Papers

AVS1996 Session AS-ThM: Insulators, Glasses and Oxides

Thursday, October 17, 1996 8:20 AM in Room 105B

Thursday Morning

Start	Invited?	Item
8:20 AM		AS-ThM-1 Enhanced Differential Charging during X-ray Photoelectron Spectroscopy as an Analysis Technique G. Halada, C. Clayton (State University of New York, Stony Brook) Charging of non-conductive or semiconductive chemical species, often on the order of one or two electron volts, tends to be a problem during X-Ray Photoelectron Spectroscopy (XPS) of non-metallic or mixed species samples. Various charge compensation techniques, such as the use of flood guns, have been used to partially overcome this problem. However, the existence of charging, and, in particular, differential charging of samples during analysis reveals important information about the electrical nature of the species present. In this study we show how differential charging and enhanced differential charging employing a sample bias technique during XPS may provide important additional information on speciation and characterization in the case of (1) superconducting perovskites, (2) mixed chromium oxides, (3) chemical species on spatially separated surface features on electronic components, and (4) organic species on metallic surfaces exposed to bacteria and organic acids.
8:40 AM		AS-ThM-2 New Approaches to the Control of Insulator Surface Charging in Surface Spectroscopy M. Kelly (Stanford University) Insulating materials present significant obstacles to a wide variety of analytical techniques, in particular to surface sensitive techniques using small probe beams: XPS, AES, and SIMS for example. Recently there has been significant progress in understanding the nature of the charging phenomena, and several effective approaches for controlling the charging have been developed, particularly for XPS. This talk will review our current understanding of non-uniform surface charging, give some examples of its control, and disucss ways in which this control can be used to provide additional information about the sample being analyzed.
9:20 AM		AS-ThM-4 Glass Surfaces C. Pantano (Pennsylvania State University) The surge of activity in flat panel display technology has increased the demand for high quality commercial flat glass. The manufacturing processes must create a clean, reproducible surface that can withstand subsequent cleaning processes, etching and deposition of thin films, and heat treatment. Related to this technology, as well as to many others, is the interaction and adhesion of organics to glass surfaces. The ability to develop and maintain a controlled adhesive bond in a polymer/glass laminate or joint, or in a polymer coated glass fiber, is intimately related to chemical interactions between the glass surface and the polymer. This paper will review important features and chemical reactions of multicomponent glass surfaces. In general, it is found that physically and chemically adsorbed water, alkali species (specifically sodium), organics, and particulate, must be controlled for most applications. In many cases, the aging, cleaning or heat-treatment of a multicomponent glass irreversibly modifies the surface. It is also important to recognize that many commercial glasses possess in-depth composition gradients that are intrinsic to the manufacturing process. Our current understanding of these surface characteristics, and some of the methods used in their evaluation, will be discussed. It will become clear that these features are difficult to quantitatively determine using conventional methods of surface analysis.
10:00 AM		AS-ThM-6 Determination of the Silanol and Molecular Water Concentration of Glass Surfaces A. D'Souza, C. Pantano (Pennsylvania State University) A novel method has been developed to semi-quantitatively determine the concentration of silanol groups and molecular water on glass surfaces. This method involves neutral beam static- SIMS analysis of heated glass surfaces. High purity, amorphous silica samples were either fractured or melted in the preparatory chamber of a UHV system and were subsequently dosed with watervapor at varying partial pressures. After dosing, the samples were heated to 250C by means of a substrate heater and static-SIMS spectra were periodically collected during the heat treatment. It was seen that for a given dosing pressure, the SiOH/Si peak area ratio decreased with increasing sample temperature and heating time before reaching an equilibrium value. The intensity of the SiOH signal observed in the static-SIMS spectra is due to SiOH groups actually present on the sample surface, and SiOH groups that are formed due to the recombination of sputtered silicon atoms and adsorbed molecular water on the silica surface. When the sample surface is heated, desorption of molecular water from the silica surface takes place, which is reflected in the decrease observed in the SiOH signal intensity. This decrease can be used to estimate the original concentration of molecular water on the sample surface. The equilibrium SiOH signal intensity obtained after the heat treatment gives the silanol concentration of the sample surface. The effect of the different dosing pressures on the molecular water and silanol concentration of the silica surface will be discussed.
