In-situ transmission electron microscopy studies of the interaction between dislocations in strained SiGe/Si(001) heterostructures

The rate of misfit strain relaxation in semiconductor heterostructures is controlled by the kinetics of misfit dislocation nucleation, propagation and interaction. Although there have been a number of detailed theoretical studies of the dislocation-dislocation interaction process in these systems, there exist limited experimental data regarding the regime of epilayer thickness and composition where these interactions affect strain relaxation. Because of its high spatial and temporal resolution, in-situ transmission electron microscopy is an ideal experimental tool with which to observe dislocation interactions. In this work, the unique capabilities of an ultrahigh vacuum transmission electron microscope have been exploited to permit direct, real-time observation of dislocation interactions. This microscope is equipped with in-situ chemical vapour deposition capabilities which allow imaging of dislocation motion during both growth and annealing of SiGe/Si(001) heterostructures. This has permitted efficient determination of the regime of epilayer thickness and composition where dislocation interactions stop the forward motion of the threading dislocation segment. At the lowest thicknesses and compositions, all dislocation interactions cause the threading segment to become blocked. Above a certain level of thickness and composition dislocation motion is halted only when the two dislocations have parallel Burgers vectors. This is due to a particular reaction between the dislocations which occurs at their intersection. Quantitative analysis of the dynamic motion of threading dislocations while in the presence of the stress field of the interfacial segment allows extraction of the magnitude of the long range interaction stresses. It is seen that the magnitude of the interaction stresses increases as the net excess stress in the epilayer increases, and that the interaction stresses are much greater when the Burgers vectors are parallel. Finally, additional experiments have indicated that dislocations that become blocked during annealing form relatively stable configurations. Following the blocking event a force greater than that associated with the long range interaction between dislocations is required to release the dislocations from their blocked configuration. This is ill contrast to theoretical models, which predict that the long range interaction force alone controls the release of dislocations. These experiments represent the first systematic study of dislocation interactions in strained heterostructures and permit improved predictive models of the kinetics of strain relaxation in these systems.