Ion bombardment-induced changes in oxide-metal interactions studied by photoelectron spectroscopy
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We have studied the influence of ion bombardment on the surfaces of MgO and A12O3 single crystals. The two oxides were sputtered with low energy Ar + ions, and x-ray photoelectron spectroscopy was used to analyze surface compositions and chemical interactions with metallic adsorbates. Expected changes in surface compositions have been calculated mathematically using first principle formulation from existing literature. The resulting effects on XPS spectra have been predicted based on concentration profile models and agree with experimental observations. Changes in surface oxygen activity were studied by evaporating Ni on these surfaces at an extremely slow rate and analyzing what fraction of the first monolayer was oxidized on adsorption. This fraction, used as a measure of surface oxygen activity, was substantially reduced in A12O3 on sputtering, but remained practically unchanged for MgO. The amount of these changes matches reasonably well with calculated changes in surface oxygen fraction. Based on these results, the relation between ion bombardment and interfacial bonding has been discussed.
I. INTRODUCTION Ion-assisted surface processing of materials is a common trend in modern technology. Depending on the mass and energy of the incident ion and the properties of the substrate, the extent of penetration, damage, and back-sputtering of target material may vary.1 In this paper, we are concerned with a medium energy (1.5 to 4 KeV), medium mass, and nonreacting ion (Ar + ) that is most frequently used for sputtering, i.e., the erosion of surface atoms for cleaning, depth profiling, deposition, and other modifications. The phenomenon of sputtering was first quantified by Sigmund2 for model elemental targets. Multicomponent systems have since then been discussed by many workers. The majority of these studies focuses on metallic alloy sputtering (such as Refs. 3, 4, and 5). These can, to a first approximation, be treated as physical mixtures of atoms whose binding energies are related to the vaporization energy of the individual elements. Ionic and covalent compounds are more complicated because of chemical bonding, electrostatic charge buildup, and related phenomena. In the recent years, the bulk of sputtering-related studies in compounds has concentrated on III-V semiconductors6'7 because of their relevance to the electronic industry. Oxides are, however, gaining interest in their use as electronic components in many devices, and ion beams are extensively used in processing them. Therefore, the influence of ion beams on oxide surfaces needs to be studied more closely at this time than it has been in the past. Some trends in the 1958
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J. Mater. Res., Vol. 8, No. 8, Aug 1993
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overall qualitative response of common oxides to ion bombardment have been studied earlier and an overview of these results was compiled by Kelly and Lam.8 Betz and Wehner9 have also discussed this phenomenon in their review where they have summarized results from all multicomponent materi
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