Mechanisms of GaN quantum dot formation during nitridation of Ga droplets*

GaN-based quantum dots (QDs) have been proposed for a variety of optoelectronic devices. Typically, QD formation is driven by a Stranski-Krastanov growth mode transition. Alternatively, the nucleation and conversion of metal droplets to QDs via nitridation, known as droplet epitaxy (DE), has emerged as a promising approach to achieve strain-free QDs. To date, GaN DE has been described as a liquid-phase epitaxy-like and/or a surface-diffusion driven process. Here, we investigate the formation mechanisms for DE GaN QDs using a combined computational-experimental approach. Our first-principles calculations of activation barriers suggest that N is immobile while Ga has a relatively high surface diffusivity, independent of the starting surface structure and chemistry. We consider the temperature and substrate dependence of the size distributions of droplets and QDs, and report on two competing mechanisms mediated by Ga surface diffusion, Ga droplet coarsening with QD formation via impinging N atoms and Ga droplet out-diffusion with QD nucleation at absorbed N surface sites. We also discuss the relative roles of nucleation and coarsening dominant growth, as well as the phase selection, on various substrates.