Morphology and strain of self-assembled semipolar GaN quantum dots in (11-22) AlN

GaN quantum dots (QDs) grown in semipolar (11-22) AlN by plasma-assisted molecular-beam
epitaxy were studied by transmission electron microscopy (TEM) and scanning transmission
electron microscopy techniques. The embedded (11-22)-grown QDs exhibited pyramidal or
truncated-pyramidal morphology consistent with the symmetry of the nucleating plane, and were delimited by nonpolar and semipolar nanofacets. It was also found that, in addition to the (11-22) surface, QDs nucleated at depressions comprising {10-11} facets. This was justified by ab initio density functional theory calculations showing that such GaN/AlN facets are of lower energy compared to (11-22). Based on quantitative high-resolution TEM strain measurements, the three-dimensional QD strain state was analyzed using finite-element simulations. The internal electrostatic field was then estimated, showing small potential drop along the growth direction, and limited localization at most QD interfaces.