We have investigated the microstructure and photoluminescent properties of Zn1-xCoxO (x <= 0.377) films deposited on (0 0 1)-oriented Al2O3 substrates using radio-frequency magnetron sputtering. A single phase with a wurtzite-like structure was found in Zn1-xCoxO films with x up to 0.105. However, a small amount of Co3O4 clusters could be observed when the doping level x reached 0.313. The PL spectra showed that Co doping in ZnO films would suppress near band edge (NBE) and violet emissions due to the degradation of material quality and the increase of nonradiative centers. For Zn1-xCoxO films with higher Co concentration (x = 0.313 and 0.377, respectively), a broad intense emission band centered at 332 nm (deep ultraviolet emission) was observed. The intense deep ultraviolet emission should be related to the O2- -> Co2+ charge-transfer process in clusters Co3O4 embedded in Zn1-xCoxO films. (C) 2009 Elsevier B.V. All rights reserved.