Ab initio study of step formation and self-diffusion on Ag(100)

Using the plane-wave pseudopotential method, we performed density-functional theory calculations on the stability of steps and self-diffusion processes on Ag(100). Our calculated step formation energies show that the (111)\-faceted step is more stable than the (110)\-faceted step. In accordance with experimental observations we find that the equilibrium island shape should be octagonal very close to a square with predominately (111)\-faceted steps. For the (100) surface of fcc metals atomic migration proceeds by hopping or exchange processes. For Ag(100), we find that adatoms diffuse across flat surfaces preferentially by hopping. Adatoms approaching the close-packed (111)\-faceted step edges descend from the upper terrace to the lower level by an atomic exchange with an energy barrier almost identical to the diffusion barrier on flat surface regions. Thus, within our numerical accuracy (≈±0.05 eV), there is no additional step-edge barrier to descent. This provides a natural explanation for the experimental observations of the smooth two-dimensional growth in homoepitaxy of Ag(100). Inspection of experimental results of other fcc crystal surfaces indicates that our result holds quite generally.