Molecular dynamics of surface diffusion. I. The motion of adatoms and clusters

The motion of isolated adatoms and small clusters on a crystal surface is investigated by a novel and efficient simulation technique. The trajectory of each atom is calculated by molecular dynamics, but the exchange of kinetic energy with the crystal lattice is included through interactions with a ’’ghost’’ atom. This atom represents surface atoms of the lattice and is subjected to random and dissipative forces that are related by the fluctuation–dissipation theorem. The diffusion process is characterized by measurements of the velocity autocorrelation function, mean square displacement, directional correlations between hops, and the mean displacement per hop. In addition, the rate of evaporation of single adatoms and the rate of dissociation of clusters are discussed. The diffusion of an isolated adatom is found to be somewhat faster than that predicted by the classical rate theory for an activated process. This effect is a result of diffusion jumps of several atomic diameters that occur preferentially at high temperatures. But Arrhenius behavior is observed over the entire range of temperatures below the melting point. Dimers and larger clusters are found to diffuse more slowly than individual atoms, but with a smaller apparent activation energy. These results do not exhibit the high-temperature anomalies that have been inferred from some experimental data on surface mass transport. In a subsequent paper the method is extended to treat mass transport in the layer of adatoms and clusters that results from a dynamic equilibrium with the vapor phase.