Mechanisms of GaN quantum dot formation during nitridation of Ga droplets

Abstract

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.__________________________ 1) H. Lu and C. Reese contributed equally to this work. 2) Corresponding Author: rsgold@umich.eduIn recent years, quantum dots (QDs) based on gallium nitride (GaN) and its alloys have been demonstrated for a wide variety of device applications, such as solar cells, 1 ,2 lightemitting diodes, 3,4,5 lasers, 6,7 and single-photon emitters. 8,9 QDs are typically grown epitaxially via a strain-induced Stranski-Krastanov (S-K) growth mode transition, which leads to a misfitstrain induced polarization in the QDs. 10 On the other hand, the nucleation and conversion of QDs via nitridation of metallic droplets, known as droplet epitaxy (DE), has attracted much attention as misfit-strain-induced-polarization is expected to be minimized. To date, DE of GaN QDs has been demonstrated on a variety of substrates, including 6H-SiC(0001), 11Si (111), 12,13,14 AlGaN/6H-SiC(0001), 15 and c-Al2O3, 16 with single electron transistor achieved on AlN/3C-SiC(001). 17 Furthermore, understanding of DE is critical for elucidation of the mechanisms for GaN growth under Ga-rich conditions, which are often argued to be a version of liquid phase epitaxy. 18 Indeed, during Ga-rich GaN growth, the low solubility of carbon in Ga and the reactivity of oxygen in Ga, with subsequent desorption of GaO at growth temperatures, leads to low carbon 19 and oxygen 20 co-incorporation. Therefore, DE GaN QDs are expected to be superior to those grown by the SK method. 21 However, conflicting results have been reported regarding the formation mechanisms of DE GaN QDs. For example, Wang et al. 17 and Gherisimova et al. 22 reported on the formation of QDs via a liquid phase epitaxy (LPE)-like process, where GaN crystallizes along the substrate/droplet interface when N supersaturates the liquid Ga. On the other hand, Debnath et al. 15 proposed a surface diffusion-driven mechanism, where Ga diffuses away from the droplets and reacts with N on the surface to form small QDs. Finally, Kawamura et al. 23 and Otsubo et al. 24 propose a formation mechanism where N diffuses along the surface to the droplet edges, and small QDs nucleate at the periphery. 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 immobi...