Thermal oxidation impact on the optoelectronic and hydrogen sensing properties of p-type copper oxide thin films

The paper presents the results of the investigation of the post-process annealing effect on selected properties of copper oxide thin films deposited by magnetron sputtering. It was found that the prepared thin films were nanocrystalline with the crystallite size smaller than 20 nm. Annealing at temperatures of 200 °C, 300 °C and 350 °C caused metallic copper oxidation to copper(I) oxide and then to copper(II) oxide. As-deposited and annealed at 200 °C thin films were composed of Cu and Cu2O. Copper(I) oxide transformation into CuO started at 300 °C, while the thin film annealed at 350 °C consisted of a single CuO phase. Scanning electron microscopy imaging revealed that annealing of the as-deposited thin films caused gradual formation of cupric oxide nanowires. Optimum annealing temperature for nanowires growth was 350 °C. X-ray photoelectron spectroscopy showed that the surface of the as-deposited thin films was oxidized to Cu2O and CuO, while in the case of annealed films it was completely oxidized to CuO. Thermal treatment also contributed to the increase in coating transparency in the infrared range. The transmission coefficient at 1500 nm of the thin film annealed at 350 °C was equal to 68. The resistivity of the as-deposited copper oxide thin film was equal to 1.9•10−3 Ωcm, while the highest resistivity of the annealed thin films was equal to 32 Ωcm for the coating annealed at 200 °C. A further increase in the annealing temperature resulted in a slight decrease in the resistivity value. Based on Seebeck coefficient measurements, it was found that all coatings were characterized by p-type conductivity. Additionally, sensing properties toward hydrogen were examined. The best sensor response exceeding 13 was observed for the thin film annealed at 200 °C.