An investigation was made of the intrinsic defects, in films grown by molecular beam epitaxy, by annealing them in O2 and N2 atmospheres. It was found that there was a good correlation between the annealing conditions and the photoluminescence characteristics. The results of annealing experiments suggested that a photoluminescence at 3.358eV was caused by excitons bound to O vacancies. A green-yellow emission at around 2.3eV was also observed in as-grown epitaxial films. This emission became weak, and the electron concentration increased, when films were annealed in an O2 atmosphere at 1000C or in a N2 atmosphere at 700C. Since the donor in undoped ZnO was related to the O vacancy, the observed green-yellow emission at around 2.3eV was suggested to be caused by defects other than O vacancies. The experimental results implied that it was related to interstitial Zn.

Intrinsic Defects in ZnO Films Grown by Molecular Beam Epitaxy. Tatsumi, T., Fujita, M., Kawamoto, N., Sasajima, M., Horikoshi, Y.: Japanese Journal of Applied Physics, 2004, 43[5A], 2602-6