The Role of Dangling Bonds in H2O-Induced Oxidation of Si(100)-2 × 1

The stability of the water-terminated Si(100)-2 × 1 surface in the 300−550 K temperature range is investigated with ultrahigh vacuum scanning tunneling microscopy experiments and density functional theory calculations. For temperatures below 450 K the surface is found to be stable from hydroxyl decomposition and surface oxidation. In the range of 450−550 K, new surface features associated with oxygen insertion into the silicon dimer bond are observed. It is found that the rate of hydroxyl decomposition and oxygen insertion does not follow simple first order kinetics with respect to the surface hydroxyl groups. Density functional theory calculations of oxygen insertion pathways point toward a catalytic effect of the dangling bonds and suggest that in the 450−550 K range the insertion events should predominantly occur next to unoccupied surface sites. A model is proposed where the dangling bonds diffuse along the dimer rows and promote hydroxyl decomposition. Kinetic Monte-Carlo simulations are used to compare the model with both experiments and density functional theory calculations, and an insertion activation barrier of 1.8 eV is found to give a good fit to the experimental data. On the basis of the findings, a strategy to increase hydroxyl group stability is demonstrated using water termination at cryogenic temperatures.