Ferroelectric And Optical Properties Of Doped Bismuth Titanate Thin Films

In a ferroelectric random access memory (FeRAM), information is stored by the polarization direction in a ferroelectric film and the stored datum is read out using the polarization reversal current. Thus, the following characteristics are desired for a ferroelectric thin film. The remanent polarization should be large, so that a large polarization reversal current can be derived from a small-area capacitor. The coercive field should be low for low-voltage operation of the FeRAM. The fatigue and imprint should be as small as possible. Fatigue describes the fact the remanent polarization becomes small when experiencing a large number of polarization reversals, while imprint characterizes the asymmetric response to opposite-polarity applied voltages.So far, many ferroelectric materials have been investigated under the above conditions, and at present the following three materials are known to be most important for the fabrication of FeRAM: PbZr_xTi_1-xO₃ (PZT), SrBi₂Ta₂O₉ (SBT) and (Bi,La)₄Ti₃O₁₂ (BLT). PZT has large Pr value, low crystallization temperature, while the largest problems were fatigue, imprint and containing Pb. The advantages of SBT are that the fatigue, imprint and retention characteristics are superior to those of PZT, while its crystallization temperature is high and Pr is small. BLT (La doped Bi₄Ti₃O₁₂ (BTO)) attributes much attention in FeRAM due to low crystallization temperature and fatigue free. In order to enhance the performance of BTO films, doped BTO films have been investigated in many elements.In this thesis, the following several aspects were studied.1. LaNiO₃ (LNO) thin films are compatible with most ferroelectric materials due to their similar perovskite structures and lattice constants. On the other hand, LNO bottom electrodes can improve the fatigue characteristics of ferroeletric films. In this thesis, based on the data of thermogravitivity (TG), by controlling pyrolysis temperature, (200)- and (110)-oriented LNO thin films were respectively obtained on Si(100) substrates by sol-gel method. It was also found that a low pyrolysis temperature favors the growth of (110)-oriented LNO grains, while a high pyrolysis temperature results in the nucleation of (200)-LNO grains. Meanwhile, at a fixed pyrolysis temperature, the effect of baking temperature on the orientation of LNO films is little.2. Different preferential orientation of BLT thin films have been prepared on (200)- and (110)-LNO bottom electrodes, and the ferroelectric properties of BLT films were investigated. It was found that the remnant polarization increases and leakage current density decreases in c-oriented BLT, while the coercive field varies little in BLT films.3. By doping indium in BTO, the optical properties of undoped and iniudm doped BTO are investigated by UV-Vis-NIR spectrophotometer. It was found that the transmittance in the Vis-NIR region increases, the refractive index n and the extinction coefficient k decrease by incorporating indium. Following the single electronic oscillator model, the single oscillator energy E₀, the dispersion energy E_d, the long wavelength refractive index n_∞, the average interband oscillator wavelengthλ₀, the average oscillator strength S₀, the refractive index dispersion parameter E₀/S₀, and the chemical bonding quantityβwere obtained and analyzed.