We report on the further investigation of the effect of post-growth thermal annealing on optical and structural properties of the high N-content GaAs0.949N0.051 layer grown on a GaAs(001) substrate by metalorganic vapor phase epitaxy (MOVPE). Photoluminescence (PL) spectroscopy was performed to measure the energy positions of the near band edge excitonic emission. The high-resolution X-ray diffraction and Raman spectroscopy were conducted to examine the lattice parameters, also the N concentration of the layers annealed at 650 degrees C with different annealing times. The layer subjected to thermal anneals exhibits an increasing of N incorporation, a strain relaxation and a blue shift of the PL peak energy. For such high N-containing layer, the interstitial N atoms generated in the growth process may replace the As atoms/vacancies on the lattice sites to become more stable substitutional N atoms through the thermal annealing process, which will produce the strain relaxation, in addition to an improvement of the alloy uniformity. Our results suggest the two major effects: (i) the reorganization of N and (ii) the strain relaxation in the GaAsN layer that can be explained the blue shift in the PL peak energy after annealing. (c) 2006 Elsevier B.V. All rights reserved.