Papers

AVS1997 Session AS-WeA: Insulators and Oxide Materials

Wednesday, October 22, 1997 2:00 PM in Room J2

Wednesday Afternoon

Start	Invited?	Item
2:00 PM		AS-WeA-1 Radiation Damage in Zirconium Oxide W.C. Simpson, T.M. Orlando (Pacific Northwest National Laboratory) Due to its high damage threshold, ZrO₂ is often applied as a protective coating to laser optics. In addition, Zr-based alloys are commonly used as cladding for nuclear fuel rods and various components in nuclear reactors, primarily because of the robust nature of the oxide that forms which is highly resistant to corrosion. Yet when hit with a sufficiently high laser fluence or when submitted to the extreme conditions present in a nuclear reactor, zirconium oxides do break down. It has been suggested that secondary electrons generated in the material by ionizing radiation lead to its degradation, but the exact mechanism is unknown. We are currently conducting a set of controlled experiments to investigate radiation damage in zirconia. Our approach is to take a well-characterized ZrO₂ surface and submit it to bombardment by a monochromatic beam of low-energy electrons or photons. By varying the energy of the incident beam while monitoring the ion and neutral flux desorbing from the ZrO₂ surface, we are able to identify the nature of the excitations that lead to its decomposition. We observe both direct and indirect mechanisms for radiation damage in zirconium oxides. The threshold for electron-stimulated desorption (ESD) of O⁺ occurs at the Zr(4p) edge, which is consistent with the Knotek-Feibelman model for ESD from oxides. The primary photon-stimulated ion desorption products observed using low-energy photons are Na⁺ and K⁺, which derive from trace amounts of these elements in the ZrO₂ crystal. We will discuss various mechanisms for the radiation-induced decomposition of ZrO₂.
2:20 PM		AS-WeA-2 Ion Scattering Studies of Epitaxially Grown Iron Oxide Films on MgO and Al₂O₃ Substrates S. Thevuthasan, G.S. Herman, Y.J. Kim, Y. Gao, S.A. Chambers (Pacific Northwest National Laboratory); N.R. Shivaparan, R.J. Smith (Montana State University); B.J. Morris, T.T. Tran (Los Alamos Technical Associates, Inc.) There have been relatively few investigations of the epitaxial growth of iron oxide thin films. The selective growth of pure and highly-oriented single crystalline iron oxide films is nontrivial. We have successfully carried out the MBE growth of iron oxide films on several substrates. Rutherford Backscattering Spectroscopy (RBS) and channeling experiments were performed to investigate the crystalline quality of epitaxially grown γ-phase Fe₂O₃ films on MgO (001), α-phase Fe₂O₃ films on Al₂O₃(0001), and Fe₃O₄ films on MgO(100). The minimum backscattering yields obtained from the angular scans for Mg and Fe show that the film crystalline quality is reasonably good for the films grown on MgO substrates. Although the film crystalline quality is good in general, the films grown on Al₂O₃ show some disordering at the interface. In addition to the channeling experiments, We have used Low Energy Ion Scattering experiments to understand the disordering at the interface by looking at the initial nucleation kinetics of the films. The RBS and channeling measurements along with LEIS analysis will be discussed. ¹Work supported by the DOE/ER/OHER and NSF Grant DMR-9409205
2:40 PM		AS-WeA-3 Valence Band X-Ray Photoemission Investigation of Surface Cleanliness of Aluminum Metal and its Alloys. N.J. Havercroft, P.M.A. Sherwood (Kansas State University) Aluminum and its alloys are used in many practical situations. The surface preparation and treatment of these materials can ignificantly impact their practical applications. This paper will present valence band and core X-ray photoemission (XPS) data on these materials, indicating in particular how the valence band region can provide an indication of differences in surface composition unobtainablein the core XPS regions, or by other surface analytical probes. In particular the aspect of surface cleanliness is vital to certain industrial applications. Therefore examples will include aluminum metal and its alloys exposed to various hydrocarbon materials and then subjected to different cleaning methods. It has been found that these procedures result in cases where substances may be adsorbed onto the surface, and even cases where trace impurities may be found to diffuse to the surface region of these materials. It is possible to interpret the valence band region by comparison with spectra calculated from molecular orbital calculations, and examples of this will be presented to aid in the analysis.
