Effect of annealing on conductivity behavior of undoped zinc oxide prepared by rf magnetron sputtering

An undoped ZnO thin film with high resistivity was prepared by rf magnetron sputtering on the quartz substrates using a ZnO (99.999% pure) as target and high pure Ar as sputtering gas. Upon annealing in a temperature range from 513 to 923 K under 10−3 Pa, the conductive properties of the film change from high resistivity, to n-type, then to p-type and finally to n-type with increasing annealing temperature and the p-type ZnO film was fabricated near 863 K reproducibly. Temperature-dependent photoluminescence (PL) of the p-type ZnO shows a dominant PL band at 3.072 eV at low temperature, which is related to Zn vacancy (VZn) acceptor. The intensity of the 3.072 eV band decreases with increasing temperature, implying increment of amount of the hole induced by VZn. X-ray photoelectron spectroscopy (XPS) and room temperature PL measurements indicate that the undoped ZnO is Zn-rich and has oxygen vacancy (Vo) and interstitial Zn (Zni) donor defects. The amount of Vo and Zni donors changes with the annealing temperature. The intrinsic p-type conduction of the undoped ZnO film is ascribed to that the VZn acceptor concentration can compensate Vo and Zni donor concentration.