Optical Properties of Cubic GaN Quantum Dots Grown by Molecular Beam Epitaxy

We have investigated the optical properties of self-assembled cubic GaN quantum dots (QDs) in a cubic AlN matrix grown in Stranski-Krastanov growth mode. Two different sample series are fabricated and optically characterized by photoluminescence measurements. Additionally, the experimental results are compared to theoretical calculations. Sample series A consists of one single QD layer with varying GaN amount. A red shift of emission energy of 140meV is observed with increasing GaN deposition time. Simulations verify this behavior and can be explained by a decrease of the QD height. Sample series B has two layers of QDs on top of each other with different QD deposition amounts of three and six monolayers, respectively. The spacer layer thickness between the QD layers is 2 and 20nm. A decreased emission intensity of the smaller QDs is measured for QD layers with smaller spacer layer thickness. This observation is a first hint of an electronic coupling between the two QD layers with thin spacer layer thickness resulting in tunneling of carriers from the smaller QDs to the larger QDs. Theoretical calculations further show that a strain induced structural coupling induces a vertical alignment of the QDs.