Oxidation kinetics of Si(111) 7×7 in the submonolayer regime

The kinetics of the initial oxidation of Si(111) 7×{}7 by O₂ in the submonolayer regime was studied using laser-induced thermal desorption, temperature-programmed desorption, and Auger-electron spectroscopy. The results showed that the oxidation of Si(111) 7×{}7 by O₂ was characterized by two kinetic processes. Initially, a rapid oxygen uptake step occurred that was followed by a slower growth process which asymptotically approached an apparent saturation oxygen coverage. The initial reactive sticking coefficient (S₀) of O₂ on Si(111) 7×{}7 decreased with surface temperature from S₀=0.2 at 200 K to S₀=0.06 at 600 K. The observed decrease in S₀ suggested that the initial oxidation of Si(111) 7×{}7 was mediated by an O₂ precursor species. In contrast, the apparent saturation oxygen coverage was observed to increase as a function of surface temperature. The apparent saturation oxygen coverage increased from approximately {Θ}=0.4 ML at 110 K to {Θ}=0.7 ML at 600 K. Experiments with preadsorbed hydrogen also demonstrated that the initial reactive sticking coefficient for O₂ and the apparent saturation oxygen coverage were reduced as a function of increasing hydrogen coverage on the Si(111) 7×{}7 surface. This behavior indicated that the oxidation of Si(111) 7×{}7 requires free dangling-bond sites.