Effect of graded buffer design on the defect structure in InGaAs/GaAs (111)B heterostructures

In this work, the defective structure of InGaAs/GaAs (111)B has been studied, using different types of substrate misorientation and incorporating different types of graded buffer layers. The formation of deformation twins has been observed to be independent of misorientation direction, but with a distribution dependent on the substrate misorientation angle. Samples grown on 2° misoriented substrates showed long twins with the majority of them formed with only one {111} variant. The use of a step-graded structure achieved a threading dislocation (TD) density improvement, although it was not able to alter the density of plastic relaxation occurring through twins. Twin nucleation occurred at the surface, being controlled by the superficial step density and not by the relaxation sequence of the structure. In the perpendicular direction to the off-cut, the linearly graded layer structures showed a higher twin density and no misfit dislocations (MD) were observed. A diffraction pattern study indicated an epilayer tilt in the off-cut direction with respect to the substrate. This additional tilt, probably due to the introduction of dislocations with a preferential Burgers vector, produced a superficial step density increase and, therefore, strain relief occurred by twins in preference to MDs.