Modifications of structural and optical properties of copper oxide thin films by thermal annealing

The structural modification of copper oxide thin films was investigated by controlling the thermal annealing atmosphere and temperature, which in turn affected their optical and electrical properties. Copper oxide thin films were deposited by spray pyrolysis deposition at 300^oC to give a uniform surface consisting of submicron-size grains with cubic Cu₂O crystalline structure. As the Cu₂O thin films were thermally annealed at less than 700^oC, they were transformed into the CuO phase. However, a mixed phase of CuO and Cu₂O was observed at temperatures above 800^oC. As the thermal annealing temperature was increased from 400 to 700^oC, the optical bandgap energy of the copper oxide thin films was decreased from 2.54 to 1.91 eV and the electrical charge carrier concentration was decreased gradually due to the improved crystalline quality. In this way, the crystalline structure of the copper oxide and its corresponding optical and electrical properties could be controlled by thermal annealing.