Phase Transition Of Polydomain PZT Ferroelectric Thin Film Under Applied Fields And Its Imprint Failure

In recent years, ferroelectric thin film has become one of the most important hotspots in material research because of the applications in nonvolatile random access memories, tunable microwave devices and micro electromechanical systems. In this paper, we mainly focus our attention on the following two questions in the application of ferroelectric thin film. Firstly, phase transition occurs in ferroelectric thin film under an external field, and the properties of ferroelectric thin film will be changed dramaticlly after the phase transition. Therefore, it is very important to understand the mechanism of the phase transition in ferroelectric thin film under applied fields, and we study this issue theoretically and experimentally. Secondly, the imprint failure is one of the important factors that hamper the extensive application of ferroelectric nonvolatile random access memories. There are a lot of mechanisms to explain the origin of the imprint failure. Based on LK theory, we present a model to investigate the imprint phenomenon in ferroelectric thin films. The main content of this paper are as follows.1. Based on nonlinear thermodynamic theory, we present a model to investigate the effect of applied stress and electric field under fixed misfit strain on phase diagrams of polydomain Pb(Zr1-xTix)O3 (PZT) ferroelectric thin films grown on dissimilar cubic substrates. The"applied stress-applied electric field"phase diagrams are constructed for polydomain PZT ferroelectric thin films with different Ti ingredients (x=0.4, 0.5, 0.6, 0.7, 0.8, and 0.9). The phase diagrams are mainly composed of a-phase, ac-phase and c-phase, and the stable area of each phase is variable with the Ti compositions. The applied stress and electric field may lead to phase transitions in polydomain PZT ferroelectric thin films even at room temperature.2. Based on nonlinear thermodynamic theory, we present a model to investigate the effect of applied electric field and misfit strain under fixed applied stress on phase diagrams and ferroelectric properties of polydomain PZT ferroelectric thin films grown on dissimilar cubic substrates. The"applied electric field-misfit strain"phase diagrams and"polarization components-applied electric field"relation curves are constructed for polydomain PZT ferroelectric thin films with different Ti ingredients (x=0.4, 0.5, 0.6, 0.7, 0.8, and 0.9). The phase diagrams are mainly composed of a-phase, ac-phase and c-phase, and the stable area of each phase is variable with the Ti compositions. The applied electric field and misfit strain may lead to phase transitions in polydomain PZT ferroelectric thin films even at room temperature. The distribution of polarization components is different before and after phase transition, and it is also variable with the Ti compositions from"polarization components- external electric field"relation curves.3. SPM is used to observe the domain structure, the domain switching and phase transition behavior of polydomain Pb(Zr0.52Ti0.48)O3 ferroelectric thin film. An applied electric field between 120 kV/cm-160 kV/cm is needed for the phase transition of a chosen crystalline grain in the ferroelectric thin film. The phase transition is investigated by the nonlinear thermodynamic theory under the same condition as the SPM observation. The result shows that the applied electric field 139 kV/cm is needed for phase transition, and it agrees with the experimental result. The accuracy of the proposed model is verified by the experimental observation, and it provides useful guidelines for future domain observation experiments.4. LK theory is used to describe the derivative of polarization with respect to time of the passive and ferroelectric layers in ferroelectric capacitor. The evolution curves of electric field and polarization in each layer are obtained by using the continuity of current in the ferroelectric capacitor, and then the hysteresis loop under different passive layer thickness of the ferroelectric capacitor is obtain by the evolution curves. The results show that the shift of hysteresis loop increases with the passive layer thickness. We believe that the internal electric field caused by passive layer results in the abnormal electric field in ferroelectric layer and the imprint failure in the ferroelectric capacitor. Comparing with the previous experimental and theoretical results, they are good agreement with the shift tendency and loop shape of the simulated hysteresis loop. It is expected to offer useful guidelines for research on imprint mechanism of ferroelectric thin film and designing ferroelectric field effect transistors with a metal-ferroelectric-metal structure as promising nonvolatile memory devices.