Papers

AVS1999 Session SS1+EM-TuA: Oxides: Growth and Structure

Tuesday, October 26, 1999 2:00 PM in Room 606

Tuesday Afternoon

Start	Invited?	Item
2:00 PM		SS1+EM-TuA-1 Noncommutative Band Offset at α-Cr₂O₃/α-Fe₂O₃(0001) Heterojunctions S.A. Chambers, Y. Liang (Pacific Northwest National Laboratory) We have measured the valence band discontinuity at artificially structured, epitaxial heterojunctions of α-Cr₂O₃(0001) and α-Fe₂O₃(0001) grown on α-Al₂O₃(0001) substrates. Oxygen-plasma-assisted molecular beam epitaxy was the growth technique, and band offsets were measured by core-level and valence-band photoemission. Layered film structures of these two materials maintain the in-plane lattice parameter of α-Fe₂O₃(0001). Thus, the α-Cr₂O₃(0001) layers are under a 2.3% tensile stress. The valence band offsets are 0.3±0.1 eV and 0.7±0.1 eV when the top layer is Fe₂O₃ and Cr₂O₃, respectively. The noncommutativity in band offset appears to be due to a growth-sequence-dependent interface dipole caused by different extents of intermixing for the two kinds of interfaces.
2:20 PM		SS1+EM-TuA-2 Ultrathin Iron Oxide Films R.L. Stockbauer, J. Karunamuni, A. Koveshnikov, R. Madjoe, R.L. Kurtz (Louisiana State University) Thin films of spin polarized conductors are receiving increased attention as potential contacts for devices such as spin-valves and high-sensitivity disk sensors. We have succeeded in producing ultra thin films of FeO and Fe₃O₄ and characterized their morphology using STM and LEED. The oxides are grown by depositing thin films of Fe on a Cu(100) surface at room temperature. The Cu substrate has terraces ca. 100Å wide with steps perpendicular to the [010] direction. The Fe grows layer-by-layer in islands uniformly distributed on the Cu terraces. The oxides are formed by heating to 840K in 10^-6 Torr O₂. The change in the morphology of the surface after oxidation is truly striking. At low Fe coverage (<2ML), FeO completely rearranges the Cu substrate. Large terraces up to 1µm wide are now observed. The FeO itself forms long stripes typically 0.2µm wide and up to 10µm long parallel to the [011] direction. Shorter stripes are observed in the perpendicular direction. The FeO stripes display two superstructures. A coarse structure consists of a series of dark bands parallel to the stripe direction and spaced at 360Å intervals. The fine structure is a 20.5Å hexagonal pattern. A comparison of LEED data and simulations indicates that the overlayer is FeO(111) with stripes aligned along the [110] directions. Oxidizing Fe films thicker than 2ML results in the oxide phase of device interest, Fe₃O₄. These patches show a 170Å banded structure and the same size hexagonal superstructure but rotated 30° from that observed for the FeO . LEED data, simulations, and thermochemistry are consistent with the overlayer being Fe₃O₄(111) rotated 15° from the [001] azimuth. Surprisingly, these highly oriented oxide films do not show strong angular dependence in photoemission. MOKE, circular dichroism, and spin polarized measurements are underway to determine the magnetic properties of the oxide films.
