The adsorption of O2 molecules at (2 x 1) reconstructed dimers on (001) surfaces was investigated by performing  ab initio  quantum chemical calculations. Detailed analyses of the potential energy hyper-surfaces in the spin triplet and quintet states revealed that the triplet state offered the lowest-energy reaction path for the oxidation process. The electronic state, as well as the atomic-level configuration of the molecularly adsorbed metastable precursor of O2 along this lowest-energy reaction path, was clarified. The molecular adsorbate was converted into an atomically adsorbed stable state by dissociation of the O2 adsorbate into O atoms. This was the insertion process of an O atom into a Si dimer bond to produce an oxide. The activation energy which was required for this conversion was calculated to be 60.4kcal/mol. This was in accord with the value of 60kcal/mol which was furnished by high-temperature experiments. By studying the temperature dependence of the reaction-rate constants, it was concluded that the reconstructed dimer was hardly oxidized at room temperature and that the origin of the natural oxide of Si(001) surfaces might be surface defects which reacted with O2. That is, the defect-free (001) surface was stable to O2 and was not oxidized at room temperature. This conclusion was consistent with recent experimental evidence that reconstructed dimers on the terraces of Si(001) surfaces were inactive with regard to the exposure to O2 molecules and that only defect sites on the same surfaces reacted with O2 molecules.

T.Hoshino, M.Tsuda, S.Oikawa, I.Ohdomari: Physical Review B, 1994, 50[20], 14999-5008