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

The kinetics of the initial oxidation of Si(111) 7×7 by ${\mathrm{O}}_2$ 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 ${\mathrm{O}}_2$ 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 ($S_0$) of ${\mathrm{O}}_2$ on Si(111) 7×7 decreased with surface temperature from $S_0$=0.2 at 200 K to $S_0$=0.06 at 600 K. The observed decrease in $S_0$ suggested that the initial oxidation of Si(111) 7×7 was mediated by an ${\mathrm{O}}_2$ 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 ${\mathrm{O}}_2$ 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.