Analysis On Ferroelectric Properties Of Bismuth-layer Ferroelectrics By Phase Field Theory

Much attention has been attached to ferroelectrics in recent years, because the pronounced properties can be used in designing many devices. However, the reliability of the ferroelectrics, such as fatigue and imprint, is an obstacle of the device application. Therefore, it is important to understand the failure mechanism for applying the ferroelectric materials in the memory unit and improving the properties of materials and microdevices. The spontaneous polarization is an important feature of ferroelectrics. Different external conditions as well as the formation and distribution of the defects will impact the capability of switching of the ferroelectric polarization, and the domain switching effect will affect the ferroelectric failure of the integrated system structure and devices. Therefore, it is necessary to research on the domain structure and the domain switching of ferroelectrics under applied conditions so as to explore the failure mechanism of ferroelectric materials and devices. Moreover, phase field simulations were conducted to understand polarization switching under external electric or/and mechanical loading. In this paper, the dynamic process of the polarization switching and the frequency effect on ferroelectric properties are investigated by phase field simulation. The passive layer is considered in the interface between the electrode and ferroelectric thin film, and the imprint failure is discussed by the phase field theory. The main results are given as follow.1. Based on the time-dependent Ginzburg-Landau (TDGL) equation, the temporal evolution of the domain structure and hysteresis loops of polarization versus electric field were simulated by a phase-field model for Bi4Ti3O12 (BIT) ferroelectric single crystal under an applied electric field. In the static electric energy induced by an applied alternating electric field, the frequency effect on the ferroelectric properties of BIT ferroelectrics is investigated. The results show the evolution of ferroelectric domain structure is a gradual process including domain nucleation, domain wall motion, domain growth and domain combination. In the boundary regions of ferroelectric domain, the new domain nucleation occurs and the old domains disappear. The coercive field increases with the field frequency, but there is no obvious change for the remnant polarization. All of these are in good agreement with the previous experiment.2. The process of the polarization switching under an invariable applied stress is analyzed by using of the phase field simulation. The results show that the domain form 90°domain wall instead of 180°domain wall under an applied stress in order to lower the elastic strain energy and then minimize the free energy of the entire system. Hysteresis loops and the butterfly curve in a ferroelectric film subjected to a strain field are simulated using the time- dependent Ginzburg-Landau equation. The results show that both the remnant polarization and strain induced by the polarization field increase with the compressive stress and decrease with the tensile stress. The results are consistent with the experiment data.3. The derivative of polarization with respect to time is described by the TDGL equation and then the hysteresis loop is simulated in ferroelectric layer and passive layer respectively. The results show the hysteresis loop exhibits negative susceptibility regions in the ferroelectric layer. It indicates that due to the existence of the passive layer, the domain is pinning in the interface between the electrode and ferroelectric thin film. Therefore, it is hard to reverse domain unless to apply an excess electric field, which results in the migration of the coercive field, namely result in the imprint failure. The distribution of the electric field in passive layer is different from that in ferroelectric layer due to the different dielectric constant. A positive dc axis is in the positive layer, and the strength of the electric filed in this layer is very large. While a negative dc axis exist in the ferroelectric layer, and the strength of the electric filed is small. Varying the thickness ratio of the passive layer in the entire layer, the larger the thickness ratio is, the more amount of space charges are. Therefore, the larger the internal field is, the more magnitude the horizontal shift. The simulation result is good agreement with the experiment. It indicates that the passive layer is of a low permittivity and weak ferroelectric polarization.