Investigations were made of the absorption and emission properties of the neutral O vacancy in ZnO. Temperature-dependent photoluminescence experiments permitted the determination of the zero-phonon-energy of the emission and its phonon-coupling parameters. The absorption to the singlet excited state was observed in photoluminescence-excitation experiments at energies above 3.1eV. Relaxation drives the system from the singlet diamagnetic excited state to the emissive paramagnetic triplet state. By applying the Mollwo–Ivey relation for the energetic position of the F-center absorption as a function of the anion–cation distance, it was found that the O vacancies in ZnO fitted perfectly into the sequence: MgO, CaO, SrO, BaO.

Oxygen Vacancies in ZnO. Leiter, F., Alves, H., Pfisterer, D., Romanov, N.G., Hofmann, D.M., Meyer, B.K.: Physica B, 2003, 340-342, 201-4