Preparation Of BDT And BYT Ferroelectric Thin Films Using For FeFET And Study Of Their Properties

With the development of modern science and technology, the ever-increasing demands for nonvolatile memories such as ferroelectric field effect transistor (FeFET) memory, have led to the intensive search for a new kind of material. At present the materials used to prepare ferroelectric thin films for FeFET memory application are mainly PZT series, they have some favorable ferroelectric properties, such as large remnant polarization (2Pr) value and low processing temperature, and some serious drawbacks such as poor anti-fatigue performance and toxicity. A new kind of ferroelectric material needs to be found to replace PZT. Rare-earth doped bismuth titanate (Bi₄Ti₃O₁₂, BIT) thin film has some favorable properties such as large remanent polarization, low crystallization temperature, and high fatigue endurance. It may be used for FeFET memory appliction. In this master thesis, Dy-doped Bi₄Ti₃O₁₂ (Bi_3.4Dy_0.6Ti₃O₁₂, BDT) and Yb-doped Bi₄Ti₃O₁₂ (Bi_3.4Dy_0.6Ti₃O₁₂, BYT) ferroelectric thin films were deposited on Pt(111)/Ti/SiO₂/Si(100) substrates by a Sol-Gel method, respectively. And their properties were studied in detail.In chapter 1 of the master thesis, the general situation and potential application of ferroelectric thin film, preparation methods and the research status for ferroelectric thin film are reviewed. On the basis of the review, the selectness of materials, aim and content of this thesis are proposed. In chapter 2, the materials and method of our experiment are introduced, and some influential factors on the experiment are also analyzed.In chapter 3 and chapter 4, the effect of crystallization atmosphere and crystallization temperature on the microstructure, ferroelectrical and electrical properties of BDT films was studied. The crystallization atmosphere was found to be important in determining the crystallization and ferroelectric properties of the BDT films. The film crystallized in nitrogen at a relatively low temperature of 650°C exhibits excellent crystallinity and ferroelectricity and a fatigue-free characteristic. While the films annealed in air and oxygen did not show good crystallinity and ferroelectricity until they were annealed at 700°C. These results indicated that the crystallization temperature is lowered significantly when the BDT films were annealed in N₂, compared with those annealed in air or O₂. Moreover, the remnant polarization of the BDT films crystallized in N₂, O₂, and air atmosphere, respectively, are not a monotonous function of the crystalline temperature, but decrease as the annealing temperature increases after an optimal crystalline temperature reaches for each case.In chapter 5, the effect of the content of Dy on BDT thin films was studied and the conclusion was that Bi_3.4Dy_0.6Ti₃O₁₂ thin film has the best ferroelectric properties. For the reason of this result, we propose one important reason is that Dy³⁺ ions substitute Bi³⁺ ions located in different site. When dysprosium content ranges in a suitable range, Dy³⁺ ions prefer to substitute Bi³⁺ ions located in Bi₂O₂ layer, which will decrease the symmetry of Bi-layered perovskite structure and improve the spontaneous polarization of BDT thin films, but excess dysprosium content can lead to Dy³⁺ ions substituting Bi³⁺ ions located at the Bi₂Ti₃O₁₀ perovskite site, which will destroy Bi₂Ti₃O₁₀ perovskite structure, and lead to the reduction of spontaneous polarization.In chapter 6, the process of the preparation of BYT thin films was introduced. Ferroelectric Bi_3.4Yb_0.6Ti₃O₁₂ (BYT) thin films were successfully deposited on Pt(111)/Ti/SiO₂/Si(100) substrates by Sol-Gel for the first time in the world, to our best knowledge, no report on BYT has been found so far. Ferroelectric properties of the as-prepared thin films annealed under different temperatures (600°C-750°C) were studied in detail.