Nature of red luminescence is less studied in the literature, according to 36 it is probably due to defect complexes V O -Zn i and V Zn V O -both seem to be possible given the availability of excess Zn, which could be included in interstices and the presence of V Zn suggested by both photoluminescence and EPR as described later. Additionally, there was no significant difference found in luminescence spectra recorded from only one single crystal and a collective of randomly selected crystals. The respective intensities of the bands and their maxima positions change slightly suggesting a certain level of inhomogeneity but the shape of the line remains similar. Doped ZnO single crystals generally exhibit a very high intensity of photoluminescence. Near band edge (NBE) luminescence can be observed at room temperature (Fig. , inset). The peak at 380 nm corresponds to the recombination of excitons; the peak at 393 nm can be attributed to the recombination of donor-acceptor pairs . This confirms formation of acceptor centers in doped samples.