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.