Influences of post-annealing conditions on the formation of delafossite–CuFeO2 thin films

In this study, post-annealing conditions, the partial oxygen pressures (pO2) and temperatures, influence on the formation of delafossite–CuFeO2 thin films is studied. The sol–gel derived films were annealed at 500 °C in air and post-annealed at 500–850 °C in pO2 = 10−2 to pO2 = 5 × 10−5 atm. The CuO and CuFe2O4 phases appeared when the sol–gel derived films were post-annealed below 800 °C for 2 h in pO2 = 10−2 atm, 650 °C for 2 h in pO2 = 10−3 atm, and 550 °C for 2 h in pO2 = 5 × 10−5 atm. Pure delafossite–CuFeO2 phase was detected as specimens were post-annealed above 800 °C for 12 h in pO2 = 10−2 atm, 650 °C for 12 h in pO2 = 10−3 atm, and 550 °C for 12 h in pO2 = 5 × 10−5 atm. The surface of post-annealed thin films exhibited a nanoparticle-like morphology when the specimens exhibited CuO and CuFe2O4 phases. However, the surface revealed granular features caused by the formation of the delafossite–CuFeO2 phase. The formation of the delafossite–CuFeO2 phase, which resulted from the chemical reaction of the CuO and CuFe2O4 phases in the post-annealing process, is consistent with thermodynamics. The optical bandgaps of delafossite–CuFeO2 thin films prepared using post-annealing ranged between 3.1 and 3.2 eV. The electrical conductivities of delafossite–CuFeO2 thin films were (1.62–6.37) × 10−1 S cm−1 and the carrier concentrations were (1.52–8.84) × 1017 cm−3. The pO2 and temperatures in the post-annealing process played primary roles in the formation of delafossite–CuFeO2 thin films in this study.