Elsevier

Surface Science

Volume 257, Issues 1–3, 1 November 1991, Pages 70-78
Surface Science

Reactivity of Pd and Sn adsorbates on plasma and thermally oxidized SnO2(110)

https://doi.org/10.1016/0039-6028(91)90780-VGet rights and content

Abstract

The surface accumulation layer that forms as a result of oxygen vacancies near the surface of an oxide semiconductor can be used as an extremely sensitive probe of phenomena occurring at the surface. By incorporating a UHV-compatible, four-point conductance measurement in our surface analysis apparatus, we have investigated the formation of metal-semiconductor interfaces on SnO2(110) with varying oxygen stoichiometry. For only a 0.1 monolayer equivalent (ML) coverage of Sn deposited onto a high-oxygen-content surface prepared by oxygen plasma treatment, we observe a 500-fold increase in conductance. Our results suggest that at this coverage, Sn reacts with the excess chemisorbed oxygen and also creates point defects by abstracting oxygen from the lattice. In contrast, Pd reacts only with chemisorbed species, as indicated by a smaller conductance change and by differences in the size-effect shifted core levels observed at the same coverage for thermal and plasma oxidized surfaces. Oxygen plasma treatment of an 8 ML Pd film is shown to be an effective room temperature oxidation procedure and is used as a basis of comparison for studies of the reaction of submonolayer coverages of Pd on plasma and thermally oxidized surfaces. The coverage dependence of the surface conductance for Pd on thermal and plasma oxidized surfaces exhibits a peak near 1 ML, possibly indicating the onset of Schottky barrier formation: barriers underneath the metal islands deplete the surface of carriers; or the formation of scattering centers. At higher coverage, inter-island conductance dominates and the conductance rises. Surfaces prepared with decreasing oxygen content (thermal, vacuum annealed, ion-sputtered) show increased apparent alloying of Pd to surface Sn.

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