Morphology and strain of self-assembled semipolar GaN quantum dots in (11(2)over-bar2) AlN

GaN quantum dots (QDs) grown in semipolar (11 (2) over bar2) AlN by plasma-assisted molecular-beam epitaxy were studied by transmission electron microscopy (TEM) and scanning transmission electron microscopy techniques. The embedded (11 (2) over bar2)-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 (2) over bar2) surface, QDs nucleated at depressions comprising {10 (1) over bar1} facets. This was justified by ab initio density functional theory calculations showing that such GaN/AlN facets are of lower energy compared to (11 (2) over bar2). 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. (C) 2010 American Institute of Physics. [doi:10.1063/1.3506686]