Basic Researches On The Ferroelectrics For The Non-volatile Random Access Memories

Integrated ferroelectrics is an intersecting discipline that deals with applications of ferroelectric thin films into microelectric devices, especially the ferroelectric random access memories (FRAMs). Attributed to the spontaneous polarizations of ferroelectric thin films, such as ABO3 structured PbZr(1-x)TixO3 (0< x<1, PZT) and bismuth layered SrBi2Ta2O9 (SBT), FRAMs are non-volatile, radiation-resistant and interference-free.Thanks to the development of thin film science and technology, low density FRAMs have been integrated using the current Si-based CMOS process. However, high density FRAM integration is underdeveloped due to the difficulties in the ferroelectric thin film growth and characterization, including the low polarization of SBT, high deposition temperature, electrical property dependences on the film texture and size, domains and their relationships with the fatigue behaviors.This dissertation is aimed to deal with the microstructures and electrical properties as well as the relations between them of PZT and SBT thin films, which are currently chosen for the applications in FRAMs. The main results are as following.(1) We modified the SBT ceramics with La3+ doping. The substitution of La3+ for Bi3+ resulted in the lattice deformation and improved the polarizations. However, the excessive La3+ doping led to partially unequivalent substitution for Sr2+, which would release the lattice deformation. Consquently, the electrical properties of La3+ doped SBT (SBLT) fell after the maximum rise at 6 at.% dopant.(2) A sawtooth-like hysteresis loop has been observed on the W6+ doped SBT (SBTW) ceramics. The substitution of W6+ for Ta5+ occurs on the B sites and is unequivalent. Therefore, defect dipoles are produced around the B sites and thought to affect the polarization switching of SBTW. The effect of defect dipoles on the polarization switching is a perturbation. On the assumption of simple harmonic pendulum oscillator for the perturbation of defect-dipole polarization, a new physic concept of relaxation angle was defined to explain the sawtooth-like hysteresis loop dependence on the measurement frequency and electric field.(3) At the understanding of the interaction between the plume and the substrate, the SBT film texture can be controlled by adjusting the deposition conditions of pulsed laser deposition processing. The texture control technique has been successfully applied on the SBLT film deposition.(4) In order to reduce the deposition temperature of PZT and SBT films, a modified chemical solution deposition (CSD) method has been proposed. In the new CSD method, the single precursor originally for the conventional CSD has been divided into two precursors. The final films have been fabricated by alternate deposition of these two precursors. The deposition temperatures have been reduced because the reaction barriers of these two layers are lowered by every very thin layer and ultra-fine grains in it.(5) A compared study has been carried on the PZT and SBT films with respect to their domain evolutions with the switching cycles and concomitant variations in the stress. It was found that domain areas as well as the stress in the PZT film increased with the repeated switching cycles while that in the SBT film did not so. The aggravation of stress in the PZT film resulted in 'pinning' of domains and 'freezing' of perovskite cell, which was thought to be the origin of PZT fatigue. On the other hand, nearly stress free status of the SBT film during the repeated switching cycles led to flexibility of polarization reorientation and then the fatigue free property. This work is aimed to suggest the important role of stress created during repeated switching cycles in the fatigue behaviors.(6) 90° domain structure as well as 90° polarization switching has been discovered in the bismuth-layered SBLT thin films, for the first time. 90° polarization switching was caused by the reduced orthorhombic distortion (b/a - 1), which resulted in an easy α-lb- axis reciprocating. However, the internal stress was created and was found to increase with the repeated switching cycles. Because of the La3+ substitutions for Bi3+, the self-regulation of (Bi2O2)2+ layers was constrained. The accumulated stress led to the fatigue behavior of SBLT thin films. The study results supported the opinion on the important role of stress in the fatigue behavior.(7) With respect to the size dependence of ferroelectric thin films, the emphasis is placed on the coercive field dependence on the film size. A parabolic curve was...