Site selectivity in the initial oxidation of the Si(111)-7×7 surface

The site selectivity of oxygen in the initial oxidation of the Si(111)-7×7 surface was investigated using a scanning tunnelling microscope. At room temperature, two different oxygen-induced features, bright and dark sites, were observed indicating two different oxygen chemisorption configurations. In particular, the major dark features were observed preferentially at the center adatom sites of the faulted half of the dimer–adatom–stacking-fault (DAS) structure over corner ones, which is different from previous results. We explain this in terms of the difference in the potential energy curves of two sites combined with the effect of the slow oxygen exposure (1×10−9 Torr) that we used. At low oxygen pressure, oxygen atoms have sufficient diffusion time to be adsorbed on the most stable sites. This proposal is supported by the pressure dependence of the site selectivity that we measured. Furthermore, when the sample was exposed to oxygen at high temperatures, the dark features still preserved the strong site preference on the center adatom sites supporting both our hypothesis and the previously reported idea that center adatom sites are thermally more stable than the corner ones.