  We have examined the formation mechanisms of GaN quantum dots (QDs) via
annealing of Ga droplets in a nitrogen flux. We consider the temperature and
substrate dependence of the size distributions of droplets and QDs, as well as
the relative roles of Ga/N diffusivity and GaN nucleation rates on QD
formation. We report on two competing mechanisms mediated by Ga surface
diffusion, namely QD formation at or away from pre-existing Ga droplets. We
discuss the relative roles of nucleation and coarsening dominant growth, as
well as the polytype selection, on various substrates. The new insights provide
an opportunity for tailoring QD size and polytype distributions for a wide
range of III-N semiconductor QDs.
