Investigation on the optical properties of annealed Mn-Co-Ni-O thin films by spectroscopic ellipsometry

Mn-Co-Ni-O thin films have been widely used for their excellent negative temperature coefficient features, but investigations about tailoring their structural and optical properties via a post-annealing (PA) process are still rare in recent years. In this Letter, we derived the visible to near-infrared optical constants of annealed Mn1.56Co0.96Ni0.48O4 (MCNO) films, and we elucidate the absorption mechanism of MCNO films by investigating the microstructure and cation distribution properties. We found that proper PA temperature will result in improved crystallinity, balanced distribution of Mn3+ and Mn4+ ions, and larger small polaron absorption. We also revealed that it is mainly the small polaron hopping, together with the charge transfer (CT) transition process involving Co2+ and O2− cations, which contributes to the absorption at 750 to 1100 nm. The process of Mn3+ occupying the octahedral sites reduces the concentration of Co2+ and the absorption peak at 1.3 eV. This Letter will improve the mechanism study of small polaron hopping and CT absorption for MCNO films, which is important to the development of optoelectronic devices based on transition metal oxides.