Theoretical Investigation On Physical Properties Of Size Effect Mechanism Of Ferroelectric Thin Films

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 have not been studied in theory. In this article, using the expression of free energy of modified Gingzburg–Landau -Devnshire (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.The research shows that an area of the absence of the spontaneous polarization existed at interface, which is the"dead layer"area, when a certain value of the thickness of the imperfect surface layer reached. With increasing the thickness of the imperfect surface layer or the density of the free energy, the area of the"dead layer"increases gradually. This indicates that the existence of the imperfect surface layer gives birth to the"dead layer", and in this case the ferroelectric thin films at the coexistence of ferroelectric and paraelectric phase.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".Under study, we also find that no matter how big is the size of ferroelectric thin film, the"dead layer"can emerge before the emergence of the critical size. The difference is that the smaller the size of ferroelectric thin film, the more remarkable role the"dead layer"plays. Thereout, we predict the phase from ferroelectric to paraelectric is drived by the"dead layer".