Chemical differences in surface diffusion: si and ge adatoms at the (formula presented) step on the hydrogenated si(100) surface

We study the adsorption and diffusion of Si and Ge adatoms near the (Formula presented) step on the hydrogenated Si(100) surfaces and their chemical effect on surface diffusion, using a first-principles total-energy method. Due to their isovalency, the Si and Ge adatoms have similar features in the adsorption sites, adsorption geometries, and diffusion pathways near the step edge. Despite the similarities, the potential energy profiles for both adatoms show a clear difference: The energy barrier for the Si adatom of the upper terrace to be bound to the step edge is 1.1 eV, but the barrier reduces to about a half, 0.6 eV, for the Ge adatom. We find its origin in the bond-energy difference between Si-H and Ge-H and in the H-mediated diffusion process. This suggests that the Si and Ge adatoms behave in different ways on the hydrogenated Si(100) surfaces.