Barrierless bond breaking and exchange diffusion on Si(100)-H

Hydrogen has long been used in epitaxial growth. We have studied adsorption and diffusion of a Si adatom on H-terminated Si(100) surfaces using first-principles total-energy method. We find that the adatom spontaneously substitutes for the H atom upon adsorption. The adatom diffusion involves the H-release from and H-capture by the diffusion species and the exchange of the adatom with a substrate Si atom. We also find that the diffusion barrier is sensitive to H coverage. Calculated results are consistent with H-induced variation in morphology of overlayers observed in epitaxial growth.