Theoretical Studies On The Polarization Properties Of Ferroelectric Thin Films

Ferroelectric thin film is a kind of dielectric thin film with spontaneous polarization, at a certain temperature, spontaneous polarization possesses two or more possible directions. The direction of the spontaneous polarization may be switched under an electric field. Ferroelectric thin films exhibit many striking phenomena and properties compared to their constituents in bulk, for example, an enhancement in remnant polarization and a large dielectric response. With the rapid development of fabrication techniques, many devices are made, such as ferroelectric memories, surface acoustic wave devices, tunable microwave filter and microwave capacitors. Therefore, ferroelectric thin films have received great attention in the research due to their potential applications.Some critical problems are often encountered in the study of ferroelectric thin films and devices, which affects the merchandising process of devices, such as ferroelectric memories. These problems mainly exhibit in following aspects:obvious fatigue in ferroelectric thin films leads to the reduction of polarization after switching many times; the phase transition properties and ferroelectricity of ferroelectric thin films have serious size effcet, namely, spontaneous polarization intensity, coercive field and dielectric permittivity, are all related to the thickness of ferroelectric thin films, etc. Researchers attribute these problems about ferroelectric thin films to the choice of electrode material and surface stress, strain between ferroelectric thin films and so on. The existence of the interface between ferroelectric thin films and substrate, the interface between ferroelectric thin films and interfacial surface of crystal grain is the main reason resulting in local inhomogeneous structure in ferroelectric thin films. In addition, many factors existing in thin ferroelectric films, for example impurities, defects and surface or interface stresses and so on, are the important cause of leading to local inhomogeneous structure due to the restriction of technique of thin ferroelectric films preparation. The physical properties of ferroelectric thin films are different from those of the corresponding bulk materials due to these factors. The new phenomena may also appear, for instance, in the process of artificial synthetical ferroelectric bilayer films or multilayers, we can find that spontaneous polarization, transition temperature and dielectric permittivity increase with the thickness of ferroelectric thin films reducing, and different compositions of ferroelectric thin film layers with suitable combination can improve the devices performance or create new functional properties. These novelty phenomena mainly come from the characters of interactions between layers. Therefore, the study on the physical properties of a ferroelectric bilayer film composed of two different ferroelectric films and the effects of the surface or interface transition layers on the properties of a ferroelectric thin film, no doubt that for basic investigation of a ferroelectric thin film not only has the significance of theoretical research but also has very practical research value. Under such background, the polarization properties of ferroelectric thin films is investigated, based on the Ginzburg-Landau-Devonshire (GLD) theory, and Landau-Khalanitkov theory.Within the framework of Ginzburg-Landau-Devonshire (GLD) theory, a ferroelectric bilayer film consisting of two different ferroelectric constituent films with the transition layer within each constituent film is considered, which is more realistic than the theoretical model assuming ferroelectric bilayer film consisting of two same ferroelectric constituent films. In practice, a ferroelectric bilayer film composed of two different constituent films can better obtain our expectant physical performance, and create new functions. Therefore, it is significative to study the properties of a bilayer film with two different constituent films in theory. We first introduce a parameter, which describes the differences of physical properties between two constituent films, to investigate the temperature dependence of the average polarization and average dielectric susceptibility of the bilayer film. The results present some interesting phenomena different from those of ferroelectric bilayer film consisting of two same ferroelectric constituent films due to the introduction of the parameter a. In this paper, we assume that the interfacial coupling between two constituent films is ferroelectric. It is shown that the ferroelectric interfacial coupling and a large a are to improve the spontaneous polarization and phase transition temperature, which result in the peaks of dielectric susceptibility and pyroelectric coefficient shifting to the higher temperature region. While the transition layer is to suppress them, the redution of spontaneous polarization and phase transition temperature leads to the peaks of dielectric susceptibility shifting to the lower temperature region. By the adjustment of parameterα, ferroelectric interfacial coupling coefficient and surfacial transition layer parameter, we can obtain physical properties of ferroelectric bilayer film under different conditions, i.e., one or two steps appear in the polarization curve accompanied by one or two dielectric and pyroelectric peaks. The characteristic can provide theoretical foundation and research direction to achieve expected physical features. The parameterαintroduced to describe the differences of physical properties between two different ferroelectric constituent films plays a crucial role in the polarization properties, dielectric susceptibility and pyroelectricity of a bilayer film.Using the Landau-Khalanitkov equation, we study the dynamic properties of ferroelectric thin films. The properties such as mean polarization, switching current and hysteresis loop are studied from two aspects. One is that the surface polarization is more than that of internal (ρ>0); The other is that the surface polarization is less than that of internal (ρ<0). The results show that on the action of amplitude and frequence of electric field, temperature and surface transition layer, the polarization will gradually reduce with the time increment until the polarization switchs. It is found that the polarization will enhance after switching and the frequence of electric field plays an obvious role in the polarization switching. When the frequence of electric field enhances, the switching time of polarization greatly accelerates and the switching current increases. The increment of amplitude and frequence of electric field makes the augment of the remanent polarization and coercive field. With the increment of temperature, the remanent polarization, the coercive field and the switching current decrease, while the switching time of polarization reduces. Whenρ>0, the mean polarization, the remanent polarization, the coercive field and the switching current all decrease with the effect of transition layer strengthening, the switching periodicity have no variation. The augment of amplitude of electric field makes the switching current increase and the switching time shorten. Whenρ<0, the action of transition layer is to enhance the mean polarization, the remanent polarization and the coercive field, and increase the switching current as well as the switching time. The increment of amplitude of electric field makes the switching current decrease and the switching time shorten. By introducing surface transition layer, we correctly disclose the physical tendency of ferroelectric thin films dynamical properties and provide more realistic theoretical basis for experimental workers.In this paper, within the framework of thermodynamic theory, we study the physical properties of the ferroelectric bilayer film consisting of two different ferroelectric constituent films and the dynamical properties of a ferroelectric film. It is successful explained the fantastic phenomena which observed in experiments, and enriched the research of ferroelectric thin film areas in this paper. The researchful results obtained in this dissertation give some guidance for the miniaturization and creation of new functional characters of ferroelectric thin film devices.