Auger-spectrum simulation techniques for surface analysis of alloys with overlapping peaks: sputtered and annealed W—Mo alloys

The surface composition of a 50% W—Mo alloy has been investigated using Auger electron spectroscopy. Problems arising from carbon contamination and the strong overlap of the intense, surface-sensitive low-energy Auger peaks have been solved. Spectra for the alloy were collected digitally and could be simulated satisfactorily using pure-element spectra. The steady-state surface composition under 500 V Ar+ ion bombardment shows an enrichment of the surface in W, caused by preferential Mo sputtering. It is observed that the surface composition is uniform over the range of escape depths of the Auger electrons. This is consistent with enhanced diffusion and mixing over the penetration depth of the bombarding Ar+ ions. On annealing, the W-enriched sputtered surface becomes Mo-enriched, consistent with surface segregation of the more weakly bonded Mo atoms. However, in contrast with the sputtered surface, it is observed that a single simulation over the range of Auger-electron energies is not possible. The Mo enrichment indicated is largest for the 120 eV peak and decreases with increasing energy (escape depth), becoming a minimum at 220 eV. This is consistent with a monolayer segregation model, which is tested using published values for electron inelastic mean free paths. The calculated monolayer composition is xMo 0.94, in agreement with the predicted ideal-solution values of 0.98 ± 0.02 for different orientations.