The defect structure of AlGaN/GaN superlattices grown on sapphire by the MOCVD method

High-resolution x-ray diffractometry and electron microscopy are used to study the defect structure and relaxation mechanism of elastic stresses in AlGaN/GaN superlattices grown by the MOCVD method on sapphire covered with a preliminarily deposited GaN and AlGaN buffer layer. Based on an analysis of the half-widths of three-crystal scan modes of x-ray reflections measured in different diffraction geometries, the density of different dislocation families is determined. For all the dislocation families, the density is shown to increase with the Al concentration in the solid-solution layers and depend only weakly on the superlattice period. From the electron-microscopic patterns of planar and cross sections, the types of dislocations and their distribution in depth are determined. It is shown that, in addition to high-density vertical edge and screw dislocations, which nucleate in the buffer layer and propagate through the superlattice layers, there are sloped intergrowing dislocations with a large horizontal projection and bent mixed dislocations with a Burgers vector $leftlangle {11overline 2 3} rightrangle $ at the interface between individual superlattice layers. The former dislocations form at the interface between the buffer layer and the superlattice and remove misfit stresses between the buffer and the superlattice as a whole, and the latter dislocations favor partial relaxation of stresses between individual superlattice layers. In samples with a high Al concentration (greater than 0.4) in AlGaN layers, there are cracks surrounded by high-density chaotic horizontal dislocations.