First-principles characterization of native-defect-related optical transitions in ZnO

We investigate the electrical and optical properties of oxygen vacancies (VO), zinc vacancies (V_Zn), hydrogenated V_Zn, and isolated dangling bonds in ZnO using hybrid functional calculations. While the formation energy of V_O is high in n-type ZnO, indicating that this center is unlikely to form, our results for optical absorption signals associated with V_O are consistent with those observed in irradiated samples, and give rise to emission with a peak at less than 1 eV. Under realistic growth conditions, we find that VZn is the lowest-energy native defect in n-type ZnO, acting as an acceptor that is likely to compensate donor doping. Turning to optical transitions, we first examine N_O as a case study, since N-related transitions have been identified in experiments on ZnO. Here, we also examine how hydrogen, often unintentionally present in ZnO, forms stable complexes with V_Zn and modifies its optical properties. Compared with isolated V_Zn, V_Zn-H complexes have charge-state transition levels lower in the band gap as well as have lower formation energies. These complexes also lead to characteristic vibrational frequencies which compare favorably with experiment. Oxygen dangling bonds show behavior mostly consistent with V_Zn, while zinc dangling bonds give rise to transition levels near the ZnO conduction-band minimum and emission peaking near 2.4 eV. Lastly, we discuss our results in view of the available experimental literature.