Effects of temperature and nitradition on phase transformation of GaN quantum dots grown by droplet epitaxy

Droplet epitaxy technique, the formation of gallium metallic droplets in the ultra-high vacuum and then nitridation with nitrogen plasma, was used to grow GaN nanodots on Si (111) by plasma-assisted molecular beam epitaxy. In this report, cubic GaN nanodots were successfully deposited on Si according to the characterizations of transmission electron microscopy (TEM), and we also investigate the influence of nitridation duration and substrate temperature on the surface roughness, crystal structure and density of GaN nanodots. The microstructures and phase transformation (from zinc-blende to wurtzite) of GaN nanodots are observed by controlling the irradiation time of nitrogen plasma (5, 7 and 10 min). Reflection high energy electron diffraction can in-situ monitor the process of droplet epitaxy. From the results of X-ray photoelectron spectroscopy, the sample grown at higher temperature and longer nitridation time had the strongest Ga-N bonding peak and the highest bonding ratio. That means GaN nanodots form more completely. The high-resolution TEM images of single GaN nanodot and selective area diffraction patterns of multi-GaN nanodots demonstrate the phase transformation of GaN nanodots from metastable zinc-blende to thermodynamically stable wurtzite structures as the nitridation time increases.