A LEELS and Auger study of the oxidation of liquid and solid tin

Abstract

Abstract The oxidation of liquid and solid tin from 25 to 240°C has been investigated using 75 eV low energy electron loss spectroscopy (LEELS) and Auger spectroscopy over an oxygen exposure range from zero to 107 L. LEELS was chosen for two reasons. First it can distinguish Sn, SnO and SnO2 from each other. Second, we show that at 75 eV incident energy LEELS has a penetration depth of only one monolayer. As a result the continuity and stoichiometry of the oxide layer could be studied as a function of thickness from submonolayer to several monolayer thicknesses. Although unable to distinguish SnO from SnO2 the larger penetration depth of the Auger technique complemented the LEELS study. From zero to one monolayer the oxide grows as islands containing both SnO and SnO2. Above one monolayer coverage the oxide is continuous and free of metallic tin with its outer most surface enriched in SnO2. Although oxide films grew more rapidly on polycrystalline tin than on single crystal tin the composition and continuity as a function of thickness remained unchanged. Very little change in oxide growth rate, continuity, or stoichiome- try was observed for solid tin up to temperatures near the melting point. However at 229°C, just 3°C below the melting point of tin, dissolution of oxygen into the metal was observed. A continuous, metal free solid oxide, primarily SnO, could be grown on liquid tin at 240°C than remained stable for 20 min after removal of the oxygen gas. Our model for the early stages of the oxidation of tin is different from that previously proposed on the basis of UPS, XPS, and 400 eV LEELS with respect to the continuity and relative ordering of the SnO and SnO2 phases. Quantitative comparison of our results with those previously reported shows that the previous results are consistent with our model for the structure and stoichiometry of the initial oxide grown on tin.