Distinct GaN islands of triangular base were formed during the annealing processes of GaN nucleation layers grown on Si-rich SiN, nanoislands pattemed sapphire substrates due to enhanced diffusion and regrowth anisotropy. Subsequent high temperature growth of GaN epilayers on the nucleation layers resulted in island coarsening and shape variations from triangular to hexagonal due to the dominating gas phase transport growth mechanism and limited diffusion length. Further growth with high V/III ratios resulted in layer-growth of atomic flatness. The atomic force microscopy, X-ray diffraction, and photoluminescence studies showed a significant improvement of the crystalline qualities and enhancement of the optical properties. (c) 2007 Elsevier B.V. All rights reserved.