THE USE OF GRADED INGAAS LAYERS AND PATTERNED SUBSTRATES TO REMOVE THREADING DISLOCATIONS FROM GAAS ON SI

We have investigated threading dislocation (TD) removal from GaAs film s on Si by introduction of additional InGaAs graded strain layers in c ombination with growth on patterned substrates. The substrate patterns consisted of mesas with 10-34 mum widths. The mesa sidewalls were eit her overhanging (concave), leading to free sidewalls for the film on t he mesas, or outward sloping (convex) sidewalls with {111} orientation . The dislocation structure was studied using transmission electron mi croscopy. It was found that the graded strained layers led to a reduct ion of dislocation density by a factor of approximately 5 in films gro wn both on mesas with concave sidewalls and on unpatterned substrates. This reduction was due to dislocation reactions leading to annihilati on of TDs. For films with graded strained layers on mesas with convex sidewalls, an additional factor of approximately 3 reduction in TD den sity was observed in the part of the film that was grown on top of the mesas. In this case all mobile TDs (TDs associated with 60-degrees mi sfit dislocations, i.e., TDs that could glide to relieve misfit stress ) were removed from the film on top of the mesas to the regions above the sidewalls and only TDs associated with 90-degrees misfit dislocati ons remained. We suggest that this is due to pinning of the TDs associ ated with 60-degrees misfit dislocations at the mesa edges and we have presented an explanation for this pinning in terms of the stress cond itions at the {111} oriented mesa edges. In addition, this leads us to suggest that in order to obtain minimum TD density it is imperative t o prevent formation of 90-degrees misfit dislocation during lattice mi smatched heteroepitaxial growth.