Real-time x-ray studies of gallium nitride nanodot formation by droplet heteroepitaxy

Self-organized gallium nitride nanodots have been fabricated using droplet heteroepitaxy on c -plane sapphire by plasma-assisted molecular beam epitaxy at different substrate temperatures and Ga fluxes. Nanoscale Ga droplets were initially formed on the sapphire substrate at high temperatures by Ga deposition from an effusion cell in an ultrahigh vacuum growth chamber. Subsequently, the droplets were converted into GaN nanodots using a nitrogen plasma source. The process was monitored and controlled using real-time grazing-incidence small-angle x-ray scattering. The samples were examined postgrowth by in situ grazing incidence x-ray diffraction and reflection high-energy electron diffraction, which confirmed the epitaxial relationship between the GaN nanodots and the sapphire surface. X-ray diffraction indicated that the wurtzite phase was dominant at higher substrate temperature (710 °C), but a mixture of wurtzite and zinc blende phases was present at a substrate temperature of 620 °C. Ex situ atomic force microscopy and transmission electron microscopy analyses showed that the dot size distribution was bimodal. A thin GaN continuous layer of ∼ three monolayers thick was observed by transmission electron microscopy on the sample grown at a substrate temperature of 620 °C, but no such layer was observed for the substrate temperature of 710 °C. This suggests that there is little mobility of Ga atoms in contact with the sapphire substrate at the lower temperature so that they cannot easily diffuse to nearby droplets and instead form a thin layer covering the surface.