Optical Transition Levels Related to Oxygen Vacancies in ZnO

Oxygen vacancies are frequently invoked to explain the experimental phenomena observed in ZnO, such as the visible emission, photoconduction, and magnetism. The levels induced by oxygen vacancies play a crucial role in reasonable interpretation on experimental data. Optical transition levels related to oxygen vacancies in ZnO have been reported in several recent papers. For the level positions relative to the band edges, there exists difference between different groups due to various methods adopted, even between the results from the same group. Original data related to oxygen vacancies in ZnO in the literatures are reviewed here. We found that the center of photoluminescence caused by VO depends on the initial states of EM, and 2.6 eV is minimum excitation energy (hν_exc) for the PL centered at 530 nm. The following conclusions are drawn. Oxygen vacancies are neutral states, and the transition energy ε (0/+) lies in the lower half of the band gap. The first ionization energy is 3.03 eV.