In contrast, when doped with Zn 3 X 2 -type dopants the luminescence spectra change dramatically and exhibit only a single symmetrical violet-blue peak (Fig. ) with the maximum between 410 and nm depending on the sample. Such luminescence in ZnO was previously attributed to Zn i defects that are formed in the presence of large quantities of Zn and seem to be the only type of defect presented in such samples. Moreover, the shape of the spectra is close to the one recorded for undoped ZnO melt-grown single crystals 27 though the peak maximum is shifted in the blue region.