Influence Of Structure Transition Layer On Statical And Dynamical Properties Of Ferroelectric Thin Films

Ferroelectric thin films have various unique characteristics, such as piezoelectricity, pyroelectricity, electro-optic, acousto-optic, photorefractive effect, non-linear optics effect and high dielectric constant and so on, compared to the bulk materials. Within ten years, the application fields of ferroelectric thin films have been broadened because of the great advancement of prepared techniques. For example, nonvolatile memory, dynamic random memory, film capacitor, micro-driver and actuator, infrared detector, pyroelectric sensor array, fiber waveguide device, optic display and memory, spatial light modular, laser frequency booster, etc. With the developments of technology increasingly and studies deeply, a number of advantages are expected from ferroelectric thin film based devices: miniaturization, high density and low-power consumption. However, the physical properties of ferroelectric thin films are different from those of the corresponding bulk materials due to some factors, such as impurities, defects, surface and interfacial stress when films are prepared, as well as the interface effects between different components and so on. In some case, such distinction is great. The new phenomena may also appear, for instance, size effects. From lots of research reports on the preparation and properties of ferroelectric thin films, we can know that all the results have individual differences, even if the technology and material elements are the same. It is indicated that the characters of ferroelectric thin films associate not only the materials and elements, but also the details of technologic process. Based on the analysis of the reported results, we consider the interfacial stress between the ferroelectric thin films and electrodes and between the different materials, as well as the defects are protogenic. The individual differences in experiments are ascribed to the protogenic stress and different distribution of the defect, which can induce the local differences in film structure. Hence, the structure transition zone, which may exist at the surface or near the interface of the films, has a great impact on the properties of ferroelectric thin films. Even the physic effects, which cannot be in the bulk materials, can be induced. The basis of experimental and theoretical research is further requested, whether the need of science cognitive process and theoretical development, or preparation of smaller scale commercial devices with high quality and stability of work. Therefore, the basic study on the surface or interface effects has both theoretical significance and actual value. Under such background, the statical and dynamical properties of a ferroelectric thin film are investigated by introducing the structure transition layer.Using the transverse Ising model and GLD phenomenological theory, respectively, the surface transition layer effects of the properties of the polarization distribution, transition temperature and pyroelectric, etc, are investigated within the framework of the mean field approximation.Functions of the intra-layerJ_a(m) and inter-layerJ_a(m) coupling are introduced to characterize the nonuniform structure of surface transition layer based on the transverse Ising model theory for the first time. It is one of the primary innovations for generalizing the use of Ising model to study a ferroelectric thin film. The results obtained are: (1) the essential reason of the properties of ferroelectric thin films different from those of bulk materials is the existence of surface transition layer which is an important factor for influence of the static properties of ferroelectric thin films. (2) surface transition layer can make the transition temperature of ferroelectric thin films higher than that of bulk materials or lower. The influence of the intra-layer pseudo-spin interaction constant on the properties of the system is more obvious than that of the inter-layer. (3) due to the influence of the surface transition layer (pseudo-spin interaction coefficient is less than the value of bulk materials) , the critical thickness will exist in the films if the pseudo-spin interaction is weak; if the thickness of the films is less than the critical thickness, the ferroelectricity of ferroelectric thin films disappears.For discussing the influence of structure transition layer on the properties of ferroelectric superlattice and multilayer thin films, the characteristics of ferroelectric bilayer films with the structure transition layer near interface and interfacial coupling between two components are discussed. The structure transition layer is introduced into a ferroelectric compound thin film firstly, and the way of the new cognition is provided for understanding the physical mechanism of abnormal characters of a ferroelectric compound thin film. Without the generality of the results, we use GLD phenomenological theory. The conclusions are: there may be an equilibrium point of the two contrary actions in the system, in the case of the interfacial coupling constant (ferroelectric coupling) advantage to the ferroelectric phase for the system and surface transition layer suppressed the system at ferroelectric phase. The abnormal behaviors of a ferroelectric bilayer film are ascribed to the competition between the surface transition layer and interfacial coupling deviated from the equilibrium point. For example, if the action of the interfacial coupling is dominant, the size-driven phase transition may occur, and the vice versa.Influence of the surface transition layer on the dynamical dielectric constant is discussed first within the framework of the mean field approximation. The following results are: (1) the action of the lattice vibrato damp reduce the peak value of the real and imaginary parts of the permittivity, and broaden the width of the peak. (2) surface transition layer is responsible for the shift of the peaks of the real and imaginary parts of the permittivity to the lower temperature, and the frequency of the soft mode reduces with increasing the influence intensity of surface transition layers.The statical and dynamical properties of a ferroelectric thin film and the polarization distribution and phase transition of a ferroelectric bilayer film have been investigated within the framework of the macroscopical thermodynamic theory and mean field approximation. The results and conclusions we obtained enrich the study contents of ferroelectric thin film fields, and may have some significance for experiments. However, a number of tasks are still further investigated, for example, the problem of depolarization field in the interior of films.