Adsorption site identification for oxygen molecules on AuSi(100) by positron annihilation induced Auger electron spectroscopy(PAES)

The top layer selectivity of PAES (positron annihilation induced Auger electron spectroscopy) was applied to study the differential adsorption of oxygen molecules on AuSi(100) surfaces. On appropriately prepared AuSi surfaces, exposure to 400–500 L of O2 at elevated sample temperature (100°C) produces significant decreases of the Si (LVV) peak intensity (30–40% decrease compared with as-deposited samples), while the PAES intensity at energies corresponding to the Au (O2,3 VV transition) undergoes only a small change (≤ ± 10%). These results strongly suggest that the O is adsorbed preferentially on top of Si atoms and that consequently the adsorbed gas is displacing the positron wavefunction away from the Si atoms but not away from the Au atoms. Evidence for changes in the Au overlayer morphology induced by thermal treatment of the AuSi surface and the corresponding differences in adsorption behavior of oxygen molecules will also be discussed.