Oxidation of the Si(001) surface was studied by the first principles calculation technique with spin-polarized gradient approximation. We have clarified the apparent barrierless reaction mechanism of the backbond oxidation of the surface Si by incoming ${\mathrm{O}}_2$ molecules. An ${\mathrm{O}}_2$ molecule does not attack directly the backbond but the oxidation occurs via metastable chemisorption states on the Si surface. We have also found that the triplet-to-singlet spin conversion is crucial in explaining the incident energy dependence of the sticking probability of ${\mathrm{O}}_2$ molecules.

• Received 23 June 1997

DOI:https://doi.org/10.1103/PhysRevLett.80.2000