Single-crystal line 350-mu m-thick GaN were grown on 2 in (0001) sapphire substrate by the hydride vapor phase epitaxy method. The free-standing bulk-like GaN crystals were obtained by delaminating from the sapphire substrate by the laser-induced lift-off technique. Mechanical polishing of the GaN crystals to remove the damage layer on their surfaces resulted in deteriorating optical properties. In order to recover the optical damages induced by the mechanical polishing, thermal annealing and N-2 plasma treatment were adopted. As the annealing temperature was increased up to 900 degrees C, the surface morphology and photoluminescence property of the polished GaN crystals improved, resulting from removal of the polishing-induced damage layer on the surface. Moreover, the N-2 plasma treatment on the mechanically polished samples effectively removed the polishing-induced damage, leading to remarkably improve the surface morphology and photoluminescence. (c) 2006 Elsevier B.V. All rights reserved.