Thermal growth ofSiO2-Si interfaces on a Si(111)7×7 surface modified by cesium

We have used photoemission spectroscopy to study the Cs-promoted formation of ${\mathrm{SiO}}_2$-Si interfaces on the Si(111)7×7 surface. A large enhancement of the oxidation rate was observed at room temperature with monolayer Cs coverage, and by annealing the initially substoichiometric oxide at moderate temperature it was transformed into ${\mathrm{SiO}}_2$; meanwhile the Cs was completely desorbed. Comparison with previous results from Si(100)2×1 indicates that differences in dangling-bond density are not of major importance for the initial oxidation, but, probably because of the more closed structure of Si(111) atomic planes, the oxide layer obtained after annealing was thinner and less stoichiometric than on Si(100)2×1. The results also suggest that Cs stays on top of the oxide, and is preferentially bonded to Si. The catalytic efficiency decreased rapidly as the Cs coverage was reduced, thereby demonstrating the strongly nonlocal nature of the catalytic mechanism.