Composition of the first two atomic layers in Au_0.2 Cu_0.8 and Au_0.8Cu_0.2 alloys

Ratios of the M_2,3VV Auger signal of copper for the alloy Au_0.2Cu_0.8 to that for Cu standard (h_Cua/h_Cus) and such a signal of gold for the alloy Au_0.8Cu_0.2 to that for Au standard (h_Aua/h_Aus), expected for particular compositions of the first and second atomic layers of alloys mentioned above and for two incidence angles (0 and 60°) of the primary electron beam in an RFA spectrometer, were calculated with the use of Monte-Carlo computer simulation of the transport of Auger electrons to the sample surface, including elastic and inelastic scattering of those electrons. These ratios were fitted to the corresponding ratios obtained from measurements, which gave the composition of the first and second atomic layers in both alloys. For the Au_0.2Cu_0.8 alloy, the relative atomic concentrations of copper atoms in the first and second atomic layer X_Cu1=0.77±0.1 and X_Cu2=0.93±0.5, respectively, were found while for the Au_0.8Cu_0.2 alloy X_Au1=0.81±0.1 and X_Au2=0.44±0.5 were obtained. These results were found to be almost unchanged when the inelastic mean free path of Auger electrons is varied from 0.3 to 0.5 nm and the acceptance angle of the RFA analyzer is varied from 25 to 35° in calculations mentioned above. On the other hand, the change of the measured ratios h_Aua/h_Aus or h_Cua/h_Cus by 3% changes the X_Au1 or X_Cu1, respectively, by about 0.1. The results obtained are compared with experimental and theoretical results (concerning the Au_xCu₁−x polycrystalline and crystalline samples) known from the literature. A good qualitative agreement is found.