Specific defects, surface band bending and characteristic green emissions of ZnO.

The green emission in ZnO can be identified as two characteristic emissions, namely high and low energy emissions, respectively. The study of band bending effect of ZnO surface demonstrates that oxygen vacancies cause both the core level and the valence band to shift to higher binding energy. The downward band bending induced by a strong accumulation layer, where the oxygen vacancies act as donors, results in the high energy green emission. ZnO with the low energy green emission has Zn 2p 3/2 core level binding energy shifted to lower binding energy. The depth of dominant oxygen vacancies plays an important role in determining the mechanisms of green emission.