} of its initial value on exposure to UV irradiation. The magnitude of the decrease depending on the annealing conditions, with the largest relative change occurring in the as-prepared sample (high initial resistance), and the smallest decrease observed in the Hydrogen-annealed film (low initial resistance). This low resistant state is surprisingly stable, having a time constant of several hours or longer to relax to the initial value after the UV illumination is removed. [Preview Abstract] |
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