Preparation And Properties Of Bi_3.25La_0.75Ti₃O₁₂ Thin Films For Ferroelectric Memory

Ferroelectric memory is considered as a new generation of memories in the future due to its unique properties such as high operation speed, low fabrication cost, low power consume, excellent radiation resistant, and non-volatility etc.. The Bi_3.25La_0.75Ti₃O₁₂ thin film is recently regarded as one of the most promising materials for ferroelectric memory devices because of its excellent ferroelectric properties. In this thesis, the research is primarily focused on the studies of the fabrication and characteristics of Bi_3.25La_0.75Ti₃O₁₂ thin films for nonvolatile memory application. The Bi₄Ti₃O₁₂ and Bi_3.25La_0.75Ti₃O₁₂ thin films are fabricated on the structures of Pt/Ti/SiO₂/Si and p-Si substrates respectively by Sol-Gel method. The microstructures, and ferroelectric properties of samples annealed at different temperatures are systematically investigated. The mechanism of the fatigue improvement of La-doped Bi₄Ti₃O₁₂ ferroelectric thin films is studied by Slow Positron Beam Technique. The main results and conclusions are performed as follows: The mechanism of polarization enhancement in La-doped Bi₄Ti₃O₁₂ thin films is investigated. In Bi_3.25La_0.75Ti₃O₁₂ compound with perovskite-type structure, La³⁺ ions prefer to substitute for Bi³⁺ ions in the unit cell, and the substitution causes the changes of both the crystal and electronic structures, and interaction between ions of Bi₄Ti₃O₁₂. The oxygen vacancies are located in both the (Bi2O2)2+ and the (Bi₂Ti₃O₁₀)2- perovskite layers. The oxygen chemical stability in the perovskite layers plays an important role in the fatigue behavior of the Bi₄Ti₃O₁₂ thin films on the metal electrodes. The oxygen ions bonding with Bi³⁺ ions is more active than those bonding with La³⁺ ions due to the volatility of the Bi³⁺ ions. The substitution improves the fatigue property, because it can enhance the stability of oxygen ions and reduces the number of oxygen vacancies in the unit cell. The covalent interaction, which originates from the strong hybridization between Ti 3d and O 2p electronic orbitals, is critical to the remanent polarization enhancement of Bi₄Ti₃O₁₂ thin films. The substitution causes less distortion of the structure, relaxes internal stresses and increases the symmetry of TiO6 octahedron. The polarization along a axis direction decreases, meanwhile the polarization along c axis increases, which shows that the polarization tilt towards the c axis. The Bi₄Ti₃O₁₂ and Bi_3.25La_0.75Ti₃O₁₂ thin films are synthesized on p-Si and Pt/Ti/SiO₂/p-Si substrates by Sol-Gel method. The effects of the anneal temperature on the crystalline phase and surface microstructure are investigated and the optimum technology parameters are determined. The results of XRD measurements exhibite that a little amount of La substitution doesn't change the crystal structures of Bi₄Ti₃O₁₂ thin films, and there is no pyrochlore phase or other second phase found in the films. The surface morphology of the films is examined to be smooth, flaws and cracks free by AFM and SEM. The results indicate that the high quality thin films are obtained repeatedly by the Sol-Gel method.For the thin films of the Pt/Bi_3.25La_0.75Ti₃O₁₂/Pt structures, the relationships among the hysteresis loops, leakage current and annealing temperatures (550～750℃) are carried out. The results show that the symmetry of hysteresis loops is well-developed. When the test voltage is 6 V, the remanent polarization (Pr) increase and the coercive voltage (Vc) decrease with the annealing temperature increasing. When the films are annealed at 750 oC, the 2Pr and 2Vc are 20.0μC/cm² and 2.7 V respectively. The insulation of the films is excellent according to the leakage current test. The leakage current densities under the test voltage 6 V are less 1.3×10-7 A/cm². The results indicated that the Bi_3.25La_0.75Ti₃O₁₂ films can be applied in the nonvolatile memories.The structures of Pt/Bi_3.25La_0.75Ti₃O₁₂/Pt and Pt/Bi₄Ti₃O₁₂/Pt are fabricated. The La doping effects on the properties of Bi₄Ti₃O₁₂ films are studied. The 2Pr with test voltage 6V for the sample annealed at 700 oC increased from 12.5μC/cm² to 18.6μC/cm², and 2Vc is still 2.8 V, when the La³⁺ ions occupied a part of Bi³⁺ sites. The Bi_3.25La_0.75Ti₃O₁₂ films showed fatigue-free behavior.The mechanism of fatigue enhancement due to La doping is studied using slow positron beam technique. The oxygen vacancies in the films with the La substitution are investigated. A decrease in the S parameter with La content increasing is observed, the results can be inferred as that the vacancy-related defects concentration decrease, and the oxygen chemical stability in the perovskite layers is enhanced. The La substitution can enhance the binding energy of oxygen in the perovskite layer and induce the Bi_3.25La_0.75Ti₃O₁₂ films to exhibit good fatigue properties. This mechanism is helpful the design of the new type Bi-layered perovskite structure ferroelectric films.