Papers

AVS1996 Session SS-FrM: Metal Oxide Surfaces: Structure and Characterization

Friday, October 18, 1996 8:20 AM in Room 204C

Friday Morning

Start	Invited?	Item
8:20 AM		SS-FrM-1 Scanning Tunneling Microscopy Studies of Thin Film Oxide Surfaces. M. Gallagher (Lakehead University); M. Fyfield (Portland State University); J. Cowin, G. Herman, C. Peden, S. Joyce (Pacific Northwest National Laboratory) Many metal oxide surfaces are unsuitable for detailed studies due to their insulating nature, the lack of large, single-phase crystals, and/or difficulties associated with surface preparation. Many of these problems can be obviated by using ultrathin films grown epitaxially on single crystalline, conducting substrates. As a real space probe, the STM can directly determine the microscopic surface structure of these films. Two examples will be presented: MgO grown on Mo(001) and TiO\sub x~1\ grown on W(110). The microscopic structures of these oxide films are found to be strongly dependent on the growth conditions. MgO is an insulator in the bulk, however, ultrathin films of MgO (up to ~30\Ao\ thick) can be imaged. At low growth temperatures, small domains of MgO uniformly cover the substrate, while at higher growth temperatures, larger domains coexist with exposed patches of the substrate. Macroscopic single crystals of the rocksalt oxide, TiO\sub x~1\, are not available. Long range, periodic structures of ultrathin film TiO, however, can be grown on the surface of W(110) substrates. A combination of low energy electron diffraction and STM has been used to determine the structure of these films. The structure of the TiO films was found to depend on both the coverage and the post-growth annealing temperature. Pacific Northwest National Laboratory is a multiprogram national laboratory operated for the U.S. Department of Energy by Battelle under contract DE-AC06-76RLO 1830.
9:00 AM		SS-FrM-3 Preparation and Characterization of Epitaxial Titanium Oxide Films on a Mo(100) W. Oh, C. Xu (Texas A&M University); D. Kim (Hallym University, Korea); D. Goodman (Texas A&M University) Ultrathin titanium oxide films have been synthesized on the Mo(100) surface. Epitaxial TiO\sub 2\ films of varying film thickness were prepared by evaporating titanium in an oxygen background (~5x10\super -7\ Torr) between 500K and 700K, followed by annealing to between 900 and 1200K in vacuum. The growth, composition and structure of the TiO\sub 2\ films have been investigated using ISS, XPS, AES, LEED, and STM. A (2x1) LEED pattern was observed after annealing from 900K to 1200K in vacuum for film thicknesses of 1.6ML, 5ML and 20ML. This (2x1) LEED pattern is in good agreement with the corresponding results for for the (011)-faceted rutile TiO\sub 2\(001) surface. LEED and STM results show that the TiO\sub 2\ films order along the <110> direction of the Mo(100) substrate. XPS data show that unannealed titanium oxide films exhibit only the Ti\super 4+\ valence state, whereas annealed titanium oxide are partially reduced to the Ti\super 3+\ state. ISS measurements of the unannealed titanium oxide films show that TiO\sub 2\ films complete the growth of first layer prior to the growth of thicker films.
9:20 AM		SS-FrM-4 Thermal Stability of Metal Oxides: A Surface Study J. Anderson, M. Kuhn, U. Diebold (Tulane University) Different surface preparation procedures applied to tranisiton metal oxides may used to modify the surface structure and composition, defect concentration, and chemical reactivity of these materials. This has motivated our investigation of the surface stability of a TiO\sub 2\(110) crystal and an MBE-grown Fe\sub 3\O\sub 4\ (001) film on Mg(001) using LEED, ISS, XPS, and STM. A nearly-perfect, stoichiometric (1x1) surface of TiO\sub 2\(110) can be prepared at room temperature and atomically-resolved images are obtained by STM. Annealing to temperatures above 700 K leads to the formation of defect sites due to the creation of single oxygen vacancies; these defects are believed to be chemically very reactive. The defect density is increased with temperature. Vacancies are found to be randomly distributed on the terraces with nucleation along step edges. An analysis of the STM images of these defect sites, along with the results of a first-principles pseudopotential calculation, strongly suggest that the STM images are dominated by uncoordinatively unsaturated Ti atoms, in contrast to geometric expectations. LEED measurements of the one micron thick Fe\sub 3\O\sub 4\ film show a p(1x1) terminated surface at room temperature which is transformed into a c(1x1) structure by annealing to >780 K in UHV; this process was found to be reversible by subsequent exposure to O\sub 2\ at 650 K. Annealing above 900 K results in the formation of extremely flat, wide (exceeding 1000 \Ao\) terraces. A large amount of Mg is found on the surface, presumably due to the thermally-induced breakdown of the MgO substrate and subsequent migration of Mg through the Fe\sub 3\O\sub 4\ lattice.
