We studied the self-diffusion at clean, flat as well as stepped Al(111) surfaces using density-functional theory. There are two important steps on fcc(111) surfaces, typically labeled according to their {111} and {100} microfacets. We calculated the formation energies of these steps, and analyzed the diffusion perpendicular and parallel to them. We discuss the general profile of the diffusing-atom potential-energy surface and identify the role of ‘‘normal’’ hopping and exchange mechanisms of diffusion at steps. From these results the equilibrium shape of islands and the temperature dependence of the island shapes under growth conditions are predicted.

• Received 9 August 1993

DOI:https://doi.org/10.1103/PhysRevLett.72.254