Phase Transition And Domain Switching Observation Of BST Mechanim Of Ferroelectric Thin Films Under Multi-Fields

Current interest in ferroelectric thin film results from the numerous potential applications in micro electronics, micro electromechanical system(MEMS) and information storing that utilize the unique ferroelectric, dielectric, pyroelectric, electro-optic,acousto-optic, and piezoelectric properties of the material. As the ferroelectric thin films structure determines the properties, the ferroelectric domain switching can determine the physical properties and applications of ferroelectric thin films. Ferroelectric domain is one of the key concerns in the research of ferroelectric thin films. Usually, researchers studied the ferroelctric domain from experimental method and theoretical simulation. In experiment,the piezoresponse force microscope(PFM) is one of the most important experimental instruments, which can be used to observe the ferroelectric thin films. Due to the practicability of this method, it has been developed as the main experimental method to study the ferroelectric domain structure and domain evolution. In theoretical simulation, the nonlinear thermodynamic theory is one of the main methods to study the ferroelectric phase transition. The reports of experiment and theoretical simulation had achieved excellent results, but there is still a bottleneck in research. At first, there are rarely reported the influence of the complex field on the ferroelectric thin film phase transition. However, the working environment of the electronic devices is more complex in the actual situation, so we need to establish a multi field model combined with the actual working environment of the device. Second, we always can obtain the polydomain structure within the single grains in the PFM experiment,thus we build a polydomain model. Combining the experimental and theoretical study on the domain transition and phase transition mechanism of ferroelectric thin films. The main contents of this paper are as follows:(1) A nonlinear thermodynamic theory is used to investigate the effect of misfit strain ratio and composition on phase diagrams and physical properties of single-domain(BaxSr1-x)TiO3(BST) thin films epitaxially grown on dissimilar orthorhombic substrates.Through the inner surface of the quantitative calculation excuse strain, we introduced an excuse anisotropic strain factor u(u= S1 / S2), which be called the misfit strain ratio. We obtained the Misfit strain ratio-Temperature phase diagram of BST50 and BST60 thin film.By the phase diagram, the misfit strain ratio can lead to ferroelectric thin film occur ferroelectric phase transition. With the misfit strain ratio becomes positive, it will induce aa tetragonal phase is more stable in phase diagram.(2) The LD-type thermodynamic theory was used to analyze the phase transition and the physical properties of ferroelectric thin films on multi fields. Firstly, we obtain the electric field-misfit srain phase diagram under the action of force and electric field; Secondly, we have calculated the electric field-the phase diagram and the dielectric curve under the action of temperature and electric field. Under the action of force and electric field, there will be a change in the phase diagram. The force field increase can easily occur a1c2 phase in phase diagram, and compressive stress will induce under the new phase paraelectric phase. Under the action of temperature and electric field, we can find that the temperature can greatly affect the regional distribution of the phase diagram. It is easy to appear paraelectric phase under higher temperature.Finally,we study the physical properties of ferroelectric thin film under multi fields.(3) Based on the observation of BST50 under the electric field, we derived the polydomain model of single crystal under electric fields. By calculation, we obtain the electric field-misfit srain phase diagram under electric field. It can be seen from the simulation results that the change of the external electric field and the misfit strain can cause the change of the phase diagram, such as the generation of the new phase or the disappearance of the phase.The theoretical results is agreement with the experiment results. There be those polydomain in the phase diagram:c2/ac/aa-phase, r1/c2-phase and c2/aa-phase, c-phase. The electric field can induce phase transition among those phases. We can obtain three ferroelectric phase transition thresholds:-120 kV/cm,-420 kV/cm and-690 kV/cm. It indicated the polydomain can be better to explain the experiment phenomenon.