The Oxidation Behavior of the √3×√3 Ag and Au/Si(111) Surfaces at Room Temperature Studied by Photoemission

Abstract

The differences of the room temperature oxidation behavior of ordered Ag/Si(111) and Au/Si(111) surfaces were studied by surface sensitive soft x-ray photoemission spectroscopy obtained with synchrotron radiation. Si surfaces covered with a monolayer of Ag or Au, once annealed to display a √3×√3 LEED pattern, were believed to be passivated against oxidation according to earlier reports. This work shows that these two surfaces oxidize but in a different way. Up to 10⁴ L O₂ exposures, the observed valence band of the Au/Si surface’s valence band electron energy distribution curve is almost identical to that of the surface before oxygen exposure. But the corresponding Si 2p core level spectrum shows a small chemically shifted component indicating an initial stage of the formation of Si oxide. This chemically shifted signal becomes a strong peak at −3.7 eV below the clean Si position, characteristic of SiO₂, after subsequent O₂ exposures up to 10¹⁰ L. The Ag/Si system behaves in a similar fashion, but oxide growth saturates at 10⁸ L, and the final oxides formed include a distribution of suboxides in addition to SiO₂. Clearly, oxide formation is not prohibited by the presence of the ordered Au or Ag metal overlayer but delayed. Although the onset of oxidation is delayed compared to that for the clean Si surface, due to the metal-silicon bonding, the oxide formation is much faster once the surface starts to oxidize.