2:40 PM		SS1+EM-TuA-3 Islanding, Facetting, and the Lattice Relaxation in the Growth of α-Fe₂O₃ on α-Al₂O₃ S.I. Yi, Y. Liang, S. Thevuthasan, S.A. Chambers (Pacific Northwest National Laboratory) The growth of α-Fe₂O₃ on c- and r-oriented α-Al₂O₃ by oxygen-plasma-enhanced molecular beam epitaxy has been investigated with reflection high-energy electron diffraction, non-contact atomic force microscopy, X-ray photoelectron spectroscopy, X-ray photoelectron diffraction, low-energy electron diffraction, and high-energy ion scattering. The epilayer is facetted due to compressional stress on the c-plane during growth of the first two monolayers. The epilayer structure is significantly distorted, with considerable in-plane strain. However, the film exhibits a constant Fe 2p binding energy with respect to the O 1s binding energy for all thicknesses, revealing the growth of phase-pure α-Fe₂O₃ throughout. A growth-mode transition begins at approximately two monolayers from strained wetting layer to the formation of clusters of rhombohedral islands when a relatively slow growth rate of 1 Å/min is used. However, this transition occurs over several monolayers at a growth temperature of 400 °C. A film with high structural order and orientational stability grows as the islands coalesce to form new and larger terraces. This transition to island-mediated growth can be kinetically impeded for higher growth rates of 0.1-0.3 Å/sec.¹ In contrast, the growth-mode transition will occur more rapidly if temperatures of 450 to 500 °C are used along with the slower growth rate. However, sustained growth at temperatures in excess of 500 °C results in the simultaneous formation of Fe₃O₄ (111) and/or γ-Fe₂O₃ (111) along with α-Fe₂O₃(0001). In contrast to growth in the c-plane, the epilayer shows only facetted growth on the r-plane, regardless of the deposition rate and substrate temperature. This result may be due to the lower surface symmetry of the r-plane compared to the c-plane. ¹ ¹ S. I. Yi, Y. Liang, S. A. Chambers, to be published in JVST A, Jul/Aug, 1999
3:00 PM		SS1+EM-TuA-4 Surface Reconstructions of Fe₃O₄(001) B. Stanka, W. Hebenstreit, U. Diebold (Tulane University); S.A. Chambers (Pacific Northwest National Laboratory) We have investigated the structural changes to the Fe₃O₄(001) surface which are induced by different sample treatments. The samples consisted of ~5000 Å thick films of Fe₃O₄(001) , grown with oxygen-plasma-assisted molecular beam epitaxy (OPA-MBE) on MgO(001) substrates. To study the surface termination, structure, morphology, and composition, scanning tunneling microscopy (STM), low energy electron diffraction (LEED), low energy He⁺ ion scattering (ISS), and x-ray photoelectron spectroscopy (XPS) were used. A (√(2) x √(2))R45° reconstruction relative to bulk-terminated Fe₃O₄ is induced by heating in oxygen (10^-6 - 10^-7 mbar) at temperatures between 250°C and 500°C after a through-air transfer from the MBE chamber. Our STM results show an appearance consistent with a B-layer termination, which consists of a layer of octrahedrally coordinated Fe and tetrahedrally coordinated O, along with one O vacancy per unit cell. This surface is autocompensated, or charge neutral, and is therefore expected to be stable. Further annealing in UHV causes a transformation to either a (1 x n) or a (2√(2) x √(2))R45 structure. In both cases, a (√(2) x √(2))R45 reconstructed surface can be regained reproducibly by annealing in oxygen. Interestingly, at no time do we observe the other autocompensated termination, which consists of one-half monolayer of tetrahedrally coordinated Fe(III), despite its observation in other laboratories. Thus, it appears that the surface termination is critically dependent on the method of surface preparation.
3:20 PM		SS1+EM-TuA-5 The Surface Structure of α-Al₂O₃(0001) Studied by LEED C.F. Walters (Sandia National Laboratories); E.A. Soares, M.A. Van Hove (Lawrence Berkeley National Laboratory); K.F. McCarty (Sandia National Laboratories) The surface structure of α-Al₂O₃(0001) has been investigated using normal-incidence low-energy electron diffraction. High quality sapphire samples were produced by ex-situ annealing in air, followed by in-situ cleaning using atomic deuterium at an elevated sample temperature. To investigate the possible occurrence of different surface structures and/or stoichiometries, the samples were processed, subsequent to the atomic deuterium cleaning, using several different procedures. These included annealing the sample in oxygen, and exposing the cooled sample to atomic deuterium. The experimental data were compared to results of calculations performed using an automated tensor LEED program. Several different models were used as the starting point for the analysis. These included termination by: an oxygen layer, a single aluminum layer, a double aluminum layer, and a "hydroxyl" covered single aluminum layer. Additionally, we have analyzed mixtures of these terminations, with the mixtures including different species, as well as the same species but containing different crystallographic planes (i.e., terraced surfaces with fractional unit cell step heights). For each sample preparation, the best structural model was determined via an analysis of the Pendry R-factor that took into account the effect of the additional fitting parameters for the models containing mixed terminations. The best-fit models derived from the tensor LEED analysis will be discussed with respect to the preparation method used, as well as compared to the results of recent first-principles calculations. This work supported by USDOE-OBES-Division of Materials Sciences.
