The green luminescence of polycrystalline samples was investigated by using diffuse reflection, steady-state or time-resolved photoluminescence, and photoconductivity methods, and was attributed to a donor-acceptor type of transition. Due to an efficient quenching by surface states, it was most effectively excited upon excitation into O defect states. This so-called dead-layer effect was used to determine the donor density (2.1 x 1018/cm3).

Luminescence and Nonradiative Deactivation of Excited States Involving Oxygen Defect Centers in Polycrystalline ZnO. Egelhaaf, H.J., Oelkrug, D.: Journal of Crystal Growth, 1996, 161, 190-4