In situ X-ray topography studies during the molecular beam epitaxy growth of InGaAs on (001) GaAs: effects of substrate dislocation distribution on strain relaxation

We report results from a novel facility constructed to enable in situ X-ray diffraction studies during the molecular beam epitaxy growth of Ill-V strained layer device structures on 50 mm diameter substrates. This new facility, used in conjunction with the Daresbury synchrotron source, permits X-ray topographic imaging of individual misfit dislocations formed during the molecular beam epitaxy growth process. The misfit dislocation growth and interactions can be imaged as a function of layer thickness, strain, growth and post-growth conditions. Our recent results show that the nature and distribution of dislocations threading up from the substrate are crucial in determining the initial pattern of epilayer relaxation. Under certain growth conditions and substrate dislocation distributions, large areas of the epilayer remain free of misfit dislocations at epilayer thicknesses significantly higher than the measured initial critical thickness tc1. We have observed in situ for the first time a second critical thickness tc2 (under certain conditions tc2>2tc1) at which there is a rapid increase in misfit dislocation density as a second misfit dislocation source(s) becomes active.