3:40 PM		SS1+EM-TuA-6 Formation of Epitaxial Al₂O₃ on Ni₃Al(111) Studied by STM, HREELS, UPS, ΔΦ, and LEED A. Rosenhahn, J. Schneider, J. Kandler, T. Pelster (Universität Bonn, Germany); R. Linke (Eindhoven University of Technology, The Netherlands); C. Becker, K. Wandelt (Universität Bonn, Germany) The formation of oxides on the Ni₃Al(111) single crystal surface has been studied in the temperature range from 300K to 1000K by STM, HREELS, UPS, ΔΦ, and LEED. The experiments reveal a strong temperature dependence of the structure and the chemical composition of the resulting oxides, which is caused mainly by a rising aluminum mobility with increasing temperature. Adsorption of oxygen at room temperature creates a disordered surface which appears rough in STM images and shows a diffuse LEED pattern. At 600K STM images show patches of different appearance on the alloy surface. At low coverages the small oxidic nuclei possess a triangular shape, and HREELS measurements show a Al₂O₃-like composition. At higher coverages two areas with different appearance can be distinguished in STM, accompanied by the observation of mixed Ni/Al-oxide vibrations in HREELS. At 800K STM measurements show highly ordered triangular shaped oxidic islands. A moiré structure can be observed which corresponds to a superstructure due to an unrotated single O/Al overlayer, that has also been found by LEED and HREELS measurements. The electronic structure of the film being very similar to that of O/Al(111). Adsorption at 1000K finally leads to the growth of an oxidic film, that proceeds from steps exclusively. The oxide exists in two rotational domains that can be detected in LEED and STM. Both HREELS and UPS measurements show the formation of a Al₂O₃-doublelayer. Only at this temperature the chemical order of the underlying Ni₃Al(111) substrate is completely restored.
4:00 PM		SS1+EM-TuA-7 Surface Structure Determination of Yttrium-Stabilized ZrO₂(001) Surface Using X-Ray Photoelectron Diffraction S. Thevuthasan, S.I. Yi (Pacific Northwest National Laboratory); Y.-J. Kim (Taejon National University of Technology, Korea); T.T. Tran, S.A. Chambers, C.H.F. Peden (Pacific Northwest National Laboratory) The physical and chemical properties of oxide surfaces are strongly influenced by several structural properties. As such, there is a growing interest in determining surface termination and relaxation in single crystal oxides. The yttrium-stabilized ZrO₂(001) surface can in principle be either O terminated or Zr terminated. However, neither of these surfaces is autocompensated as a (1x1) structure. The Zr terminated √2x√2)R45° reconstruction is autocompensated. It was recently reported that the CeO₂(001)surface, which is similar to ZrO₂(001), is terminated with 0.5 monolayers of oxygen.¹ We have carried out low-takeoff-angle x-ray photoelectron diffraction (XPD) measurements using Zr 3d, Y 3d and O1s core levels on a oxygen-plasma-cleaned ZrO₂(001) surface to determine the surface structure. Low-energy electron diffraction data show broad (1x1) spots that are characteristic of many oxide surfaces, and no half-order spots are visible. Currently single and multiple scattering simulations and R-factor analysis are being performed using model clusters generated using both O and Zr terminated surfaces. Preliminary analysis indicates that the surface is Zr terminated with the first layer spacing contracted by 23% relative to the bulk interlayer spacing. Detailed analysis is in progress and results will be presented at the meeting. ¹ ¹G.S. Herman, Phys. Rev. B Rapid Comm. (in press). Work supported by the U.S. Department of Energy (DOE), Offices of Basic Energy Sciences, Division of Chemical Sciences and the DOE's Environmental Management Science Program.
4:20 PM		SS1+EM-TuA-8 Epitaxial Growth and Characterization of Zirconium-Doped CeO₂(111) Thin Films on Yttria-Stabilized Zirconia(111) T.T. Tran (Pacific Northwest National Laboratory); Y.-J. Kim (Taejon National University of Technology, Korea); S. Thevuthasan, G.S. Herman (Pacific Northwest National Laboratory) Doping of the ceria lattice with Zr has been shown to enhance the oxygen storage capacity and the rate of oxygen release, and thus the catalytic properties of noble metals supported on these oxides.¹ The ability to obtain a detailed knowledge of the effect of the surface structure and zirconium doping levels on oxygen exchange rates requires high-quality single crystalline materials. In a previous study, pure and Zr-doped CeO₂(001) thin films were grown on SrTiO₃(001).² Due to the good lattice match and identical crystal structure, yttria-stabilized zirconia (YSZ) was chosen as the substrate for the growth of CeO₂(111) in this study. Both pure and Zr-doped CeO₂(111) thin films were grown on YSZ(111) by oxygen-plasma-assisted molecular beam epitaxy. After deposition, the films were compositionally and structurally characterized by in-situ reflection high-energy electron diffraction (RHEED), low-energy electron diffraction (LEED), x-ray photoelectron diffraction (XPD), and x-ray photoelectron spectroscopy (XPS), as well as by ex-situ Rutherford backscattering spectrometry. RHEED, LEED, and XPD results indicated that the materials are well-ordered at the surface for zirconium doping levels up to 40%. Furthermore, a +4 oxidation state was confirmed for both Zr and Ce using XPS. RBS results showed that Zr was found to incorporate substitutionally at cation sites in the ceria lattice. ¹ P. Fornasiero, G. Balducci, J. Kaspar, S. Meriani, R. Di Monte, and M. Graziani, Catal. Today, 29, 47 (1996); Hamada, H., Catal. Today, 22, 21 (1994); Ozawa, M., M. Kimura, and A. Isogali, J. Alloys Comp., 193, 73 (1993). ² Y. Gao, G.S. Herman, S. Thevuthasan, C.H.F. Peden, and S.A. Chambers, J. Vac. Sci. Technol. (in press).
