Application of low energy ion scattering to oxidic surfaces

Abstract

Low-energy (0.1-10 keV) ion scattering (LEIS) is used for the determination of the atomic composition and structure of oxide surfaces. Its extreme surface sensitivity enables the selective analysis of the outermost atomic layer. It is precisely this layer that is largely responsible for many chemical and physical properties of oxides. Lowering of the surface energy provides a strong driving force for segregation to the surface. Surface segregation is very important at temperatures that are high enough to enable diffusion. Since the surface enrichment is often restricted to the outermost atomic layer, the unique capability of LEIS to analyze this layer selectively, is one of the main applications of the technique. Other applications pertain to studies of the mechanism of oxidation of metals, semiconductors and to that of the growth of oxides on itself and on other materials. For equilibrated surfaces of spinels (powders or sintered) both LEIS and chemical methods show that, while cations in octahedral sites are accessible, cations in tetrahedral sites are not. This suggests that the equilibrium surfaces of these oxides are dominated by only one or two crystallographic planes. For single crystals, blocking, shadowing and focussing effects of the incident ions or scattered ions have been used to determine the location of surfce atoms as well as the presence and annealing characteristics of surface defects. Although the number of such studies are very limited, the results show great promise for the understanding of oxide surfaces.