10:20 AM		AS-ThM-7 A Scanning Tunneling Microscopy and Tunneling Spectroscopy Study of the Reduced LiNbO\sub 3\ (0001) Surface D. Carroll, M. Wagner, O. Kienzle, M. R\um u\hle (Max-Planck-Institut f\um U\r Metallforschung, Germany) The effect of ultra high vacuum reduction (UHV) on the LiNbO\sub 3\ (0001) surface is investigated using scanning tunneling microscopy (STM) and tunneling spectroscopy coupled with other, conventional surface analysis techniques. The morphology of the reduced surface was found to be dominated by inhomogeniously shaped structures of approximately 10nm to 30nm in diameter which are closed packed across the substrate. Auger data suggest a change in stoichiometry, as measured by Li/O and Nb/O peak height ratios, that indicates the precipitation of a second phase at the surface. Tunneling spectra clearly exhibit a shift in the Fermi level towards the conduction band, indicating an n-type surface, as expected, as well as the existence of a band-gap. Further, analysis of the tunneling spectra taken at different ramping rates reveals high frequency response, which is related to the dielectric function of the surface, and low capacitance of the junction. This is being interpreted in terms of a reduction in the ferroelectric response of the precipitate relative to stoichiometric LiNbO\sub 3\. These results are compared to those for the same reduced crystals which have been repolished to remove this nonstoichiometric layer and flash heated to reform a near stoichiometric surface. This represents the first demonstration of STM and tunneling spectroscopy on the LiNbO/sub 3/ surface, suggesting the technique may be used as a powerful analytical tool in atomic scale identification of structure on these surfaces.
10:40 AM		AS-ThM-8 Structure and Composition of Oxidized Aluminum / NiO (100) Interfaces S. Imaduddin, R. Lad (University of Maine) The nature of interactions occurring at oxide/oxide interfaces can be investigated by depositing ultra-thin films on oxide single crystal substrates. In this study, we deposited aluminum onto stoichiometric NiO(100) in O\sub 2\ pressures of 10\super -5\ Torr. Photoemission (XPS and UPS) measurements following depositions at substrate temperatures of 250\super o\C indicate the creation of a ternary Ni-Al-O phase at the interface, as a result of interdiffusion between the deposited layer and the underlying NiO substrate. In situ RHEED patterns show only a gradual attenuation of the underlying NiO pattern as a function of overlayer thickness, indicating that the resulting films are amorphous. RHEED patterns obtained for depositions carried out at substrate temperatures of 800\super o\C show the creation of a distinct crystalline NiAl\sub 2\O\sub 4\ spinel phase; this spinel phase is also formed via post-deposition annealing of the amorphous films. These observations are in contrast to aluminum deposition in ultra-high vacuum which yields a Ni\sub 3\Al phase at the interface. Our results demonstrate that oxygen activity and kinetics during ultra-thin film growth have a large influence on the structure and composition of the resulting interface.
11:00 AM		AS-ThM-9 Growth of SiO\sub 2\ by Tetraethoxysilane Pyrolysis on MgO Thin Films and Mo(100) T. Jurgens-Kowal, J. Rogers, Jr. (University of Washington) The deposition of silicon dioxide on MgO thin films and on Mo(100) by tetraethoxysilane (TEOS) pyrolysis has been investigated at temperatures between 300 and 860 K using X-ray photoelectron spectroscopy (XPS), temperature-programmed desorption (TPD), low-energy electron diffraction (LEED), and reflection-absorption Fourier-transform infrared spectroscopy (IRRAS). To approximately 600 K, TEOS adsorbs forming a di-ethoxysilyl intermediate, whereas at higher temperatures SiO\sub 2\ is formed during the initial exposure, as evidenced by both XPS and IRRAS. Surprisingly, MgO thin films are more reactive toward TEOS than the metal surface and more SiO\sub 2\ is deposited on the oxide for equivalent exposure conditions. Heating the TEOS-exposed surfaces to 1000 K yields ethylene as the primary gas-phase decomposition product, and improves both the stoichiometry and the order of the films as indicated by an increase in the stretching frequency of the Si-O IRRAS peaks. A decrease in the amount of ethylene desorption is observed as the deposition temperature increases from 300 to 600 K, and no significant gas phase products were detected at the higher deposition temperatures. Carbon contamination is minimal in these SiO\sub 2\ films. *We would like to acknowledge Battelle Pacific Northwest National Laboratories and the Intel Foundation for support of this work.
11:40 AM		AS-ThM-11 XPS Studies of Crystalline and Amorphous Alkali-Aluminosilicates V. Bojan, J. Hamilton, D. Voigt, S. Brantley, C. Pantano (Pennsylvania State University) The ability to predict and understand groundwater and soil porewater chemistry is an important tool in the study of practical environmental problems. The models used in groundwater and soil porewater chemical studies depend, fundamentally, upon the weathering and dissolution mechanisms of the various alkali and alkaline-earth aluminosilicate minerals (primarily feldspars) in the earth's crust. Typically, the results of controlled laboratory dissolution experiments are used to develop and test models for natural weathering processes. XPS is a very useful tool for characterizing feldspar surfaces both before and after laboratory leaching experiments. Characterization of the sample surface before a laboratory leaching experiment is an especially important step in controlled laboratory dissolution studies. In this study, XPS has been used to characterize the effects of sample preparation on the surface composition of samples which are being used in laboratory leaching experiments. These samples include crystals, powders, and glasses prepared by melting feldspar minerals and related aluminosilicate compositions. The effects of sample grinding, polishing, washing, and UV ozone cleaning have been determined. The effects of sample form (i.e. powder vs. crystal) on the accuracy and precision of the compositional measurements will also be discussed. It has also been found that long exposure to Mg K\alpha\ x-rays from the non-monochromatic x-ray source can have deleterious effects on the measurement of Na concentration. The effects of sample form (powder vs. crystal) and structure (crystalline vs. glass) will be discussed with respect to the measurement of Na concentration in the surfaces of these samples.