Composition Dependence of Optical Properties and Band Structures in p-Type Ni-Doped CuO Films: Spectroscopic Experiment and First-Principles Calculation

Despite much attention on the photoelectronic device applications of CuO-based materials, a thorough analysis of optical properties and electronic band structure of Ni-doped CuO films is still necessary. Here, the calculation based on the density functional theory revealed a strong hybridization of O 2p and Cu 3d orbits near the conduction band minimum (CBM) and valence band maximum (VBM) of CuO films. The Ni addition is found to enhance the carrier mobility, because the weaker localization of O 2p states at the VBM is observed in 50 atom % doped CuO. To confirm the theoretical results, the ellipsometric spectra of solution-processed CuO films doped by Ni ions (from 0 to 50 atom %) were fitted, and the optical constants were uniquely extracted. The optical conductivity has a linear increase with the Ni doping concentration, which results from the decreased electron traps. Besides, the band gap was found to be modulated in a range of 2.22–2.37 eV owing to the quantum confinement effects. The variation trend is confirmed by the first-principles calculation, where the computational indirect band gap is 1.27 and 1.79 eV for pure CuO and 50 atom % Ni-doped CuO. Four electronic transitions are observed at ∼2.75, 3.27, 4.01, and 4.90 eV, and the physical origins have been discussed.