Zn interstitials and O vacancies responsible for n-type ZnO: what do the emission spectra reveal?

Zinc interstitials and their complexes with oxygen vacancies, harbouring a lone pair of electrons, have been discussed as suspects for an n-type character and the ensuing conductivity of nominally undoped ZnO. ZnO nanorods, with different Zn concentrations, have been studied using photoluminescence and photoluminescence excitation spectroscopy to evidence the presence of defect states, their correlation with excess Zn and their viability as donors. It is shown that each sample may emit intense violet-blue, green-orange and red emissions, as decided by the excitation energy and presence of excess Zn. Intensity variation of these individual emissions vis-a-vis their excitation energies, in the range 4.0-2.7 eV, indicates the presence of defect states not only within the band gap but crucially within the conduction band itself, but only in a Zn rich environment. The study provides the experimental signature of the theoretical predictions of the latter states and demonstrates why Zn interstitials may be responsible for the n-type doping and conductivity of ZnO. The origin of the emissions, their interdependence and tunability are further discussed.