Synthesis And Optoelectronic Properties Of Doped Na_0.5Bi_0.5TiO₃Ferroelectric Films

Sodium-bismuth titanate Na0.5Bi0.5TiO3(NBT) was first reported by Smolensky in1960. Sodium bismuth titanate is a ferroelectric complex perovskite-structure compound with two different ions at the A site of the ABO3structure. Pure NBT is a ferroelectric material with a relatively high Curie temperature of TC-320℃, a relatively large remanent polarization of Pr=38μC/cm2, and a coercive field of Ec=73kV/cm at room temperature. It was considered to be one of the most promising candidates for lead-free ferroelectric and piezoelectric devices. However, pure NBT shows low piezoelectric coefficients and electric-field-induced strain. In order to improve this situation and make it a more favorable substitute, chemical substitution/doping can be used to improve the piezoelectric and ferroelectric properties. Besides, a reduced leakage current and enhanced ferroelectric properties have been obtained in a (Ce, Fe)-codoped NBT film. Therefore, it is necessary to further investigate the growth, optoelectronic and electrical properties of (Na0.5Bi0.5)1-xCex(Ti0.99Fe0.01)O3(NBCTFx;0≤x≤0.10) films to emplore some promising devices.In this work,(Ce, Fe)-codoped NBT films (NBCTFx;0<0.10) were prepared by sol-gel method and spin coat technology for its low cost and simplicity. The microstructure, surface morphology, optoelectric properties at room temperature and the optical properties and phase transition properties at various temperatures have been investigated by X-ray diffraction, atomic force microscopy, scanning electron microscopy, X-ray photoelectron spectroscopy, spectroscopic ellipsometry, ferroelectric test system and electrometer. The main works and innovations of this dissertation are listed in details as following:(1) The crystalline structure, surface morphology and element composition of NBCTFx films which were prepared by sol-gel technique have been investigated by X-ray diffraction, atomic force microscope, scanning electron microscopy and X-ray photoelectron spectroscopy. From the X-ray diffraction spectra can be known that all the films are polycrystalline without impurity phases. The grain size decreases with increasing cerium composition. From the AFM and SEM images can be learned that the surface and cross-sectional morphology were dense and uniform, they presented a similar pattern. Elemental compositions and chemical states of the NBCTFx nanocrystalline films were investigated by XPS measurements.(2) The optical properities of NBCTFx films at room temperature were investigated by Spectroscopic Ellipsometry (SE). The SE spectra were studied with a multilayer model with the four-phase-layered structure (air/surface rough layer (SRL)/NBCTFx/Pt) to obtained the dielectric functions and optical constants of the NBCTFx films deposited on the (111) Pt/TiO2/SiO2/Si substrates. The results indicated that the optical band gap of the films decreases with increasing cerium composition.(3) The electric properties of films were characterized by a ferroelectric test system and an electrometer. The relationship between the ferroelectric properties, leakage current and the doped cerium concentration were obtained after analyzing the polarization-electric field (P-E) hysteresis loops and the current-voltage characteristic (I-V) of the films. The leakage current mechanism was discussed in detail. The results showed that the leakage current density of the films decreases with increasing cerium composition, and the NBCTF0.10film performed the best in ferroelectric properties.(4) The optical properties and phase transition of NBCTF0.08and NBCTF0.10films at various temperatures were investigated by Spectroscopic Ellipsometry. Temperature dependent dielectric functions near infrared ultraviolet range (0.6-6.4eV) can be derived by the Tauc-Lorentz model. The results indicated that both the real part of dielectric function (ε1) and optical band gap (Eg) showed an abrupt variation near200and340℃, respectively, which indicates abnormal variation of the electronic structure near the phase transition region. This phenomenon could be explained by the appearance of reversal of polarization.