A typical PL spectrum of ZnO single crystals doped with group-V elements or their oxides is shown in Fig. . The spectrum contains a very broad band of defect-associated luminescence that extends from about 400 nm to about 800 nm. The band has a discernible "shoulder" in the high-energy range, an absolute maximum around 600 nm and a long "tail" in the low-energy range. While the procedure of Gaussian deconvolution of PL bands is not always correct due to asymmetric nature of individual bands 33 , in our case it is justified since the band is clearly composed of at least three bands of substantially different origin.