Non-isotropic surface diffusion of Pb on Cu(110): a molecular dynamics study
Résumé
By using numerical simulation we have studied the diffusion of isolated lead atoms on Cu(110). The calculations rely on a phenomenological many-body potential derived in the framework of the second-moment approximation of the tight-binding method, with parameters fitted on the physical properties of the bulk crystals of copper and lead and to the copper–lead phase diagram. Static calculations, at T=0 K, provide the energy and relaxed atomic positions of the equilibrium and saddle-point configurations of various possible diffusion mechanisms. In spite of the large miscibility gap present in the lead–copper phase diagram, we find that insertion of a lead adatom into the uppermost copper surface layer is a thermodynamically favoured process. Molecular dynamics calculations show that inserted lead atoms diffuse via an exchange mechanism with copper adatoms and via jumps in adatom position along the open [10] direction. These results confirm previously published experimental observations. They also confirm the validity of a statistical model that was developed to account for these observations. The quantity governing the variation of diffusion anisotropy with temperature is the difference Er−Ej between the activation energies for the insertion of a lead atom in the copper plane and for its jumps in adatom position. The value of this difference, as determined in the static simulations, compares very well with what can be deduced from experimental observations. The agreement is also very good concerning the value of the main diffusion barrier, which is the energy associated with the de-insertion of a lead atom. Simulations performed in the temperature range 400–700 K show that multiple jumps occur frequently. Their frequency increases with temperature, thus leading to lead diffusion that is more anisotropic and more steeply dependent on temperature than could be expected from the static calculations.