3:00 PM		AS-WeA-4 The Valence Region in Photoelectron Spectroscopy: A New Approach to Chemical Information for Non Conductive Systems T.L. Barr, P.D. Shah, E.E. Hoppe, E. McAdam, T.H. Dugall (University of Wisconsin, Milwaukee) It is not uncommon for practioners of XPS to totally ignore the valence spectral region with its broad (generally low cross sectioned) bands (V.B.). Users of UPS and/or synchrotron radiation based systems do the opposite, generally ignoring core lines (even those near the V.B.) and are also unable to examine many practical systems with their charging and cleanliness problems. To alleviate these problems, we have begun a study addressing such questions as: What information is contained in valence bands and core levels? Is the information always the same? When should both be monitored? What constitutes the valence band and what the core levels? When should near valent versions of the latter be integrated into the former? As examples of this problem we will describe the contribution of O (2s) and N (2s) to the valence bands of select oxides and nitrides. Also we examine cases when near core metal lines seriously perturb the V.B. and thus alter the description of the bonding and behavior of key oxide systems. In this study we present both experimental and theoritical evidence and include consideration of charging and final state relaxation effects. General models for valence bands are proposed.
3:20 PM		AS-WeA-5 Self Texture in the Initial Stages of Zinc Oxide Film Growth R.N. Lamb, M.H. Koch, A.J. Hartmann (The University of New South Wales, Australia) The growth of thin (sub-micron), ordered zinc oxide films has been achieved using single source chemical vapour deposition of zinc acetate. The growth was carried out under high vacuum conditions with H₂O background vapour. The film structure and properties are highly dependent on the ambient H₂O pressure with optimum conditions resulting in crystalline and c-axis oriented films. An interesting feature of this growth is the preferred structural ordering or self texturing. This effect indicates a relatively high degree of both film stoichiometry and purity and appears to be independent of substrate. The origin of this latter effect has been investigated and the results are presented. The initial stage of ZnO film (non epitaxial) growth on Si(100) was examined in situ during film growth using X-ray photoemission spectroscopy (XPS) and X-ray absorption spectroscopy (XAS). Surface extended X-ray absorption fine structure (EXAFS) and high resolution transmission electron microscopy have been employed to investigate the film structure near the interface. Near edge X-ray absorption fine structure (NEXAFS) spectroscopy studies indicated that for film thicknesses above 5 nm the adsorbed species have a preferred orientation which results in c-axis orientation for thicker ZnO films. The thinner (amorphous) transition layer appears to be composed of a mixture of byproducts of the decomposition reaction and zinc oxide. The origin of this "buffer" layer and its effect on subsequent film growth is discussed.
3:40 PM		AS-WeA-6 Surface Structure and Dynamycs of KTaO₃ by Helium Atom Scattering. J.A. Li, J.G. Skofronick (Florida State University); R.L. Banerjee (Florida State University (Presently at Univ. of Moncton, Canada)); E.A. Akhadov, S.A. Safron, T.W. Trelenberg (Florida State University); D. Bonart (Arizona State University); L.A. Boatner (Oak Ridge National Laboratory) This work is concerned with the progress on HAS measurements on the surface structure and dynamics of KTaO₃(001). The surface is prepared by cleaving in situ, and its lifespan varies from a few days to more than a month, depending on experimental conditions. Three general types of measurements have been made: 1) angular distributions (AD) which provide information on the surface structure and any phase changes which occur on the surface, 2) time-of-flight (TOF) measurements, which provide the dynamics and 3) "drift spectra" which gives information on the step heights distribution. Our ADs show the 1x1 expected diffraction pattern after the crystal is cleaved at room temperature. When the crystal is cycled in temperature to 50K and then back to 270K, a 2x1 structure evolves at about 250K and remains until 365K where it disappears. However, the temperature cycling is repeatable for this structure. The TOFs, mostly done at 190K, on the 2x1 phase and in the <100> direction, have most of their intensity in two Einstein-like modes at about 13.5 meV and 27 meV. The data in the spectra show features lying below the Einstein modes, but the peak intensities are weak and poorly resolved at present. A shell model analysis indicates that the Rayleigh mode intensity from the KO surface is weak in agreement to the TOF data. On the other hand the TaO₂ surface appears to be unstable within the present shell model and its calculated intensities match poorly the experimental data. Drift spectra measurements on this surface indicates that the surface is composed of both KO and TaO₂ planes. Supported by US DoE Grant No. DE-FG05-85ER45208, Lab Contract DE-AC05-84ER21400, NSF Grant No. DMR-9510182 and the Feodor Lynen program of the Humboldt Foundation.
