Despite nearly sixty years of research, several fundamental issues surrounding ZnO remain unresolved. Chief among these have been the difficulty of p-type doping and the role of compensating native defects. Oxygen vacancies (V_O), V_O complexes, Zn interstitial-related complexes, and residual impurities such as hydrogen and aluminum are all believed to be donors in ZnO, while Zn vacancies (V_Zn) and their complexes are considered to be acceptors. Although their impact on carrier compensation is recognized, the physical nature of the donors and acceptors dominating carrier densities in ZnO is unresolved. It remains a challenge to correlate the commonly observed 1.9-2.1 eV “red” and 2.3-2.5 eV “green” luminescence emissions with specific native defects. Previous optical absorption, photoluminescence, electron paramagnetic resonance, and depth-resolved cathodoluminescence spectroscopy (DRCLS) studies indicate a correlation between the “green” optical transition and O vacancies (V_O). Still controversial, however, is how such visible emissions correlate with the energetics of Zn/O vacancies, interstitials, and their complexes overall. This work clearly identifies the physical nature of the defects dominating optical features of this widely studied semiconductor and, in turn, these defects provide a consistent explanation for ZnO’s effective free carrier densities on a local scale.
We have used depth-resolved cathodoluminescence, positron annihilation, and surface photovoltage spectroscopies to determine the energy levels of Zn vacancies and vacancy clusters in bulk ZnO crystals. Here we augment the depth-resolved luminescence of energy level transitions involving native defects with recent positron annihilation spectroscopy (PAS) results to determine the energetics of V_Zn and their complexes in ZnO over both surface and near surface regions (7~1500 nm) in ion (Li or N) implanted and annealed bulk ZnO. The correspondence between these PAS native defect distributions and the DRCLS intensity distributions versus depth permits us to identify the luminescence energy associated with isolated VZn defects (~1.6 eV) as well as the energy shift due to vacancy cluster (1.9~2.1 eV) formation. Surface photovoltage spectroscopy (SPS) yields the positions of these levels with respect to the ZnO band edges. We associate the remaining deep level DRCLS emission (2.3~2.5 eV) with positively charged V_O-related defects, which are not detected by PAS, and describe how the balance between these donor and acceptor defects accounts for depth-dependent resistivity in these irradiated crystals. Taking these depth-resolved techniques altogether, we clearly identify the optical transitions and energies of V_Zn and vacancy clusters, the effects of different annealing methods on their spatial distributions in ion-implanted ZnO, and the contribution of V_Zn and V_O to near-surface resistivity.  L. J. Brillson et al., Appl. Phys. Lett. 90, 102116 (2007).  Y. Dong et al., Phys. Rev. B 81, R081201 (2010).