Interface Control Of Self-Polarized BiFeO₃-based Thin Films

Due to its simultaneous exhibition of ferroelectricity and antiferromagnetism at room temperature,BiFe O₃（BFO）,as a lead-free multiferroic material,has aroused great interest.It has been considered as the most promising alternative for Pb（Zrx,Ti1-x）O₃ lead-based materials.Its high Curie temperature makes it potential in the application of piezomicroelectronics and pyroelectric.However,for pyroelectric devices,the ferroelectric thin films with high Curie temperature should be manually pre-poled before its use.Comparing with lead-based ferroelectric thin films,it is more difficult to pole BFO film due to its high coercive field and large leakage currents of BFO film,and thus will definitely consuming more energy during pre-poling procedure.The pre-poling can be saved if as-prepared BFO films have sufficient self-polarization,contributing to the reduction of production costs and the enhancement of efficiency.According to the previous studies,the self-polarization is resulted from the internal field in the film which may originate from many factors,such as strain,defects and contact potential in the interface of thin film and electrode etc.As been reported by Ren et al.,the defect dipoles have a tendency to align along the direction of spontaneous polarization.With the respect of BFO film,the films start aging during deposition procedure due to the annealing temperature far below its Curie temperature.If the film exhibits high degree of self-polarization,the aging effect will play important role in stabilizing the self-polarization;if the film exhibits partial self-polarization,the aging effect will make the film difficult to be poled or easy to be reversed to its original state after the removal of electric field.And this will deteriorate the stabilization of the macro-piezoelectricity and pyroelectric property.As we known,the domain backswitching mainly involes three steps,namely nucleation、longitudinal growth and horizontal expansion.The domain with opposite directions always starts at the top and bottom interfaces of the films.And the growth can be accomplished within very short time and the nucleation step dominates the whole growth.Therefore,eliminating the nucleation of domains with opposite direction is crucial for inhibiting domain backswitching.In this work,we prepared self-polarized BFO-based thin films using metal organic decomposition method and layer-by-layer annealing process.We systematically investigated the ferroelectric properties of self-polarized BFMO thin film and through interface control the ferroelectric and piezoelectric properties are dramatically improved by inhibiting domain backswitching.The major research work is listed as follows:The self-polarized BFMO film was prepared on Pt（111）/TiO₂/SiO₂/Si substrate with high degree of（012）-preferred orientation.The influences of ferroelectric test on the ferroelectric property are systematically investigated and new explanation of the formation of gap in ferroelectric hysteresis loops was proposed.The effect of defect dipoles on the stabilization of self-polarization was well demonstrated in the study of aging of the film with time.BFWO layers with different W⁶⁺ doping content were prepared.The best W⁶⁺ doping content was chosen as 1% by investigating the influences of（top）BFWxO and BFWxO（bottom）on the structure and ferroelectric property of BFMO film.Through the combination of volume effect and interface effect,the nucleation of domain backswitching was effectively inhibited.By contrasting the roles of（top）BFWxO and BFWxO（bottom）on BFMO film,it has been demonstrated that the BFWxO/BFMO/BFWxO is the most efficient structure for eliminating domain backswitching and dramatically enhancing the ferroelectric and piezoelectric properties of BFMO film,especially the superior piezoelectric of which is comparable to that of lead-based ones.