High-resolution X-ray diffractometry and electron microscopy were used to study the defect structure and relaxation mechanism of elastic stresses in AlGaN/GaN superlattices grown by the metal-organic chemical vapour deposition method on sapphire covered with a preliminarily deposited GaN and AlGaN buffer layer. Based on an analysis of the half-widths of 3-crystal scan modes of X-ray reflections measured in different diffraction geometries, the density of different dislocation family was determined. For all the dislocation family, the density was shown to increase with the Al concentration in the solid-solution layers and depend only weakly upon the superlattice period. From the electron-microscopic patterns of planar and cross sections, the types of dislocations and their distribution in depth were determined. It was 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 were sloped intergrowing dislocations with a large horizontal projection and bent mixed dislocations with a Burgers vector, <1123>, 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 favoured partial relaxation of stresses between individual superlattice layers. In samples with a high Al concentration (greater than 0.4) in AlGaN layers, there were cracks surrounded by high-density chaotic horizontal dislocations.

The Defect Structure of AlGaN/GaN Superlattices Grown on Sapphire by the MOCVD Method. R.N.Kyutt, G.N.Mosina, M.P.Shcheglov, L.M.Sorokin: Physics of the Solid State, 2006, 48[8], 1577-83