Abstract
Theoretical considerations indicate that surface segregation in alloys should be confined to the outer few atomic layers. A model in which the compositions of only the first two atomic layers can differ from the bulk composition was used to interpret in situ Auger measurements on clean Cu−Au(111) evaporated alloy films. Exponential attenuation lengths independent of alloy composition were assumed to be 4, 8, and 30 Å for Au peaks at 72, 239, and 2024 eV. After normalization of alloy Au Auger intensities to those from pure Au, the data were analyzed to determine the compositions of the first two layers and the bulk, including a correction for the change in lattice parameter with bulk composition. The results indicate that segregation in Cu−Au is confined essentially to the top layer, which is gold rich for the large range of compositions studied. Uncertainties associated with backscattering effects, limitations in knowledge of escape depths, and possible lattice parameter changes near the surface are discussed. The observed gold enrichment agrees qualitatively with predictions based on the lower surface energy of gold.
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