The site selectivity of oxygen in the initial oxidation of the Si(111)
-7 X 7 surface was investigated using a scanning tunnelling microscope
. At room temperature, two different oxygen-induced features, bright a
nd dark sites, were observed indicating two different oxygen chemisorp
tion configurations. In particular, the major dark features were obser
ved preferentially at the center adatom sites of the faulted half of t
he dimer-adatom-stacking-fault (DAS) structure over comer ones, which
is different from previous results. We explain this in terms of the di
fference in the potential energy curves of two sites combined with the
effect of the slow oxygen exposure (1 X 10(-9) Torr) that we used. At
low oxygen pressure, oxygen atoms have sufficient diffusion time to b
e adsorbed on the most stable sites. This proposal is supported by the
pressure dependence of the site selectivity that we measured. Further
more, 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 report
ed idea that center adatom sites are thermally more stable than the co
rner ones. (C) 1998 Elsevier Science B.V.