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 $\mathrm{Si}\left(111\right)-(7\times 7)$ surface. We find that hydrogen binding is stronger at rest-atom than at adatom sites by $\sim 0.2\mathrm{eV}$, in good agreement with desorption experiments. This result together with a detailed analysis of H diffusion paths and barriers indicates that $R\to A\to R$ jumps provide the mechanism of H diffusivity at low coverages. The computed barrier for these jumps agrees well with experiment.