Mechanism of GaN quantum dot overgrowth by Alo.5Gao.5N: Strain evolution and phase separation

摘要

The capping of GaN quantum dots (QDs) with an Al_(0.5)Ga_(0.5)N layer is studied using transmission electron microscopy and atomic force microscopy in combination with theoretical calculations. The capping process can be divided into several well-distinguishable stages including a QD shape change and a local change of the Al_(0.5)Ga_(0.5)N capping layer composition. The phase separation phenomenon is investigated in relation with the capping layer thickness. Amount of the chemical composition fluctuations is determined from separate analysis of scanning transmission electron microscopy and high-resolution transmission electron microscopy images. , The local distortion of atomic lattice in the QD surroundings is measured by high-resolution electron microscopy and is confronted with theoretically calculated strain distributions. Based on these data, a possible mechanism of alloy demixing in the Alo.5Gao.5N layer is discussed.