Structures of Steps and Appearances of {311} Facets on Si(100) Surfaces

First-principles total-energy calculations for structures and reactivities of single-, double-, and quadrilayer steps on Si(100) surfaces are presented. It is found that an activation energy for diffusion of an adatom on an upper terrace increases near a step edge and that rebonded and nonrebonded steps show bistability. A new reaction pathway in which the rebonded double-layer steps bunch and lead to a {311} facet is found to be energetically favorable.