Ion modification for improvement of electrical properties of perovskite-based ferroelectric thin films fabricated by chemical solution deposition method

Ion modification for various perovskite-based ferroelectric thin film using rare-earth cation was attempted for improving the electrical properties. Strategy for controlling the electrical properties is mainly based on two concepts, that is, (i) substituting the volatile cations such as Pb²⁺ and Bi³⁺, and (ii) controlling the crystal anisotropy of perovskite unit cell. In this study, the influences of ion-modification conditions (i.e., amount, species and occupying site of substituent cations) on the electrical properties of perovskitc-bascd ferroelectric films fabricated by a chemical solution deposition were investigated, Substituting volatile cation in simple-perovskite oxides, such Pb²⁺ in Pb(Zr,Ti)O₃ and Bi³⁺ in BiFeO₃, for the rare-earth cations like La ³⁺ and Nd³⁺ reduced the leakage current density of these films due to suppressing the metal and / or oxygen vacancies, as well as in layered-perovskite oxides, such as Bi₄Ti₃O₁₂ films [i.e., strategy (i)]. Also, crystal anisotropy of perovskite-bascd oxides could controlled by varying the species and the occupying site of substituent cations [i.e., strategy (ii)]; for example, the ciystal anisotropy of Pb(Zr,Ti)O₃ lattice was elongated by Ti- and Zr-site (B-site) substitution using rare-earth cations whose ionic radii locate on the smaller part of rare-earth scries (such as Y³⁺, Dy³⁺), that resulted in enhancing the spontaneous polarization from 20 to 25 μC/cm, We concluded that the strategy for controlling the electrical property mentioned in this study would be applicable for a various kind of perovskite-based ferroelectric films.