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 III-V strained layer device structures on 50 mm diameter substrates. T his new facility, used in conjunction with the Daresbury synchrotron s ource, permits x-ray topographic Imaging of individual misfit dislocat ions formed during the molecular beam epitaxy growth process. The misf it dilocation growth and interactions can be imaged as a function of l ayer thickness, strain, growth and post-growth conditions. Our recent results show that the nature and distribution of dislocations threadin g 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 t(c1). We have observed in si tu for the first time a second critical thickness t(c2) (under certain conditions t(c2) > 2t(c1)) at which there is a rapid increase in misf it dislocation density as a second misfit dislocation source(s) become s active.