Evolution of CuO thin films through thermal oxidation of Cu films prepared by physical vapour deposition techniques

Abstract

Understanding the conversion process of Cu thin films into copper oxide via thermal oxidation reaction is critical to determine their environmental stability as well as to prepare copper oxide thin films and nanostructures. This study reports the evolution of copper oxides through the thermal oxidation of Cu metal thin films deposited using two different processes i.e., DC sputtering and vacuum evaporation. The sputtered Cu films contain a fine-granular morphology with some porosity while the evaporated Cu films have a compact, coarse-granular surface. The oxidation of deposited Cu films was performed in ambient air at different temperatures ranging from 200 to 400 °C for 2–5 h. The polycrystalline cubic phase of Cu completely converts into a polycrystalline, cubic phase of Cu2O at 300 °C and the complete conversion of Cu2O phase into thermally stable, single-phase monoclinic CuO is achieved at 400 °C. The Cu2O and CuO films having a crystallite size of ∼15 nm exhibit an optical band gap of ∼2.3 and ∼1.8 eV, respectively. It is found that the CuO phase evolves through Cu2O intermediate phase via solid-state-reaction between Cu and oxygen. It is further noticed that the sputtered Cu films with smaller grains oxidize faster than the evaporated films having large-sized grains.