The effect of surface steps on the critical thickness for spreading of threading dislocations in thin epitaxial films

Experimentally, there is a critical thickness of an epitaxial thin film, below which misfit dislocations are not observed and above which threading dislocations are found to glide. There are several theoretical critical thicknesses that may or may not coincide with the observed value. We show that the presence of a properly aligned surface step can strongly influence the motion of a threading dislocation, enhancing or retarding its motion. Anisotropic elastic and elastic inhomogeneity effects are considered. The results are discussed in terms of the critical thickness concept. The anisotropic elastic prediction of the critical thickness for spreading agrees well with the experimental results for a Ni film on Cu.