The spatial variation of the optical properties of hydride vapor-phase epitaxial GaN layers of various thickness has been studied using scanning cathodoluminescence microscopy. A strong improvement of these properties with film thickness is observed in plan view. Cross-sectional studies show a strong redshift of the luminescence in the vicinity of the substrate within a typical thickness of about 2 μm, reflecting a high local impurity content. Above this initial growth region, a strong blueshift is observed up to the energy of fully relaxed high-purity GaN, indicating vertical strain relaxation as well as depletion of residual donors. This is accompanied by a sharp increase in the lateral spectral homogeneity, indicative of a significant improvement in crystalline quality. © 2001 American Institute of Physics.