Theoretical Investigation On Properties Of Composite Ferroelectric Thin Films

Although researchers have known a lot of excellent characteristics offerroelectric substance, as it is difficult for ferroelectric crystal materialsto turn it into thin film, and it is incompatible with semiconductor andmetal , so this kind of materials doesn't play an important role inmaterial and information domains. As the technologies of film preparation have developed, particularlyin 1980's the technologies of preparing ferroelectric films made greatprogress. As ferroelectric films have a lot of excellent characteristics,such as ferroelectricity, piezoelectricity, pyroelectricity and nolinearoptics, so it is used widely. The study on ferroelectric films aredeveloping towards applications, and it has become active in the highand new-techonology material fields.Since the increase of the kinds of the artificial composite ferroelectricfilms and its better properties than the single bulk material, it has beenapplied widely. But due to the limitation of the preparing technologies,there are many factors that influence the properties of the multilayerferroelectric thin film, such as, impurities, defects, stress and imperfectin crystal lattice. These factors are the causes of appearance of imperfectinterface in which the structure is different from bulk material, and makethe spontaneous polarization distribution inhomogeneous. So the study ofthe influence of the imperfect surface layer or interface of the artificialcomposite ferroelectric films are of great significance.Within the framework of thermodynamic phenomenological theory, ageneralized GLD free energy has been developed to describe theferroelectric thin film of second-order phase transition. In this work, theauthor studies the properities of ferroelectricity, dielectricity andpyroelectricity of the composite ferroelectric films. Through thenumerical calculation we investigated the polarization, pyroelectricity,hysteresis loop and electric susceptibility.During the study of properties of ferroelectric thin film, we foundthat the composition of two bulk materials make the system nonuniform,which cause the appearance of a transition layer between the two bulkmaterials. An inhomogeneous structure with a very small range in a thinfilm can influence the spontaneous polarization in a large range. Theinfluence of the change of the transition-layer width on the polarizationdistribution is insensitive.The weaker ferroelectric material provides a strong influence on thepolarization. It lowers the average polarization and the phase-transitiontemperature of the ferroelectric multiplayer films. It is also the cause ofthe appearance of the two peak values in the pyroelectric coefficient.When the reverse electric field is improved to the coercive field of theweaker ferroelectric material, which will cause the materials occurfield-induced phase transition.The electric susceptibility is reduced when the applied external fieldis enhanced. But it will increase under elevated temperature. When theparameter δ which reflects the intensity of the ferroelectricity of thematerial Ⅱ increased, the electric susceptibility will also increase.During the investigation of the properties of the model (There isanother ferroelectric material Ⅱ between two same ferroelectric materialⅠ interlayers), we found that there is only one peak value in thepyroelectric coefficient. The ferroelectric material Ⅱ doesn't influenceon the pyroelectric coefficient of the ferroelectric multiplayer films.Our work lays the theoretical foundation for the preparing ofcapacitor and pyroelectric materials with good properties.