Diffusion Mechanism of Si Adatoms on a Double-Layer Stepped Si(001) Surface

Using the ab initio molecular dynamics approach we investigate the diffusion mechanism of Si adatoms on a rebonded and a nonrebonded double-layer stepped Si(001) surface. The rebonded $D_B$ step shows two Schwoebel barriers, whereas the nonrebonded step reveals a single barrier. This is due to the severe tensile strain on the surface layers parallel to the dimer row near the rebonded step edge. Adatom-step interaction is more favorable on a nonrebonded step than on a rebonded one near the ascending step. Several pathways for diffusion of an ad-dimer to the rebonded step edge from an upper terrace are examined. We find that the exchange and crossing-over processes are favored over the rolling-over process.