Electronic transition and electrical transport properties of delafossite CuCr1−xMg<i>x</i>O2 (0 ≤ <i>x</i> ≤ 12%) films prepared by the sol-gel method: A composition dependence study

Abstract

Highly transparent CuCr1−xMgxO2 (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 CuCr0.94Mg0.06O2 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 CuCr1−xMgxO2 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 CuCrO2 films are promising for optoelectronic device applications.