Diffusion mechanisms of a Si adatom on H-terminated Si(100) surfaces

We present features in the adsorption and diffusion of a Si adatom on H-terminated Si(100) surfaces based on first-principles total-energy calculations. We find that the adatom spontaneously substitutes for a H atom upon adsorption. The resulting diffusion species on the surface is no longer a single atom and a typical approach based on a single potential energy surface is invalid. As an alternative, we trace the diffusion pathways of the adatom-H unit, simultaneously examining the effect of H release and capture by the adatom. Extensive investigations on the (Formula presented) phase show that the adatom releases the H atom bonded to it and captures another H atom as it migrates on the surface. In addition, the adatom penetrates into the surface, breaking a surface backbond. This results in exchange between the adatom and a surface Si atom. The mechanisms of H release and capture and of adatom exchange are also expected on other phases of the H-terminated Si(100) surfaces.