Defect formation in self-assembling quantum dots of InGaAs on GaAs: a case study of direct measurements of local strain from HREM.

The growth and defect structures in free-standing self-assembled In0.6Ga0.4As quantum dots (QDs) grown on (001) GaAs by solid source molecular beam epitaxy has been investigated. The QDs are elongated along [1; 1 0]. At a nominal thickness of eight monolayers defect complexes, associated with intrinsic stacking faults, have been generally observed on both sides of a QD in (1; 1 0) cross-section. The total defect vector of such defect complexes is a/3 <111>. Local strain components on {111} slip planes in the QDs without defects have been measured directly from digitized high-resolution electron microscopy images. The distortion on the two sets of {111} planes of a (1; 1 0) cross-section is different owing to elastic relaxation. The results of strain measurements suggest that a 60 degrees dislocation nucleates first on the set of {111} planes of higher contractive shear strain, i.e. (111) planes on the right side of the QDs, and (1; 1; 1) planes on the left side. A 30 degrees partial dislocation forms subsequently on the other set of {111} planes, i.e. (1; 1; 1) planes on the right side of the QDs and (111) planes on the left side, when the 60 degrees dislocation glides down towards the In0.6Ga0.4As/GaAs interface, as a result of the additional strain field of the 60 degrees dislocation. The efficiency of the defect complexes in strain relaxation of the QDs has been shown by strain measurements in QDs with the presence of defects.