Rutherford backscattering spectrometry (RBS) was performed on GaN layers grown on sapphire and annealed at temperatures between 500 and 1100 degreesC. Protons of energy 2 MeV were used for nanoscale depth-resolved RBS measurements. The simulation package SIMNRA was used to extract quantitative information from RBS results. Our results describe quantitatively the complete evaporation of GaN surface layers followed by partial evaporation of gallium and nitrogen atoms from successive layers along with incorporation of oxygen from the ambient during annealing. The formation of micron-sized islands or terraces on GaN surface during annealing has been explained using RBS and atomic force microscopy results. (C) 2003 Elsevier B.V. All rights reserved.