GaN Quantum Dots Grown at High Temperatures by Molecular Beam Epitaxy

ABSTRACTIn this paper we report the growth by MBE of GaN quantum dot superlattices (QDSLs) with AlN barriers on (0001) sapphire substrates at relatively high temperatures (770 °C) by the modified Stranski-Krastanov method. TEM studies indicate that the GaN QDs are truncated pyramids. We find that the height distribution of the dots depends strongly on the number of GaN monolayer coverage on the top of AlN. Specifically, we find that the height distribution consists of two Gaussian distributions (bimodal) for coverage of 3 and 4 MLs, and becomes single Gaussian distribution for 5 and 6 MLs of coverage. Furthermore, we find that the density of quantum dots increases with the degree of coverage and saturates at 2×1011 dots/cm2. The number of stacks in the superlattice structure was also found to lead to bimodal height distribution of the QDs. Ordering of the quantum dots was accomplished by thermal annealing of the sapphire substrates at 1400 °C prior to the growth of GaN QDs. The annealing process reveals the vicinal steps due to the miscut of the substrates and the GaN QDs were found to line up along those steps. Photoluminescence studies show a broad luminescence spectrum centered at 3 eV which is red shifted with respect to that of bulk GaN and is consistent with internal fields due to polarization (Quantum Confined Stark Effect). Furthermore, we find that the luminescence intensity increases with the number of stacks in the superlattice structure due to higher spatial density of QDs.