The influence of high temperature buffer layers on the structural characteristics of GaN grown by hydride vapour phase epitaxy on sapphire was investigated. Strain relaxation as well as mismatch-induced defect reduction in thick GaN layers grown on AlN buffer was microscopically identified using cathodoluminescence and micro-Raman spectroscopy in cross-section of the films. The results were correlated with photoluminescence and Hall-effect data of layers with different thicknesses. These relaxation processes were suggested to account for the specific defect distribution in the buffers revealed by high-resolution X-ray diffraction and transmission electron microscopy.