Self-organization mechanisms promoting elimination of cracks in thick GaN layers grown on sapphire substrates are considered on the basis of the experimental results on the fabrication of the layers by Hydride Vapor-Phase Epitaxy on MOCVD-grown GaN/Al2O3 templates. The obtained data support the supposition on the closure of tensile stress-related cracks via diffusion processes and demonstrate the strong contribution of bulk diffusion in addition to surface diffusion discussed earlier.