Characterization and optoelectronic properties of sol-gel-derived CuFeO2 thin films

In this study, CuFeO2 thin films were deposited onto quartz substrates using a sol-gel and a two-step annealing process. The sol-gel-derived films were annealed at 500 degrees C for 1 h in air and then annealed at 600 to 800 degrees C for 2 h in N-2. X-ray diffraction patterns showed that the annealed sol-gel-derived films were CuO and CuFe2O4 phases in air annealing. When the films were annealed at 600 degrees C in N-2, an additional CuFeO2 phase was detected. As the annealing temperature increased above 650 degrees C in N-2, a single CuFeO2 phase was obtained. The binding energies of Cu-2p(3/2), Fe-2p(3/2), and 0-1s were 932.5 +/- 0.1 eV, 710.3 +/- 0.2 eV and 530.0 +/- 0.1 eV for CuFeO2 thin films. The chemical composition of CuFeO2 thin films was close to its stoichiometry, which was determined by X-ray photoelectron spectroscopy. Thermodynamic calculations can explain the formation of the CuFeO2 phase in this study. The optical bandgap of the CuFeO2 thin films was 3.05 eV, which is invariant with the annealing temperature in N-2. The p-type characteristics of CuFeO2 thin films were confirmed by positive Hall coefficients and Seebeck coefficients. The electrical conductivities of CuFeO2 thin films were 0.28 S cm(-1) and 0.36 S cm(-1) during annealing at 650 degrees C and 700 degrees C, respectively, in N-2. The corresponding carrier concentrations were 1.2 x 10(18) cm(-3) (650 degrees C) and 5.3 x 10(18) cm(-3) (700 degrees C). The activation energies for hole conduction were 140 meV (650 degrees C) and 110 meV (700 degrees C). These results demonstrate that sol-gel processing is a feasible preparation method for delafossite CuFeO2 thin films. (C) 2012 Elsevier B.V. All rights reserved.