AB-INITIO PERIODIC PSEUDOPOTENTIAL HARTREE-FOCK CALCULATIONS OF O-2 DISSOCIATION ON PERFECT SI(100) SURFACE

This paper investigates the cleavage of O-2 adsorbed on clean Si(100) surfaces with ab initio periodic pseudopotential Hartree-Fock calculat ions. For the adsorption of the O-2 molecular precursors at coverage t heta = 1 (one O-2 per dimer), the Si(100) surface remains reconstructe d and O-2 are adsorbed on the dimers, parallel to the surface; the sit es between the dimers are vacant. For the atomic adsorption at the sam e coverage (representing then one O per surface Si) the surface recons truction nearly disappears; the oxygen atoms are adsorbed over the suc cessive spaces between adjacent surface Si atoms along the (110) direc tions (those of the elongated dimers and those between them) in asymme tric fashion. The dissociation of O-2 requires the breaking of two ban ds, the SiSi and OO bonds and the migration of an oxygen atom. It ther efore implies the presence of a high activation energy. Two paths are investigated for the O-2 dissociation and the migration of an atomic o xygen on the surface. The migration of O-2 from the best adsorption mo de to least favourable one where the dissociation is easier is also di scussed.