Optical properties of Mn-Co-Ni-O thin films prepared by radio frequency sputtering deposition

Mn1.4Co1.0Ni0.6O4 (MCN) thin films are prepared by RF sputtering deposition method on amorphous Al2O3 substrate. Microstructure and X-ray photoelectron spectroscopy analyses suggest improvements in crystallinity and stoichiometry for MCN films with post-annealed process. Infrared (IR) optical constants of the MCN films are obtained by IR spectroscopic ellipsometer (SE) in the range of 1500 cm−1 to 3200 cm−1 (2.8–6.7 μm). The derived effective charge supports the increase of the oxidation after annealing. The dielectric function of the films is also extracted by SE in the range of 300–1000 nm adopting a double Lorentz model together with a Tauc–Lorentz model. The mechanism in electronic transition process is discussed based on the variation observed in the optical absorption spectra of the as-grown and post-annealed samples. The optical absorption peaks located at 1.7 eV, 2.4–2.6 eV, and 3.5–4 eV are attributed to the charge-transfer transitions of 2p electrons of oxygen ions and 3d electrons of Mn and Co ions. Our results are very important to understand the optoelectronic mechanism and exploit applications of metal oxides