Study Of The Thermodynamic Properties In Graded Ferroelectric Films

In recent years, functionally graded ferroelectric films have received considerable attention as they exhibit some new behavior and properties that are not observed in homogeneous bulk or thin film ferroelectrics. It has been possible to form graded ferroelectrics from a variety of material systems, by grading the composition of the ferroelectrics, by imposing a temperature gradient normal to the electrode surfaces, and by imposing stress gradient on the material normal to their electrode surfaces. Many experiments show that one of the most notable properties of these functionally graded ferroelectrics is the offset in hysteresis loop along the polarization axis when excited with an alternating electric field. Furthermore, the position and direction of the offsets are found to be determined by the polariztion gradient, the temperature, the strain and the magnitude of the periodic excitation field. Besides, functionally graded ferroelectrics display improved dielectrc properties and tunability, as well as giant pyroelectric coefficient. Therefore, the graded structures have potential applications in infrared detection, actuation, sensors, energy storage, and tunable microwave components, such as tunable filters, tunable oscillators, and phase shifters. The study of the thermodynamic properties in graded ferroelectric films is not only of theoretical importance but also valuable for the technological application in material physics, condensed matter physics and microelectronics . The purpose of our work is to study the polarization, dielectric and pyroelectric properties of graded ferroelectric films. The main results of our study are listed as follows: 1. Polarization offset and pyroelectric effect of compositionally graded ferroelectric films Compositionally graded ferroelectric films have been prepared by a variety of thin film deposition techniques including rf-magnetron sputtering, sol-gel, metal-organic chemical vapor deposition, and laser ablation process. While much is known experimentally concerning compositionally graded ferroelectric structures, theoretical description and analysis of these materials have been significantly lacking. Firstly, we have developed the modified transverse Ising model (TIM) taking into account four-spin interaction and the double-peak probability distribution functions to investigate the polarization, phase transition and pyroelectric properties of compositionally graded BST thin films. Our model can be applied to explain the experimental results very well. The calculated results exhibit that the magnitude and the sign of the polarization gradient are not only dependent on the compositional gradient, but also related to the temperature. We can obtain a critical Sr2+ compositional concentration for an up-graded ST→BST film, where crossover from the second-order phase transition to the first-order phase transition occurs. This phenomenon is different from the downgraded BT→BST film. We also introduce the Poisson equation to calculate the charge offset and investigate the impact of the temperature and compositional gradient on the charge offset. Secondly, we have extended the first-principles approach to investigate the pyroelectric properties and polarization offset of compositionally graded PZT films. We can adjust an appropriate compositional gradient and thickness gradient to acquire a large value of the effective pyroelectric coefficient, which can shed light on how to produce good pyroelectric devices. Besides, we explore the origin of the polarization offset, which can be mainly attributed to the polarization gradient. 2. Stress effect and dielectric properties of compositionally graded ferroelectric films Internal stresses in thin films arise due to a variety of reasons including the lattice mismatch between film and the substrate, differences between thermal expansion coefficients of film and substrate, polarization switching, external mechanical loading and phase transformations. It was shown that the choice of both substrate and the film thickness can be used to adjust the internal stress level and thus to tune physical properties. The latter is due to the thickness dependence of stress relaxation by misfit dislocationformation at the deposition temperature. Before investigating the impact of stress on the thermodynamic properties of the graded ferroelectric films, we discuss the effect of homogeneous ferroelectric films. Firstly, We have concentrated on the influence of the choice of substrate material, film thickness and deposition temperature on the piezoelectric coefficients of epitaxial Ba0.6Sr0.4TiO3 thin films by use of the modified Landau-Devonshire-type phenomenological thermodynamic theory. We find that the selection of the substrate and film thickness can be chosen as design parameters to manipulate the effective piezoelectric coefficient to achieve optimum nonlinear piezoelectric response. It is obvious that there is much stronger electric field dependence for a lower deposition temperature. Secondly, the long-range interaction in ferroelectric material usually was neglected in the previous studies. We investigate the impacts of both film stress and long-range interaction on the spontaneous polarization, phase transition temperature, critical thickness and pyroelectric coefficient by using the finite difference method, based on the Landau-Devonshire-type phenomenological theory. We can conclude that both the compressive stress and long-range interaction increase the polarization and the Curie temperature, whereas the tensile stress reduces the polarization. Thirdly, we have discussed the effect of internal stress on the dielectric properties of the compositionally graded BST thin films by the modified TIM. We for the first time introduce a position-dependent stress term into describe the stress distribution in TIM. The compressive stress not only is useful to good temperature stability of dielectric susceptibility in a wide temperature range for the downgraded BST film , but also improves the dielectric tunability of the upgraded one. 3. Thermodynamic properties of temperature graded ferroelectric films Effects of quantum fluctuation and temperature gradient on the spontaneous polarization, phase transition and pyroelectric coefficient of a temperature graded BaTiO3 ferroelectric film have been investigated, based on the TIM with the four-spin interaction. We for the first time introduce the quantum fluctuation to the temperature gradedferroelectric films. It is found that the quantum fluctuation plays an important role on the polarization of the graded film, especially away from the phase transition temperature. There is a broad and smooth peak of the pyroelectric coefficient occurring at a low temperature except for a sharp pyroelectric peak at the phase transition temperature. Moreover, the four-spin interaction not only increases the mean polarization but also makes the smooth pyroelectric peak at low temperature sharper. We find that the graded film displays first-order phase transition when the four-spin interaction parameter is larger than a critical value, which is not only related to temperature gradient but also related to the quantum fluctuation. On the other hand, experimental results have shown that polarization graded ferroelectrics can be formed from homogeneous ferroelectric films of lead strontium titanate by imposing stress gradients on the material normal to their electrode surfaces. However, up to now, little consideration has been given to the stress-induced polarization-graded ferroelectrics. In the future work, I will focus on the effect of internal stress on the dielectric and pyroelectric properties of the stress-induced polarization-graded ferroelectric films. We will try to look for the effective approach to enhance the dielectric properties, improve dielectric tunability and pyroelectric coefficient of the graded structure by adjusting the internal stress level.