Surface composition of polycrystalline Au–Cu alloys as a function of temperature

Abstract

The surface composition of polycrystalline Au–Cu alloys is studied by means of Auger electron spectroscopy (AES) in the composition range from 7.5 to 56.3 at. % Au. Well equilibrated surfaces show predominant Au segregation in the temperature range from 200° to 550°C as observed by the low-energy Cu (58/60 eV) and Au (66/69 eV) Auger transitions. The Au segregation decreases linearly with increasing temperature. Quantitative evaluation, based on internal and external calibration shows Au enrichment up to 100%. The deeper layers, studied by means of the high-energy Cu (920 eV) and Au (2024 eV) Auger transitions, generally do not show Au but Cu enrichment. This result is consistent with depth profiling by Ar+ sputtering. Segregation of S or O impurities at T?600°C leads to strong, chemically induced Cu enrichment on the surface as identified by the appearance of fine structure in the sulfur Auger transition. Current theories of segregation for pure alloys are discussed. Lowering of surface and lattice distortion energy are found to be important, whereas the heat of sublimation has no influence on segregation in Au–Cu alloys.