9:40 AM		SS-FrM-5 Chemical and Spectroscopic Surface Science Studies of MoO\sub 3\ and Oxide Supported MoO\sub 3\ S. Street, D. Goodman (Texas A&M University) Molybdate (MoO\sub 3\) and molybdate supported on highly ordered ultra-thin films of various metal oxides, e.g. Al\sub 2\O\sub 3\, MgO, SiO\sub 2\, have been studied using high resolution electron energy loss spectroscopy (HREELS), electronic energy loss spectroscopy (ELS), and temperature programmed desorption (TPD) of various probe molecules. These well-characterized ultrathin films are supported on a refractory metal substrate, e.g. Mo or Ta, allowing homogeneous temperature control and the use of charge particle probes on the insulating surfaces. The vibrational features associated with the terminal Mo=O and the bridging M-O-Mo in tetrahedral molybdate have been identified by HREELS. The intensity of the Mo=O loss (~ 1000 cm\super -1\) of molybdate decreases following the successive adsorption/desorption of methanol. This particular structure is essential for the catalytic activity of molybdate in alcohol oxidation, as determined previously by in-situ Raman spectroscopy studies. The M=O feature appears in the MoO\sub 3\/alumina system only after annealing to >800 K. Concurrent with the rise in intensity of the vibrational loss feature at 1000 cm\super -1\, a feature in the ELS spectrum appears at ~ 1.6 eV. The influence of support oxides on this electronic feature and the desorption characteristics of probe molecules will also be discussed.
10:00 AM		SS-FrM-6 The Effect of Sulfur on the Surface Structure of SrTiO\sub 3\ (100) X. Li, D. Bonnell (University of Pennsylvania) Single crystals of SrTiO3 are frequently used as substrates for the growth of high quality high Tc thin films; therefore an understanding of the surface structure is very important. The effect of thermochemical variations on the evolution of surface morphology is only beginning to be studied. We report here the effect of annealing on a sulfur doped SrTiO3(100). The initial surface that is atomically smooth with 2 angstroms steps can be obtained by annealing the surface at 550 oC for 10 h followed by 900 oC for 20 sec at about 1 x 10-9 Torr. The result indicates that the surface is terminated alternatively by Sr-O and Ti-O layers. Our finding agrees with the fact that Sr-O and Ti-O surfaces have very similar surface energies; however, the alternating terminations have not been observed in other STM studies on the SrTiO3(100) surface. Continued annealing results in a high density of small clusters of 100 angstroms diameter and 20 angstroms height that are preferably located along the steps. In conjunction with STM, AES and XPS were applied to study the compositional and chemical environmental changes of the surface upon the annealing. Based on the AES and XPS results, we suggest that sulfur segregation is charge limited and describe a model of clustering.
10:20 AM		SS-FrM-7 A Combined Medium Energy Ion Scattering and Photoemission Study of Pt Deposition on SrTiO\sub 3\(001) and Ba\sub x\Sr\sub (1-x)\TiO\sub 3\ Surfaces M. Copel, T. Shaw, R. Tromp (IBM T.J. Watson Research Center) Perovskite titanate compounds have recently found use as dielectrics in prototype ferroelectric memory devices. However, thin film Pt/ferroelectric/Pt structures exhibit capacitances far below expected values, possibly because thin films have a much lower polarizability than bulk material. It is likely that a key factor in determining capacitance is the effect of Pt metallization on the ferroelectric. We have investigated the interaction of Pt with single-crystal SrTiO\sub 3\(001) and polycrystalline Ba\sub x\Sr\sub (1-x)\TiO\sub 3\ thin films. Using medium energy ion scattering, we observe that the substrates are inert to Pt. But with x-ray photoemission spectroscopy we observe band bending due to metallization, as would be expected in the formation of a Schottky contact. We will also report medium energy ion scattering results for the structure of the clean SrTiO\sub 3\(001) surface. There is an extensive reordering of the surface, with a severe distortion of the Ti sublattice, and a bulk-like termination of the Sr sublattice.
