Reversible ultraviolet-induced photoluminescence degradation and enhancement in GaN films

UV-induced modifications in undoped metalorganic chemical vapor deposition grown GaN on sapphire are observed from 9 to 160 K. The photoluminescence i ntensities of bound excitons (3.476, 3.482 eV), the yellow band (2.2 eV) an d the blue band (2.9 eV) change with time when a fresh sample is irradiated by 325 nm (He-Cd laser). The free exciton peak at 3.488 eV is unchanged by laser irradiation. Initially the blue and donor-bound exciton emission deg rade rapidly and the yellow luminescence increases, each at the same rate. Later, the yellow luminescence degrades and the donor-bound exciton emissio n increases very slowly, at the same rate. Mechanisms are proposed that may explain the luminescence pathways and defects involved. (C) 1999 American Institute of Physics. [S0021-8979(99)06116-2].