Effect of high temperature annealing on strain and band gap of GaN nanoparticles

Black-coloured GaN nanoparticles with an average grain size of 50 nm have been obtained by annealing GaN nanoparticles under flowing nitrogen at 1200 oC for 30 min. XRD measurement result indicates an increase in the lattice parameter of the GaN nanoparticles annealed at 1200 oC, and HRTEM image shows that the increase cannot be ascribed to other ions in the interstitial positions. If the as-synthesised GaN nanoparticles at 950 oC are regarded as standard, the thermal expansion changes nonlinearly with temperature and is anisotropic; the expansion below 1000oC is smaller than that above 1000 oC. This study provides an experimental demonstration for selecting the proper annealing temperature of GaN. In addition, a large blueshift in optical bandgap of the annealed GaN nanoparticles at 1200 oC is observed, which can be ascribed to the dominant transitions from the C(Γ₇) with the peak energy at 3.532 eV.