High-quality Zn1-xCoxO (ZnCoO) thin films with different Co concentrations (x=0-0.07) were synthesized on Al2O3 substrates using a radio-frequency magnetron sputtering process. The structural, chemical, optical, and electrical properties of the films were characterized using various techniques. High-resolution X-ray diffraction measurements showed that the ZnCoO films had a c-oriented hexagonal wurtzite structure with no extra phases. The lattice constants of the films increased slightly with increasing the Co concentration ratio. X-ray photoelectron spectra show the incorporation of Co2+ ions in the ZnO host without the formations of Co clusters. Hall measurements revealed that the films were n-type with a charge carrier density of similar to 2.6 x 10(18) per cm(3). A neutral-donor bound exciton peak ((DX)-X-0) from all specimens was observed at 3.36 eV using photoluminescence measurements at 10 K. The physical and chemical properties of the films suggested that their ferromagnetic properties originated from a direct exchange interaction between magnetic Co ions. (C) 2012 Elsevier B.V. All rights reserved.