Ab initio surface reaction energetics of H2X (X = O or S) on Si(0 0 1)-(2 × 1)

First principles pseudopotential calculations within a simple dynamically constrained scheme have been performed to investigate the surface mediated reaction of 0.5 monolayer H2X (X = O or S) with the Si(0 0 1)-(2 × 1) surface. H2S is found to adsorb with a partial dissociation, and H2O adsorbs molecularly followed by a partial dissociation. For both molecules two fully dissociated structures are discussed: a symmetric chain-like structure where the Si(0 0 1)-(2 × 1) dimer is broken and a H-Si-X-Si-H formation takes place; and an interstitial structure where the Si(0 0 1)-(2 × 1) dimer is intact but the species X is situated in one of the dimer back-bonds. A comparison of the classical migration barrier rate and a semi-classical barrier tunnelling model is also discussed and it is found that for the dissociation of both molecules, proton tunnelling is dominant even at room temperature.