Effect of well/barrier thickness ratio on strain relaxation in GaN/AlN superlattices grown on GaN/sapphire template

In this study, the authors investigate the influence of increasing well-to-barrier thickness ratio (from one up to ten times) on the growth mechanism and strain relaxation of 20-periods GaN/AlN superlattices (SLs) on GaN(0001)/sapphire template. High-resolution x-ray diffraction was performed to determine the strain state and structural parameters of the SL. The total thickness of the SL and the individual thicknesses of the quantum well and the barrier layers were additionally determined by strain insensitive x-ray reflectometry. A slight difference between the measured and designed thicknesses of quantum well and barrier layers was observed. The surface modifications of the samples were investigated with atomic force microscopy. It was observed that the growth mechanism of the GaN/AlN SLs changes with the increase of well-to-barrier thickness ratio, from the columnar to the planar two-dimensional layer-by-layer growth. High density of cracks and/or threading dislocations are observed on the surfaces of ...