Structure and nucleation mechanisms of misfit dislocations in epitaxial FCC thin films with positive and negative mismatches

Three-dimensional molecular dynamics simulations of epitaxial 〈1 1 1〉 growth of FCC aluminum films with positive and negative mismatch of 4%, respectively, were carried out. An embedded atom method (EAM) potential was employed. The results show that, in the film with the positive mismatch, an edge dislocation is formed through a glide mechanism. The Burgers vector of the dislocation has angle of 60° with the misfit strain. In the film with the negative mismatch, an edge dislocation is formed by a squeeze-out mechanism, in which a tetrahedron consisting of four atoms is squeezed out of the surface, which then generates a disorder zone, which incubates the dislocation. The Burgers vector of the dislocation is parallel to the misfit strain. The critical thickness of the film with the positive mismatch is found to be significantly larger than that with the negative mismatch. Presence of adatoms and other surface irregularities was found to favors the nucleation processes, and its role is discussed based on an analysis of the effect of single adatom on the structure of a flat film, with the positive or the negative mismatch.