EPITAXY ON SURFACES VICINAL TO SI(001) .1. DIFFUSION OF SILICON ADATOMS OVER THE TERRACES

We have studied the migration of silicon adatoms over the flat, reconstructed Si(001) surface with molecular-dynamics techniques, using the empirical Stillinger-Weber potential. We have mapped out the potential-energy surface seen by a single adatom over the surface. The binding of an adatom to the Si(001) surface was found to be quite strong-its value of 3.0 eV is a sizable fraction of 4.33 eV, the energy of a bulk silicon atom. In agreement with other studies, we find that the diffusion of single adatoms over the surface is anisotropic, with the fast diffusion direction being along the dimer rows. We carried out a direct measurement of a diffusion coefficient by studying the motion of individual adatoms over a time period of 300 ps. The motion of individual adatoms conforms quite well to the underlying energy surface. However, we did observe frequent exchange events between adatoms and atoms near the surface, which gives an additional contribution to the rate of diffusion. In addition, we mapped out the potential-energy surfaces seen by a silicon adatom over Si(001) surfaces that were under compressive and tensile strain. In these situations, the binding was found to increase or decrease depending upon whether the surface was compressed or expanded. Little change was observed in the anisotropy of the diffusion constant. We also looked at diffusion over the Si(001) surface in which the compression is either along or perpendicular to the dimer axis. Compression of the surface perpendicular to the dimer axis decreases the diffusional anisotropy, while compression of the surface parallel to the dimer axis increases the diffusional anisotropy.