INITIAL-STAGES OF OXYGEN-ADSORPTION ONTO A SI(111)-7X7 SURFACE STUDIED BY STM

The initial stage of oxygen adsorption onto a Si(111)-7 x 7 surface ha s been studied by scanning tunneling microscopy. The substrate was exp osed to oxygen under a partial pressure of 1 x 10(-9) Torr at room tem perature during the observation. Dark features, which have been report ed as a main channel for oxidation, appeared exclusively in the faulte d halves of the DAS structure though bright features appeared in both halves. This difference can be explained by the adsorption site of oxy gen. That is, the greater energy difference of the dangling-bonds of a datoms between both halves than that of the back-bonds causes the excl usive appearance of the dark feature, which has an oxygen atom on on-t op site in addition to the other in the back-bond.