Role of hydrogen for adsorption and diffusion of a Si adatom on monohydride and dihydride Si(0 0 1) surfaces

We investigate adsorption and diffusion of a Si adatom on monohydride and dihydride Si(0 0 1) surfaces using first principles calculations. We find that adsorption and diffusion of a Si adatom on hydrogenated Si(0 0 1) surfaces are drastically altered from those on a clean surface in several ways. Both positions and number of adsorption sites vary with different coverages of hydrogen, leading to completely different diffusion behavior from the clean surface. The Si adatom is stabilized by capturing surface hydrogen without potential barrier during adsorption. The H atom hops back and forth exothermally to the adatom during diffusion and thus, the diffusion species is no longer Si adatom itself. It reveals different diffusion features that typical potential energy surface with a single adatom fails to describe. Differences in the adsorption and diffusion of Si adatom between monohydride and dihydride phases are discussed. The role of hydrogen as a surfactant on hydrogenated surface will be further discussed based on our calculations.