Structural, electronic band transition and optoelectronic properties of delafossite CuGa1−xCrxO2 (0 ≤ x ≤ 1) solid solution films grown by the sol–gel method

Abstract

Pure phase CuGa_1−xCr_xO₂ (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_0.2O₂ 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_0.2O₂ 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_xO₂ films.