10:40 AM		SS-FrM-8 The Behavior of Interface States in SrTiO3 by Photo Assisted STM A. Frye, D. Bonnell (University of Pennsylvania) The technique of photo-assisted tunneling spectroscopy (PATS) is applied to directly measure deep level defect states at the boundary of SrTiO3 bicrystals. These traps give rise to the special functionality of polycrystalline barrier-layer electroceramics (i.e., ZnO varistors and BaTiO3 thermistors). The details of non-stoichiometry at the grain boundary (GB) core are not resolvable from the chemistry of the space charge region by conventional methods such as STEM X-ray analysis. In the pursuit of tracing the electronic properties of GBs to their microscopic origin, the PATS technique may provide detailed electronic structural information to which the resultant GB electrical activity may be correlated.
11:00 AM		SS-FrM-9 Scanning Tunneling Microscopy Image Calculations of an Incommensurate Metal Oxide Overlayer: FeO on Pt(111) H. Galloway (Lawrence Berkeley National Laboratory); P. Sautet (Laboratoire De Chimie Th\aa e\orique, ENS and Institut de Recherche sur la Catalyse, France); M. Salmeron (Lawrence Berkeley National Laboratory) The surface of iron oxide thin films on Pt(111) has been investigated by LEED [1], STM [2], and XPD [3]. These techniques indicate that the monolayer film is a bilayer of Fe and O similar to that in the (111) planes of cubic FeO. The layer is incommensurate with the Pt since the unit cell dimensions are 3.1 and 2.77 \Ao\ respectively, giving rise to Moir\aa e\-type structures. In this work we present an analysis of the STM images using the Electron Scattering Quantum Chemistry (ESQC) method. The calculations show that the observed topography in the images is not simply related to the topographic variations due to the incommensurate overlayer. The amplitude of the atomic scale corrugation however, does have a simple dependence on the physical topography and only a lesser dependence on the coordination with respect to the substrate. The atomic scale corrugation is predominantly due to electronic states near the Fermi level composed of O 2p\sub z\ and Fe 3d\sub z2\ orbitals, with maxima over O positions for a Pt tip and over Fe positions for an oxygen terminated tip. The atomic scale corrugation is strongly dependent on the Fe-O bond length, with our calculations showing good agreement with the experimental result of Kim, et al [3]. [1] G.H. Vurens, V. Maurice, M. Salmeron and G.A. Somorjai, Surf. Sci. 268, 1-3 (1992) 170. [2] H.C. Galloway, J.J. Ben\aa i\tez and M. Salmeron, Surf. Sci. 298, 1 (1993) 127. [3] Y.J. Kim, C. Westphal, R.X. Ynzunza, H. Xiao, Z. Wang, H.C. Galloway, M. Salmeron, M.A.Van Hove and C.S. Fadley, in preparation (1995)
11:20 AM		SS-FrM-10 Oxygen Plasma Assisted Molecular Beam Epitaxial Growth of Different Crystal Faces of \alpha\-Fe\sub 2\O\sub 3\ and Fe\sub 3\O\sub 4\ Films Y. Kim, Y. Gao, S. Chambers (Pacific Northwest National Laboratory) The need to synthesize well-defined, single-crystal surfaces of the less common oxides will become increasingly more critical as the surface science of these materials matures. Magnetite (Fe\sub 3\O\sub 4\) and hematite (Fe\sub 2\O\sub 3\) are among the most important oxides for fundamental studies of surface geochemistry. Yet, these oxides are not readily available as bulk single crystals. We have used oxygen-plasma assisted molecular beam epitaxy to synthesize pure, crystallographically well-defined surfaces of these materials. \alpha\-Fe\sub 2\O\sub 3\ has been grown on \alpha\-Al\sub 2\O\sub 3\(0001), (11-20), and (1-102), and Fe\sub 3\O\sub 4\ was grown on MgO(001), (110), and \alpha\-Al\sub 2\O\sub 3\(0001). The resulting epitaxial films and surfaces have been characterized by RHEED, LEED, XPS, XPD, XRD, and AFM. The selective growth of these phases depends critically on the choice of substrate, the iron and oxygen fluxes, and the substrate temperature. In general, we find that lattice matching, minimization of interface chemistry, oxygen sublattice continuity across the interface, and careful flux control during growth are extremely important for successful epitaxy. The growth of \alpha\-Fe\sub 2\O\sub 3\ is achieved using a low iron-to-oxygen flux ratio compared with that used to grow Fe\sub 3\O\sub 4\. Fe\sub 3\O\sub 4\ must be grown at the relatively low substrate temperature of 250\super o\C on MgO to avoid Mg outdiffusion. In this talk, we discuss recent progress in our laboratory in the growth and detailed characterization of these important oxide surfaces with special emphasis on Fe\sub 3\O\sub 4\(001)/MgO(001).