Effect of annealing temperature on structural, optoelectronic properties and interband transitions of CuCrO₂ nanocrystalline films prepared by the sol-gel method

Transparent conductive CuCrO₂ oxide films were prepared on sapphire substrates by the sol-gel method using copper (II) acetate monohydrate and chromium (III) nitrate hydrate as raw materials. The highly c-axis orientation and optical transparency (60-80%) in the visible region were obtained. The microstructure, vibration modes, optical and electrical transport properties as a function of annealing temperatures have been systematically discussed. With increasing annealing temperature, the crystalline quality and the c-axis orientation of CuCrO₂ films were improved, which induced the increase of Raman phonon amplitude with the decrease of the linewidth. But the film compactness and density decrease with the increase of average particle size. In order to study the intrinsic mechanism of the optical response behavior, a three-phase layered structure (air/film/substrate) was applied to simulate the transmittance spectra of CuCrO₂ films. Three electronic transitions can be uniquely assigned and it is found that the direct transitions are the dominant effect for CuCrO₂ optical properties. The temperature dependence of electrical conductivity was studied and the electrical conduction mechanisms of the thermal activation behavior and the three-dimensional variable range-hopping transport were observed. In addition, the conductivity of CuCrO₂ films decreases with increasing annealing temperature induced by its microstructure and the grain boundary scattering. Thus, the film annealed at 600°C has the largest electrical conductivity of 0.49 S m^-1 at room temperature. The present results could be crucial and provide theoretical support for future applications of p-type semiconductor and optoelectronic devices.