The surface composition of Au–Sn alloys determined by Auger electron spectroscopy

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Abstract
The Auger spectra of clean and equilibrated Au–Sn alloy ingots of bulk composition ranging between 50 and 99 at% Au have been obtained. The intensity ratios of Auger peaks of various different energies were normalized by comparison with the corresponding intensity ratios from pure Au and Sn. The normalized intensity ratios are used to obtain the surface atom fractions by applying a model which takes full account of the attenuation of the Auger electrons. It is found that for a one phase alloy with overall composition of 50 at% Au there is little or no segregation at the surface, while for a one phase alloy with bulk composition of 86 at% Au there is marked segregation of Sn (surface monolayer composition of about 43 at% Au). For the alloys of bulk compositions between 50 and 86 at% Au in which both phases are present, the surface composition is given by applying the lever rule to the one phase surface compositions. For 1 at% Sn dissolved in the Au lattice there is also considerable enrichment of the surface with Sn (surface monolayer composition of about 46 at% Au). These surface compositions are related to the properties of the bulk phase diagram and are explained by the interplay of the differences in atomic size and in heats of sublimation of pure Au and Sn and of the heats of mixing of the alloys.