The Film Surface as the Site for Spontaneous Nucleation of Dislocation Half-Loops in Strained Heteroepitaxial Systems

Abstract

A study of dislocation formation in the initial stage of coherency breakdown, i.e. the transition from coherent to incoherent epilayer growth, is presented. It is assumed that half-loop nucleation at the growing surface is the dominant process for the spontaneous misfit dislocation generation in strained single heteroepitaxial layers. Based on the classical energy equilibrium theory, expressions with respect to critical loop radius R c and critical nucleation energy E hl,c have been derived for different loop habits where the dissociation of dislocations into partials has been taken into account for layers grown under both, compressional as well as tensional strain conditions. In addition, the stress dependence of the separation width between Shockley partials was considered. Since the activation energy is too great for spontaneous half-loop nucleation at a flat surface we propose the half-loop nucleation at large surface steps acting as stress concentrators. Due to the stress increase there the activation energy for half-loop nucleation is decreased considerably and the formation of misfit dislocations is more feasible in this way. The calculations have been performed for the heteroepitaxial system In 1-x Al x P/(001)GaAs.