Periodic Hartree-Fock calculation of the oxidation of Si(111)

We present Hartree-Fock calculations of the adsorption of oxygen on a Si(11 1)-(2 x 2) surface, which models the Si(111)-(7 x 7) structure, including i ts two main sites (adatom and rest atom sites). In the most stable configur ations, molecules dissociate leading to atomic oxygen bridging the adatom b ackbonds. The molecular adsorption is less favorable. The dioxygen, however , is a local minimum of the potential surface in two cases: (i) in the "gri f" geometry; (ii) bridging between a rest atom and an adatom. It then repre sents a possible precursor for the early stage of oxidation. The presence o f an oxygen atom already adsorbed on the surface enhances the heat of adsor ption of other oxygen (atomic or molecular) on the same site.