Oxidation of the Si(001) surface: lateral growth and formation of P(b0) centers.

More than 100 oxygen-adsorbed configurations with oxygen coverage of up to two monolayers (ML) were studied through first-principles calculations. It was found that oxidation proceeds almost laterally. When the coverage exceeds 1.25 ML, oxygen atoms introduced between the second and third layers are captured at a bridging site in the second layer, generating twofold-coordinated Si atoms. Emission of such twofold-coordinated Si atoms leaves weakly bonded Si pairs in the fourth layer. When such pairs happen to be generated close to each other, they transform into a chain of Si trimers with one P(b0) center at each end.