Dislocation nucleation in strained epitaxial layers

The mechanisms of misfit dislocation nucleation in strained epitaxial layers are not well understood. While the conditions under which strained layers will relax and the kinetics of the relaxation are generally well described, traditional dislocation nucleation theories do not effectively explain how these layers relax. Recent observations of a rapid anomalous relaxation, which occurs prior to the full relaxation of a strained film, cast further doubt on explanations based on nucleation sources such as heterogeneous sources and pre-existing dislocations. This dissertation demonstrates that strained epitaxial layers relax through the introduction of misfit dislocations, and that these dislocations may be introduced through a mechanism similar to one active in brittle-ductile transitions (BDTs). A recent theory on the BDT sheds new light on the issue of the generation of misfit dislocations in strained epitaxial layers. The Khantha-Pope-Vitek (KPV) model of the BDT states that thermal fluctuations in the lattice, assisted by an applied stress, can result in the spontaneous generation of dislocations. Application of the KPV model to the problem of epitaxial layers predicts that at a critical temperature and stress, a sudden onset of dislocation generation will occur. Using a wafer curvature technique, real-time strain relaxation curves were generated as a function of temperature. Misfit dislocation and mobile dislocation densities were extracted from the relaxation curves through simple relationships. The relaxation data shows a sudden onset of relaxation of 5 to 22% of the epitaxial strain upon reaching temperatures in the range of 500 to 540C. Atomic force microscopy (AFM) studies confirmed that the relaxation event occurred through the introduction of misfit dislocations, as opposed to film surface roughening. Misfit dislocation densities measured with the wafer curvature technique were validated through AFM measurements of the dislocation density. The strong temperature dependence of the initial relaxation event supports our hypothesis that a KPV-type mechanism is the source of the massive dislocation generation necessary to relax the film strain