Temperature dependent phonon evolutions and optical properties of highly c-axis oriented CuGaO₂ semiconductor films grown by the sol-gel method

Transparent conductive CuGaO₂ oxide films were prepared on sapphire substrates by the sol-gel method. The highly c-axis orientation and optical transparency (60-80) in the visible region were obtained. It indicates that the A_1g phonon mode shifts about 20 cm^-1 with the temperature due to the thermal expansion of the lattice and anharmonic phonon coupling. Moreover, temperature-dependent dielectric function has been investigated and three electronic transitions located at about 1.05, 2.67, and 3.99 eV can be uniquely assigned. It was found that the optical band gap of the CuGaO₂ film decreases with the temperature, which mainly originated from the electron-phonon interactions.