Phase Field Simulation Of The Impact Mechianism Of Microstructure On Domain Structure Of Ferroelectric

Owing to its excellent properties, nonvolatile ferroelectric memory is honoured as the most promising memory device. However, ferroelectric memory has a series of failure problems to be solved, such as fatigue, imprint and retention loss problems. These failure problems all have relations with the polarization(domain structure) of ferroelectric film. Therefore, study of the main impact factors on domain structrue and their underline mechanism are an indispensible approach to solve the reliability problem of ferroelectric memory. The microstructure, such as interface miasfit strain and dislaocation, have an important impact on domain structure of ferroelectric thin film. Previous studies disclosed that the properties of the ferroelectric domain wall is partly determined by the misfit strain at the interface. However, the intrinsic affect mechanism of misfit strain are still unclear. Therefore, numerical models and simulations that can reveal the intrinsic affect mechanism are still in urgent need. Dislocations, one of the most common defects, are always potential nucleation sites of new domains during polarization switching. There are always large strain gradients around dislocation. The huge strain gradients could couple with polarization via flexoelectric and produce a siginificant flexopolarization, which then affect the domain structure dramatically. The flexoelectric effects are always ignored in existing dislocation phase field models, which lead to underestimation of thr effect of dislocation. Therefore, it is necessary for us to consider the flexoelectric effects. the simulation results is not accurate. The primary researches of this thesis are as follows:(1) A finite element method based phase field model is established. In the proposed model, the misfit strain is applied directly at the interface. The impact of interface misfit strain on the movement, tilt angle, strain gradient and energy of the domain wall have been studied. It is certified that the shape of domain and the properties of domain wall could be modulated by misfit strain at the interface.(2) A dislocation phase field model is established. We have investigated the influence of different flexoelectric coupling coefficient11 f,12f,44 f on the polarization switching around dislocation. The simulated results are compared with the experimental results. The results show that all three flexoelectric coefficients play important role on the polarization around dislocation.(3) The influences of single dislocation on polarization are investigated. The single dislocation local interface of single c domain, inside the c domain, inside a-dmain or c-domain of a/c/a domain structure are considered, respectively. The results indicat that around dislocation, new smal domain will form.(4) The influences of periodic dislocations on polarization and polarization response are investigated. We discuss the effects of periodic dislocation localed at different places on various domain structure. Then the impact of periodic dislocation density on polarization response is discussed. Due to the strong polarization pining around periodic dislocations, the polarization switching becomes difficult and the remnant polarization decreases.