4:00 PM		AS-WeA-7 SIMS Measurements of Mobile Ion Concentration Depth Profile in Insulating Layers A.A. Saleh, M.R. Sardela, Jr. (Charles Evans & Associates) The performance of oxide and other dielectric layers in electronic device is often compromised by the existence of low-level mobile ion (Na, K and Li) contamination. Secondary Ion Mass Spectroscopy (SIMS) measurements offers excellent detection limits for these contaminants (5E13 at/cc) at the levels of interest to the semiconductor industry, and thus serves as a reliable choice to characterize insulating layers. In a contamination reduction effort, SIMS serves a key role by providing feedback regarding the impact of the various processing stages that are involved in device fabrication. In conventional magnetic-sector-based SIMS, the measurement involves applying a high potential to the sample which results in the displacement of the mobile ions during the sputtering process. This is particularly a problem for Na depth profiling where the concentration profile is significantly distorted. Also, this arrangement makes it somewhat difficult to compensate for sample charging during the analysis. Alternatively, quadrupole-based SIMS measurements, where the sample is grounded during the analysis, provides a more accurate depth profile. In this study, we show that proper charge compensation as well as the choice of sputtering rate both play a critical role in minimizing the displacement of the mobile ions within insulating layers. Various analytical conditions were used to measure the concentration depth profiles of Na and K implanted oxide-layers and device dielectric layer stacks in an effort to investigate the involved factors.
4:40 PM		AS-WeA-9 High Resolution XPS Investigations of the Interactions of a Resole Phenolic Resin Matrix with Silanised E-glass Surfaces T. Choudhury, F.R. Jones (Sheffield University, United Kingdom) The surface chemistry and the interactions of a resole phenolic resin has been investigated to determine its reactivity with a range of silane coupling agents. The silane coupling agents are a component of size or finish applied to glass fibres which act as adhesion promoters in reinforced composites. However, currently there is no general formulation for glass sizing which is phenolic compatible. Thus a knowledge of the reactions between the matrix phenolic resin and the coupling agent at the glass/silane interface is of extreme importance to the reinforced plastic industry in optimising the phenolic/silane systems for improved mechanical performance. High resolution X-ray Photoelectron Spectroscopy (XPS) has been employed to study the interactions of the phenolic resin with three different silanised surfaces. These silane coupling agents are γ-aminopropyltriethoxysilane (APS), γ- mercaptopropyltrimethoxysilane (MPS) and γ- glycidopropyltriethoxysilane (GPS). A comparison of these systems also provides the fundamental understanding required to study relevant silanes which do not contain readily detectable elements. Curve-fitting of the carbon and the oxygen spectral lines can differentiate the reactivity of the differing silanes. The high resolution spectroscopic capability has been extremely valuable in achieving this. The sulphur and nitrogen give confidence to the conclusions that there is coupling between the resin and the E-glass when the silanes APS and GPS are used, however no significant interactions are seen with MPS.
5:00 PM		AS-WeA-10 Surface Characterization of Passivation Films on Aluminum R. Bastasz (Sandia National Laboratories) Thin, self-limiting passivation layers readily form on the surface of aluminum and can protect the metal from further chemical reaction. In addition to the ubiquitous air-formed oxide layer, other passivating layers, such as the fluoride, readily form. Oxide and fluoride passivation layers on clean Al surfaces have been studied using three low-energy ion beam analysis methods: ion scattering spectroscopy (ISS), direct recoil spectroscopy (DRS), and secondary-ion mass spectrometry (SIMS). Helium ISS of Al reveals a strong inelastic loss feature below the elastic scattering energy. This feature is affected by the passivation layer and appears to contain chemical state information. DRS is useful as a profiling tool for determining the layer thickness. SIMS data show the emission of numerous cluster species. With fluorine, the lack of multiple isotopes allows clear identification of the cluster type. The AlF₄ and Al₂F₇ negative secondaries are especially prominent and, when correlated with ISS and DRS data, may provide information about surface bonding in the passivating layer. This work was supported by the US Department of Energy under contract DE-AC04-94AL85000.