Nanocomposite Co₃O₄-ZnO Thin Films for Photoconductivity Sensors

Thin nanocomposite films based on zinc oxide (ZnO) added with cobalt oxide (Co₃O₄) were synthesized by solid-phase pyrolysis. According to XRD, the films consist of a ZnO wurtzite phase and a cubic structure of Co₃O₄ spinel. The crystallite sizes in the films increased from 18 nm to 24 nm with growing annealing temperature and Co₃O₄ concentration. Optical and X-ray photoelectron spectroscopy data revealed that enhancing the Co₃O₄ concentration leads to a change in the optical absorption spectrum and the appearance of allowed transitions in the material. Electrophysical measurements showed that Co₃O₄-ZnO films have a resistivity up to 3 × 10⁴ Ohm∙cm and a semiconductor conductivity close to intrinsic. With advancing the Co₃O₄ concentration, the mobility of the charge carriers was found to increase by almost four times. The photosensors based on the 10Co-90Zn film exhibited a maximum normalized photoresponse when exposed to radiation with wavelengths of 400 nm and 660 nm. It was found that the same film has a minimum response time of ca. 26.2 ms upon exposure to radiation of 660 nm wavelength. The photosensors based on the 3Co-97Zn film have a minimum response time of ca. 58.3 ms versus the radiation of 400 nm wavelength. Thus, the Co₃O₄ content was found to be an effective impurity to tune the photosensitivity of radiation sensors based on Co₃O₄-ZnO films in the wavelength range of 400-660 nm.