Cobalt doping effects on structures and electrical properties of lead-free ferroelectric K_0.5Na_0.5NbO₃ films

Lead-free ferroelectric K_0.5Na_0.5NbO₃ (KNN) films doped with different amounts of cobalt (Co) were prepared by using a chemical solution deposition method. The amounts of Co were ranged from 0 mol% to 3.0 mol% and the thicknesses of the films were all ∼1.6 μm. All the KNN films showed single perovskite phase, smooth surface and dense micro-structures. The average grain sizes of the Co doped KNN films were smaller than that of the un-doped KNN film, indicating a poor crystallinity after Co doping. However, the leakage current densities of the KNN films were obviously decreased after doping. As a result, the Co doped KNN films showed improved electrical properties. The lowest leakage current density of ∼10 ^-7 A/cm² at 100 kV/cm and the best electrical properties were observed in the 2.0 mol% Co doped KNN film. For all KNN films, the I-V characteristic is ohmic at low electric fields and space-charge-limited at high electric fields. The XPS analyses suggested that the Co ions entered the lattice of KNN structure and that Co doping promoted the formation of KNN perovskite phase.