Electronic transitions of the transparent delafossite-type CuGa_1-xCr_xO₂ system: first-principles calculations and temperature-dependent spectral experiments

The structure and optical properties of the CuGa_1-xCr_xO₂ (CGCOx) system with 0 ≤ x ≤ 1 have been investigated by combining theoretical calculations and optical experiments. Density functional theory within the generalized gradient approximation (GGA) was utilized to calculate the electronic structure of the CGCOx system. It reveals strong hybridization between the 3d states of the transition metal ions and 2p states of the O element, which has an important effect on the electronic transitions of CGCOx materials. Moreover, to confirm the theoretical results, CGCOx films with different Cr compositions were deposited via a sol-gel method and the optical properties were measured directly by temperature dependent UV-Vis transmittance and infrared reflectance spectroscopy. The frequency of two acoustic modes (E_u and A_2u) gradually increases, whereas the values of the electronic band gap decrease linearly with increasing Cr composition, which can be attributed to the stronger Cr-O covalent interaction. Remarkably, an additional direct electronic band gap has been observed for the CuGa_0.75Cr_0.25O₂ film, which shows an abnormal behavior in a low temperature region. It can be assigned to the p-d electron hybridization at the top of the valence band. These results show that the first-principles calculations agree well with the experimental data and can be used to explain the microscopic origin of the interband transitions for CGCOx films. The present work further improves the potential applications of delafossite-type oxides in the field of optoelectronic devices.