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 films on Si by introduction of additional InGaAs graded strain layers in combination with growth on patterned substrates. The substrate patterns consisted of mesas with 10-34 mum widths. The mesa sidewalls were either overhanging (concave), leading to free sidewalls for the film on the mesas, or outward sloping (convex) sidewalls with {111} orientation. The dislocation structure was studied using transmission electron microscopy. It was found that the graded strained layers led to a reduction of dislocation density by a factor of approximately 5 in films grown both on mesas with concave sidewalls and on unpatterned substrates. This reduction was due to dislocation reactions leading to annihilation of TDs. For films with graded strained layers on mesas with convex sidewalls, an additional factor of approximately 3 reduction in TD density 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 misfit 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 dislocations remained. We suggest that this is due to pinning of the TDs associated with 60-degrees misfit dislocations at the mesa edges and we have presented an explanation for this pinning in terms of the stress conditions at the {111} oriented mesa edges. In addition, this leads us to suggest that in order to obtain minimum TD density it is imperative to prevent formation of 90-degrees misfit dislocation during lattice mismatched heteroepitaxial growth.