Formation Mechanism of p-Type Cu₂O Thin Films via Intermediate Cu⁰ Species Derived from Cu(II) Complex of Ethylenediamine-N,N,N′,N′-Tetraacetic Acid

The kinetics of Cu₂O thin film fabrication by the molecular precursor method was investigated by means of X-ray diffraction. The thermal reaction of the precursor film, which consisted of a dibutylammonium salt of a [Cu(edta)]–complex, produced metallic Cu species in Ar gas containing less than 10 ppm of air. The Cu phase appeared gradually, and the amount of the phase could be determined from the area of the (111) peak of Cu. The activation energy of the metallic Cu species formed from the Cu(II) complex (1.5 × 10² kJ mol^–1) was obtained by the Arrhenius plot in the temperature range 230–250 °C. Above this temperature range, the Cu₂O phase was formed by the oxidation reaction of the Cu phase in the Ar gas flow. The amount of the Cu₂O phase could be determined from the area of the (111) peak of the phase. The activation energy of the Cu₂O formation from the Cu phase (1.4 × 10² kJ mol^–1) was obtained by the Arrhenius plot in the temperature range 400–450 °C. The complete removal of nitrogen and carbon atoms from the Cu film was analyzed by Auger electron spectroscopy and X-ray photoelectron spectroscopy spectra. In order to examine the stability of the formed Cu₂O phase, the reaction rate of oxidation from Cu₂O to the CuO phase in the identical atmosphere was also measured in the temperature range 450–475 °C. The activation energy of the oxidation reaction from Cu₂O to the CuO phase was determined to be 1.0 × 10² kJ mol^–1. It was elucidated that the quality of the p-type Cu₂O thin film is strongly dependent on the formation mechanism by comparing the current method with the sol–gel method. The importance of the co-presence of nitrogen and carbon atoms in the intermediate Cu phase is discussed.