Doping Modification Of Bi₄Ti₃O₁₂ Ferroelectric Thin Films Prepared By Sol-gel Method

Rare-earth element doped Bi4Ti3O12 (BTO)-based ferroelectric thin films possess excellent ferroelectric and anti-fatigue properties, showing promising applications in devices such as non-volatile ferroelectric memory etc.. In this thesis, A-site (La, Nd, and Tb) substitution and A/B-sites (Nd/Mn) cosubstitution of BTO-based thin films were prepared by Sol-gel method. The doping effects of element on the structure and electrical properties of the BTO thin film were studied. In addition, the influences of different seeding-layers on the structure and ferroelectric properties of Bi3.25La0.75Ti3O12 (BLT) thin film were investigated. Finally, by using BNT film as a good barrier layer, the BiFeO3/Bi3.15Nd0.85Ti3O12 (BFO/BNT) multilayers films were prepared and the ferroelectric properties of the multilayer films were studied. The main contents of this dissertation are as follows:1. Ferroelectric BLT and BNT thin films were fabricated on Pt/TiO2/SiO2/Si substrates by a modified sol-gel technique. X-ray diffraction indicated that these films were of single phase with random polycrystalline orientations. The surface morphologies of the films were observed by scanning electron microscope, showing uniform, dense films with grain size of 50-100 nm. Well-saturated hysteresis loops of the films were obtained at an applied voltage of 400 kV/cm, giving the remanent polarization (2Pr) and coercive field (2Ec) values of the films of 25.1μC/cm2 and 203 kV/cm for BLT, and 44.2μC/cm2 and 296 kV/cm for BNT, respectively. Moreover, these capacitors did not show fatigue behaviors after up to 1.75×1010 switching cycles at a test frequency of 1 MHz, suggesting a fatigue-free character. The influences of La3+ and Nd3+ doping on the properties of the films were comparatively discussed.2. The high-valence Tb-doped BTO film, Bi4-xTbxTi3O12(BTTx) film, was prepared by sol-gel method on a Pt/TiO2/SiO2/Si substrate for the first time. The BTTX film is of pure phase with a polycrystalline perovskite structure. It displayed a saturated hysteresis loops. The BTTx film exhibited enhanced ferroelectric properties than pure BTO with:(i) a saturated electric hysteresis loop with a large Pr of 60μC/cm2 and a low Ec of 298 kV/cm at Emax～540 kV/cm, (ii) a fatigue-free behavior after being subjected to 1.0×1010 switching cycles. These results suggested that the Tb4+ doping improved apparently the ferroelectric and antifatigue properties of BTO film, which may be attributed to the distorted crystal lattice and possible decreasing number of oxygen vacancies in BTTx thin film.3. Ferroelectric thin films of Nd and Mn co-doped bismuth titanate, Bi3.15Nd0.85Ti3-xMnxO12(BNTM)(x=0,0.01,0.03,0.05, and 0.1), were fabricated on Pt/TiO2/SiO2/Si substrates by a sol-gel technique. The BNTM films had a polycrystalline perovskite structure with uniform and dense surface morphologies. A lattice distortion was observed in the BNTM films due to Mn ion doping. The ferroelectric measurement of the films indicated that the values of coercive field (Ec) decreased gradually with the increase of the Mn content (x), however, the remanent polarization (Pr) increase firstly and then decrease with the increase of x. The sample with x= 0.05 had optimum electrical properties and a maximum 2Pr value. The 2Pr and 2Ec values of the film at a maximum applied electric field of 400kV/cm were 38.3μC/cm2 and 180 kV/cm, respectively. The dielectric constant and dissipation factor were about 290 and 0.08 at 1MHz. Moreover, this BNTM capacitor did not show fatigue behaviors after 1010 switching cycles, suggesting a fatigue-free character.4. Ferroelectric BLT thin films with Bi2O3, TiO2, and BTO as seeding layers, respectively, were fabricated on Pt/TiO2/SiO2/Si substrates by sol-gel method. X-ray diffraction indicated that the different seeding layers had totally different effects on the preferred orientation of BLT films. The surface morphologies of the films with different seeding layers were different, in consistent with the XRD results. The leakage current density of the BLT thin films with TiO2 seeding layer was lower than that of the films with the other seeding layers. Ferroelectric properties analysis showed that the BLT thin films with BTO seeding layer exhibited the largest remnant polarization (2Pr= 62.6μC/cm2). Fatigue tests showed that the BLT films with TiO2 seeding layers had the best anti-fatigue properties.5. The BFO/BNT multilayer films were prepared on Pt/TiO2/SiO2/Si (100) substrates by Sol-gel method. The microstructure, leakage current, ferroelectric and fatigue properties were investigated at room temperature. Relative to the pure BFO thin film, BFO/BNT multilayer film could withstand the higher electric field and be fully polarized due to the lower leakage current, which showed excellent ferroelectricity. The 2Pr value of the BFO/BNT film at an applied electric field of 350kV/cm was 55μC/cm2 and the leakage current density of this film was two orders of magnitude lower than that of the pure BFO thin film at an applied voltage of 6V.