Defect-rich GaN interlayer facilitating the annihilation of threading dislocations in polar GaN crystals grown on (0001)-oriented sapphire substrates

Abstract

The interaction of microstructure defects is regarded as a possible tool for the reduction of the defect density and improvement of the crystal quality. In this study, this general approach is applied to reduce the density of threading dislocations in GaN crystals grown using high-temperature vapor phase epitaxy directly on (0001)-oriented sapphire substrates. The GaN crystals under study were deposited in three steps with different process temperatures, growth rates, and ammonia flows. The first GaN layer accommodates the lattice misfit between sapphire and gallium nitride. Thus, it contains a high number of randomly distributed threading dislocations. The next GaN layer, which is internally structured and defect-rich, bends and bunches these dislocations and facilitates their annihilation. The uppermost GaN layer mainly contains bunched threading dislocations terminating large areas of almost defect-free GaN. In order to be able to visualize and to quantify the microstructure changes in individual parts of the sandwich-like structure, the samples were investigated using nanofocused synchrotron diffraction, confocal micro-Raman spectroscopy, and transmission electron microscopy. The transmission electron microscopy provided information about the kind of microstructure defects and their mutual interaction. The synchrotron diffraction and the micro-Raman spectroscopy revealed the depth profiles of dislocation density and lattice parameters.