Structural, electronic band transition and optoelectronic properties of delafossite CuGa _1-xCr _xO ₂ (0 ≤ x ≤ 1) solid solution films grown by the sol-gel method

Pure phase CuGa _1-xCr _xO ₂ (0 ≤ x ≤ 1) films were prepared on (001) sapphire substrates by the sol-gel method. The structure, vibration modes, and compositions of the films were analyzed by X-ray diffraction, scanning electron microscopy, Raman spectroscopy, and X-ray photoelectron spectroscopy. It was found that the Cr-substituting induced the increase of the film's roughness, changed the film's internal structure, and made more crystal defects and grain boundaries. Due to the interatomic potential becoming weaker between Cu and O atoms with increasing the Cu-O bond length, the peak positions of the A _1g and A _g phonon modes shifted toward a lower frequency with increasing x. The optical transmittance of the films approached about 60-80% in the visible region and the values of the direct band gap linearly decrease from 3.56 to 3.09 eV with increasing x. The Cr-introduction effects on the electronic band transition have been investigated in detail. The new energy state located at 0.17 eV above the top of the valence band is observed in the CuGa _0.8Cr _0.2O ₂ film, which can be derived from the defect energy level. It can induce the increment of the hole in the valence band, contribute to the electrical conductivity, and lower the thermal activation energy. Moreover, the CuGa _0.8Cr _0.2O ₂ film is found to be of the larger electrical conductivity of 0.071 S cm ^-1 at room temperature, which shows the promising application values, as compared to other CuGa _1-xCr _xO ₂ films.