Atomic and electronic structures of oxygen on Si(100) surfaces: Metastable adsorption sites

We have explored the mechanisms of oxygen adsorption on Si(100) reconstructed surfaces by first-principles total-energy band-structure and force calculations for slab geometries within the local-density approximation. The oxygen and the surface Si atoms are fully relaxed, according to the calculated forces acting on the atoms, toward the total-energy–optimized configurations. We have found three (meta)stable adsorption sites for the oxygen atom. Upon oxygen adsorption, the original dimer is preserved, twisted, or decomposed, depending on which site the oxygen is adsorbed. The adsorption energy for the (meta)stable configurations are so large that an ${\mathrm{O}}_2$ molecule dissociates exothermically on the Si surfaces. In fact, the ${\mathrm{O}}_2$ molecule is shown to dissociate on the Si dimer due to electron transfer from the Si dangling bond to the antibonding orbital of the ${\mathrm{O}}_2$ molecule. Comparison of the calculated valence density of states and vibrational frequency with existing experimental data indicates that the most stable atomic adsorption site is realized in typical experimental conditions. We propose, however, that other metastable configurations manifest themselves at early stages of the oxygen adsorption.