Binding Sites, Migration Paths, and Barriers for Hydrogen on Si(111)-(7 × 7)

We present density functional calculations of the potential energy surface for the binding and diffusion of a hydrogen atom on a Si(111) adatom structure with two adatom and one rest-atom dangling bonds per unit cell, which we use to model the Si(111)-(7×7) surface. We find that hydrogen binding is stronger at rest-atom than at adatom sites by ~0.2eV, in good agreement with desorption experiments. This result together with a detailed analysis of H diffusion paths and barriers indicates that R->A->R jumps provide the mechanism of H diffusivity at low coverages. The computed barrier for these jumps agrees well with experiment.