Surface chemical bonding states and ferroelectricity of Ce-doped BiFeO₃ thin films prepared by sol–gel process

Bi_1−x Ce _x FeO₃ (x = 0, 0.05, 0.1, 0.15 and 0.20) (BCFO) thin films were deposited on Pt/TiN/Si₃N₄/Si substrates by sol–gel technique. Crystal structures, surface chemical compositions and bonding states of BCFO films were investigated by X-ray diffraction and X-ray photoelectron spectroscopy (XPS), respectively. Compared to BiFeO₃ (BFO) counterparts, the fitted XPS narrow-scan spectra of Bi 4f_7/2, Bi 4f_5/2, Fe 2p_3/2, Fe 2p_1/2 and O 1s peaks for Bi_0.8Ce_0.2FeO₃ film shift towards higher binding energy regions by amounts of 0.33, 0.29, 0.43, 0.58 and 0.49 eV, respectively. Dielectric constants and loss tangents of the BCFO (x = 0, 0.1 and 0.2) film capacitors are 159, 131, 116, 0.048, 0.041 and 0.035 at 1 MHz, respectively. Bi_0.8Ce_0.2FeO₃ film has a higher remnant polarization (P _r = 2.04 μC/cm²) than that of the BFO (P _r = 1.08 μC/cm²) at 388 kV/cm. Leakage current density of the Bi_0.8Ce_0.2FeO₃ capacitor is 1.47 × 10⁻⁴ A/cm² at 388 kV/cm, which is about two orders of magnitude lower than that of the BFO counterpart. Furthermore, Ce cations are feasibly substituted for Bi³⁺ in the Bi_0.8Ce_0.2FeO₃ matrix, possibly resulting in the enhanced ferroelectric properties for the decreased grain sizes and the reduced oxygen vacancies.