ADATOM-DIMER INTERACTION ON THE SI(001)-2X1 SURFACE

We use a modified form of the Stillinger-Weber potential to obtain the binding sites and diffusion barriers of a Si adatom in the vicinity o f single F and B type dimers on the Si(001)-2 X 1 surface. We find tha t both kinds of dimer provide good sinks for adatoms and are therefore ideal nucleation sites, provided the temperature is not too high as t o induce dimer breaking. Our results also show that adatoms can be tra pped in non-lattice sites surrounding the F type dimer, leading to a d isordering of the growing epitaxial film. Monte Carlo simulated anneal ing indicates that adatoms at these ''defect'' sites are vertically di splaced with respect to those adsorbed on the epitaxial sites, giving rise to step structures that closely resemble those proposed by Falta and Henzler [Surf. Sci 269/270 (1992) 14] to account for their SPA-LEE D results.