Fast rate oxidation to Cu2O, at room temperature, of metallic copper films produced by the argon-plasma bombardment of CuO films

Abstract

Using a powerful argon plasma equipment, 400 °C sintered CuO thin films were rapidly reduced to metallic copper for treatment times less than 35 s. The transformation caused the crystallographic plane of Cu to appear at the expense of the CuO ones. Likewise, transmittance spectra showed a rapid decline while reflectance spectra exhibited a fast increase towards the infrared region. The last effect was caused by the production of free electrons of metallic copper, which was also evidenced by a low electrical resistivity of 30 μΩ∙cm. The plasma-produced Cu samples displayed a fast oxidation rate at room temperature and atmospheric conditions, which has been rarely mentioned in literature. Interestingly, instead of CuO, the oxidation produced Cu2O films, which is a p-type semiconductor oxide with high potential for several applications. Even though it only requires some hours, the oxidation was allowed to take place for one week. X-ray diffraction, X-ray photoelectron spectroscopy, transmittance-reflectance, and resistivity measurements demonstrated the oxidation of Cu to Cu2O. Similar phenomena were confirmed for lower sintering temperatures of the CuO films. Nevertheless, the oxidation tendency of plasma-obtained Cu films showed a dependence on CuO sintering temperature and on plasma processing time. A stable form of metallic copper, that does not oxidize, was produced at lower sintering temperatures and longer plasma processing times. At other conditions, a metastable form of metallic copper was obtained, which readily transformed to Cu2O. Based on the results, a transformation pathway scheme was elaborated. The scheme can provide with a route to rapidly produce high quality Cu2O films on glass with tunable electrical and optical properties.