It was noted that several of the optical transitions in these materials appeared to have a similar origin, and exhibited considerable overlap in the energy regimes where they occurred. In particular, the donor-acceptor pair transitions and the so-called yellow band (GaN) and analogous green band (ZnO) exhibited marked similarities. It was concluded that it was likely that the transitions in these 2 materials were defect-related and shared a common mechanism.

Similarities in the Bandedge and Deep-Centre Photoluminescence Mechanisms of ZnO and GaN. Reynolds, D.C., Look, D.C., Jogai, B., Morkoç, H.: Solid State Communications, 1997, 101[9], 643-6