Rf Power dependence on the chemical and structural properties of copper oxide thin films obtained at various oxygen fractions

Copper oxide (CuO _x ) thin films were deposited on flexible polyethyleneterephthalate (PET) substrates by rf magnetron sputtering and the effects of the rf power and the O ₂ fraction on the physical and chemical properties of the films were examined. The obtained films were characterized with a surface profiler, X-ray diffraction (XRD), scanning electron microscopy (SEM), four points probe, and X-ray photoelectron spectroscopy (XPS). The trend of surface resistance of CuO _x films was almost identical as rf power varied from 200 to 300 W. The surface resistance of the films increased as the O ₂ fraction increased up to 14.3%. After this point it decreased. The propensity of surface resistance of the films, however, monotonically increased as a function of O ₂ fraction at 400 W of rf power. XRD study revealed that the preferential phase of CuO _x changed from metallic cubic Cu(1 1 1) through cubic Cu ₂ O(1 1 1) to monoclinic CuO(1̄11) as the O ₂ fraction increased. The compositional ratio of Cu species with different oxidation state in CuO _x films was investigated by X-ray induced Auger electron spectroscopy (XAES). XAES study revealed that the compositional ratio of Cu was dependent with the rf power. The higher rf power applied, the more Cu ⁺ species and less Cu ²⁺ species formed in CuO _x films.