Theoretical Investigation On Properties Of Ferroelectric Thin Films With Dead Layer

With the development of the modern techniques, ferroelectric (FE) materials have attracted much attention due to their excellent ferroelectric, piezoelectric, dielectric and pyroelectric properties, and wide applications in many fields. With further investigation of the properties of ferroelectric thin films, the people gradually take note of the serious deviation from the bulk for some physical properties of ferroelectric thin films as the thickness of films decreases. At the present time, a widely accepted view for the reason of this behavior is owing to the existence of the"dead layer"(nonferroelectric active layer) between ferroelectric thin film and the electrode. However, the being of the mechanism of the"dead layer"and the effect of which on ferroelectric thin films has not been studied in theory.There are many factors that influence the configuration of the ferroelectric thin film, such as impurities, defects and imperfect in crystal lattic, which causes dead layer between ferroelectric thin film and the electrode, and the transition layer and dead layer make the spontaneous polarization distribution inhomogeneous. At the surface of ferroelectric thin film, the ferroelectric disappeared. This region corresponds to a nonferroelectric dielectric layer, which is called dead layer. Completely with the bulk, the influence of dead layer for ferroelectric thin film is not ignoring. The study of influence of this configuration for the property of ferroelectric thin film can guide us preparing needed thin film in experiment.In this article, using the expression of free energy of modified Gingzburg- Landau -Devonshire (GLD), the being of the"dead layer"and the effect of which on ferroelectric thin films are investigated from the theoretical point of view, within the frame work of the phase transition of Landau theory. Through the numerical calculation we investigated the polarization, the average polarization, the dielectric susceptibility, pyroelectricity, hysteresis loop and so on.During the study of properties of ferroelectric thin film, we found that the transition layer and dead layer influence obviously on the properties of ferroelectric thin film. The transition layer and dead layer lead to descent of spontaneous polarization .The existing of them leads to obvious boundary effect, which enhance the difference of polarization between thin film center and surface. This leads to the polarization of thin film surface is less than that in the center, and the polarization is less following the increasing of dead layer and transition layer thickness.During the study of hysteresis loop of ferroelectric thin films, we found that the coercive field and remanent polarization decrease and the slope of the hysteresis loop increases due to the existence of the"dead layer", that is to say, the hysteresis loop loses their squareness. The results are in good agreement with the experimental data. Therefore, we believe the fatigue is attributed to the existence of the"dead layer".The transition layer and dead layer lower the phase- transition temperature. For the same transition layer, the transformation temperature decreases with increasing the thickness of dead layers.The pyroelectric coefficient peak value lowers and the working temperature region shrinks when the transition layer and dead layer are existed. This is disadvantageous for the expanding of temperature region of pyroelectric material. In practice, we should enhance sufficiently the pyroelectric coefficient and expand enough the working temperature region by controlling the transition layer and dead layer thickness.The average dielectric susceptibility distribution is changed with the transition layer and dead layer .They leads to improve the dielectric susceptibility. Average dielectric susceptibility and the temperature of the average dielectric susceptibility peak value reduce.Through the numerical calculation we investigated properties of ferroelectric thin film with the transition layer and dead layer. This shows that the influence of the dead layer on the properties of ferroelectric thin film is more evident than that of the transition layer layer. Based on the experiment, we development the theoretical of ferroelectric thin film. Our work lays the theoretical foundation for the preparing of thin film device with good property.