GaN quantum dot superlattices grown by molecular beam epitaxy at high temperature

In this paper, we report the growth of GaN quantum dot superlattices (QDSLs) with AlN barriers on (0001) sapphire substrates by molecular beam epitaxy at relatively high temperature (770°C) using the modified Stranski-Krastanov growth mode. Observations with atomic force microscopy show that the height distribution of the dots depends strongly on the number of GaN monolayers (MLs) grown on the AlN barriers. Specifically, the height distribution consists of two Gaussian distributions (bimodal) for coverages of 3 or 4 ML, and becomes a single Gaussian distribution for 5 and 6 ML of coverage. Furthermore, the density of quantum dots increases with the degree of coverage and saturates at 2×1011dots∕cm2. An increase in the number of stacks in the superlattice structure with 4 ML coverage also leads to a more pronounced bimodal height distribution. Electron microscopy observations indicate that the GaN QDs are truncated pyramids faceted along the {11¯03} planes and suggest that larger dots are associated with threading dislocations which presumably provide low-energy nucleation sites. Transmission electron microscopy studies also indicate that most of the larger dots are nucleated next to edge-type dislocations, while most of the smaller dots are located in dislocation-free regions. These GaN QDSLs were also studied by grazing-incidence small angle x-ray scattering and grazing-incidence x-ray diffraction methods. The average lateral deviation and the vertical correlation length between QD positions for two successive layers were determined to be 1.4nm and 190nm, respectively. A GaN QD growth model is proposed to explain the phenomenon.