Electronic transition and electrical transport properties of delafossite CuCr_1-xMg_xO₂ (0 ≤ x ≤ 12%) films prepared by the sol-gel method: A composition dependence study

Highly transparent CuCr_1-xMg_xO₂ (0 ≤ x ≤ 12%) films were prepared on (001) sapphire substrates by sol-gel method. The microstructure, phonon modes, optical band gap, and electrical transport properties have been systematically discussed. It was found that Mg-doping improved the crystal quality and enhanced the (00l) preferred orientation. The spectral transmittance of films approaches about 70%-75% in the visible-near-infrared wavelength region. With increasing Mg-composition, the optical band gap first declines and climbs up due to the band gap renormalization and Burstein-Moss effect. The direct and indirect band gaps of CuCr_0.94Mg_0.06O₂ film are 3.00 and 2.56 eV, respectively. In addition, it shows a crossover from the thermal activation behavior to that of three-dimensional variable range hopping from temperature-dependent electrical conductivity. The crossover temperature decreases with increasing Mg-doping composition, which can be ascribed to the change of spin-charge coupling between the hole and the local spin at Cr site. It should be noted that the electrical conductivity of CuCr_{1- x}Mg_xO₂ films becomes larger with increasing x value. The highest electrical conductivity of 3.85 S cm^-1 at room temperature for x = 12% is four-order magnitude larger than that (8.81 × 10^-4 S cm^-1) for pure CuCrO₂ film. The high spectral transmittance and larger conductivity indicate that Mg-doped CuCrO ₂ films are promising for optoelectronic device applications.