4:40 PM		SS1+EM-TuA-9 Structure and Dynamics of KTaO₃(001) via Elastic and Inelastic Helium Atom Scattering¹ J.A. Li, T.W. Trelenberg, E.A. Akhadov, S.A. Safron, J.G. Skofronick (Florida State University); L.A. Boatner (Oak Ridge National Laboratory) Although bulk KTaO₃ is considered an incipient ferroelectric which remains cubic at all temperatures, He diffraction on the (001) surface, cleaved in situ, shows reconstruction of the (1x1) surface to (2x1). The surface behavior, however, seems to depend on its thermal history as, for example, in the following observations: 1)After the sample is cleaved at about 300K and cooled to about 190K, He diffraction reveals a (1x1) surface which appears to be stable indefinitely. 2)If the target is then warmed from 190K to about 220K, reconstruction to the (2x1) phase takes place gradually over a period of about 2 days, whereas if it is warmed to about 260K, the (2x1) phase becomes evident after about 3 hours. 3)When the target is instead taken from 190K to approximately 60K and then to about 260K, the (2x1) surface is apparent immediately. 4)Restoration to (1x1) can be achieved (a) in a few hours or (b) immediately by warming the surface to about 330K or to about 360K, respectively. In addition, there is considerable hysteresis in the intensities of the He diffraction spots for the (2x1) surface over the thermal cycle from 260 K to 50 K to 260 K. Helium time-of-flight (TOF) spectra were obtained to determine the surface phonon dispersions of the (1x1) and (2x1) surfaces for several surface temperatures between 80K and 300K. These appear very similar over the entire surface Brillouin zone. The dominant peaks in the TOF spectra correspond to a dispersionless mode at about 13.5meV, which stiffens slightly as the temperature is raised. However, the lowest energy branch which is quite flat at low temperatures seems to soften near the zone center as the temperature increases. ¹Supported by U. S. Department of Energy grant number DE-FG02-97ER45635.
5:00 PM		SS1+EM-TuA-10 Deposition of Highly Oriented Mg₃(VO₄)₂ Thin Films for Use in Catalysis Studies J.A. Ruffner, A.G. Sault (Sandia National Laboratories) Magnesium vanadates are potentially important catalytic materials for the conversion of alkanes to alkenes via oxidative dehydrogenation. However, little is known about the active sites at which the catalytic reactions take place. It may be possible to obtain a significant increase in the catalytic efficiency if the effects of certain material properties on the surface reactions could be quantified and optimized through the use of appropriate preparation techniques. Given that surface reactivity is often dependent upon surface structure and that the atomic level structure of the active sites in these catalysts is virtually unknown, we desire thin film samples consisting of a single magnesium vanadate phase and a well defined crystallographic orientation in order to reduce complexity and simplify the study of active sites. We report on the use of reactive RF sputter deposition to fabricate very highly oriented, stoichiometric Mg₃(VO₄)₂ thin films for use in these surface analysis studies. Preliminary depositions of Mg₃(VO₄)₂ onto amorphous substrates resulted in amorphous thin films. However, deposition of Mg₃(VO₄)₂ onto well-oriented, lattice-matched thin film "seed" layers such as Ti (0002), Au (111) or Pt (111) resulted in very strong preferential (042) crystallographic orientation (pseudo-hexagonal planes parallel to the substrate). This strong preferential growth of the Mg₃(VO₄)₂ suggests epitaxial (single-crystal) growth of this mixed metal oxide on the underlying metal seed layer. The effects of the substrate material, seed layer material, deposition temperature, and post-deposition anneal temperatures on thin film properties such as stoichiometry, crystallographic orientation, and chemical interactions will be reported.