Optoelectronic and ferroelectric properties of cerium-doped (Na _0.5Bi_0.5)(Ti_0.99Fe_0.01)O ₃ nanocrystalline films on (111) Pt/TiO₂/SiO ₂/Si: A composition-dependent study

The optical and ferroelectric properties of (Na_0.5Bi _0.5)_1-xCe_x(Ti_0.99Fe _0.01)O₃ (NBCTFx; 0 ≤ x ≤ 0.10) nanocrystalline films deposited on platinized silicon (Pt/TiO₂/SiO₂/Si) substrates using a sol-gel method were investigated. The microstructure, surface, and cross-sectional morphology and compositions of the films were analyzed by X-ray diffraction, scanning electron microscopy, and X-ray photoelectron spectroscopy, respectively. The X-ray diffraction patterns indicate that all films are polycrystalline and show the single perovskite structure. The dielectric functions of the NBCTFx films can be uniquely extracted by fitting the measured ellipsometric spectra with a four-phase-layered model (air/surface rough layer/NBCTFx/Pt) in the photon energy range of 0.6-6.4 eV. The Tauc-Lorentz model was successfully applied and reasonably describes the spectral response behavior of ferroelectric NBCTFx films in the light-frequency region. It was found that the optical band gap and grain size decrease with increasing cerium composition because of the introduction of disorder and defects. The electrical results show that the leakage current density of the films was decreased with increasing cerium composition by reducing the density of oxygen vacancies and forming the defect complexes. The optimal ferroelectric properties were obtained in the film doped with x = 0.10, whose remnant polarization and coercive field values are 14.9 μC/cm² and 217.3 kV/cm, respectively. The present results could be crucial for future applications of lead-free ferroelectric and optoelectronic devices.