The initial stage of oxygen adsorption onto a Si(111)-7×7 surface has been studied by scanning tunneling microscopy. The substrate was exposed to oxygen under a partial pressure of 1× 10-9 Torr at room temperature during the observation. Dark features, which have been reported as a main channel for oxidation, appeared exclusively in the faulted halves of the DAS structure though bright features appeared in both halves. This difference can be explained by the adsorption site of oxygen. That is, the greater energy difference of the dangling-bonds of adatoms between both halves than that of the back-bonds causes the exclusive appearance of the dark feature, which has an oxygen atom on on-top site in addition to the other in the back-bond.
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