Study On The Aging Mechanism Of BiFeO₃-based Multilayer Ferroelectric Thin Films

Various application performances of ferroelectric materials would also decrease with time when no external mechanical or electrical load was applied,which called aging.Aging could be observed in almost all ferroelectrics,and the"hard ferroelectrics"doped by the host had more severe degree of aging than the"soft ferroelectrics"doped by the donor.Aging strongly affected the application of ferroelectric components,especially the reliability of ferroelectric components and dielectric devices.It was a fundamental and important problem in application and had received extensive attention in the past few decades.It was natural to speculate that ferroelectric aging was caused by some relaxation process.In order to further explore the microscopic origin of aging effect on the basis of previous studies,we innovatively researched from the perspective of the influence of different orientation on the aging properties of polycrystalline films.The Bi Fe_0.95Mn_0.05O₃(BFMO)thin films with orientation of<100>,<110>and<111>were epitaxial grown on Sr Ru O₃/La Al O₃(SRO/LAO)and Sr Ti O₃(Nb:STO)single crystal substrate via sol-gel method.The aging mechanism was explored in ferroelectric materials that may exhibit relaxation and severe aging.The structure and electrical properties of the BFMO/SRO/LAO films with different orientation had been studied.Through XPS test and analysis,different orientations had no obvious effect on the element valence and defect state in the samples.XRD,Raman,SEM and AFM techniques were used to analyze the structure of the films.It was found that the rhombohedral R3c phase with orthorhombic Pnma phase was formed in the three different oriented films.The average grain size of<110>oriented sample was larger.The domain structure of the samples was observed by PFM measurement.It was found that the sample with<100>orientation grew more typical 109°striped domain,the sample with<110>orientation had less domain content and smaller domain wall density,and the sample with<111>orientation showed island domain.Different orientations had different internal influences on piezoelectric response,dielectric response,ferroelectric response and aging degree due to the different number of<111>polarization equivalent components and the different angles between the equivalent components and the direction of the applied electric field:(1)The film with<100>orientation possessed the highest piezoelectric displacement(D?50pm),and the<111>oriented film had the lowest piezoelectric displacement(D?12pm).(2)The value of<100>and<111>oriented samples at10⁴Hzwas 115 and 185,respectively.(3)The remnant polarization(2Pr)of<100>and<111>films was 63 and 106,respectively.However,the external contribution of various properties was greatly affected due to the larger grain size and less domain of<110>oriented sample,which leaded to that the performance characterization parameters did not conform to the intrinsic polarization.We also studied the structure and properties of BFMO/Nb:STO films with different orientations.PFM test showed that Nb:STO<110>substrate was not conducive to the generation of ferroelectric domain in BFMO thin films.Imprinting,as a behavior that occurred early after the preparation of films,caused the degradation of ferroelectric properties,and the mechanism of this behavior was also explored.By comparing the internal electric field(E_i)between the BFMO films grown on SRO/LAO and Nb:STO substrates,it was found that the former had a more serious imprinting behavior.We believe that the imprinting behavior of the BFMO films in this thesis was dominated by both volume screening and interface screening mechanisms.It was found that there was almost no aging behavior in the<111>oriented sample through the comparison between aging behaviors of the six film samples in this article before and after aging for 30 days and 120 days at room temperature.The model for mechanism had been established to explain the effect of different crystal orientations on aging behavior of the BFMO films.The reason may be that defect dipoles pinned the domains gradually with time driven by symmetry consistency,which is a kind of volume effect.