Preparation And Properties Of BiFeO₃ Based Multilayer Composite Ferroelectric Thin Films

BiFeO₃?BFO?is one of the few single-phase multiferrite materials at room temperature.Due to its excellent ferroelectric,piezoelectric and ferromagnetic properties,BiFeO₃ has wide application potential in the fields of microelectronics,micromachining,integrated optics and so on,so it is widely concerned by researchers.However,the large leakage current caused by Fe³⁺and the intrinsic weak magnetism limit the multifunctional application of BFO.In many ways to reduce the leakage current,it is easier to form a double-layer or multi-layer film,which belongs to the nanoscale structure design and is a more subtle micro change.Therefore,researchers pay more and more attention to it.Moreover,the coupling and interaction between different functional layers can be used to regulate the carrier concentration at the interface of composite films to strongly affect the growth and performance of the films.It is found that SBT film has lower leakage current and no obvious fatigue phenomenon.In this paper,BFO based double-layer composite films with SBT as the transition layer were prepared by sol-gel process and layer by layer annealing.The effects of SBT transition layer thickness on the structure and properties of the bilayer composite films were systematically studied,and the related mechanism was analyzed.1.The effect of the thickness of the SBT transition layer on the properties of bilayer composite films was studied.All the thin film samples formed bismuth layered perovskite structure.The BS80 sample exhibits good crystallization,an improved density and a uniform minimum grain size;the Fe²⁺content and oxygen vacancy concentration are lowest.The 80nm thick transition layer improves the remanent polarization,dielectric constant and reduces the leakage current density,dielectric loss of the samples.The leakage mechanism changes with the increase of electric field strength,in low electric field?E<100 kV/cm?is ohmic conduction,and in high electric field?E>100 kV/cm?is F-N tunneling effect.2.Combining Mn doping and constructing double layer structure,a series of BFMO/SBT double layer composite films with different transition thickness were prepared via a sol-gel method,and the effect of the thickness of SBT as transition layer on the performance of the BFMO/SBT double layer composite films was researched.According to the XRD and SEM analysis,the perovskite structure was formed in all film samples.The BS40 sample had good crystallization,good density,uniform and small grain size.It can be seen from the XPS results that the Fe²⁺and(_OV_2-)^??concentration were the lowest in this sample.In addition,the 40nm transition layer enhanced the piezoelectric(d₃₃¹21pm/V)and ferroelectric properties?2Pr⁹9?C/cm²@1250kV/cm,2Ec⁸46kV/cm?and reduced the leakage current density(J⁵.21×10^-5A/cm²@350kV/cm)of the sample due to the coupling between SBT and BFMO layers and the action of stress.The ferromagnetic properties of the sample decreased derived from the grain size effect after the SBT transition layer was introduced.Since the degree of aging depended on the rearrangement of the defect dipoles affected by the oxygen vacancy concentration,the samples with the transition layer were weak in the aging degree.3.The effect of Ca doping on the properties of SBT ferroelectric films was investigated.The results show that proper Ca doping can improve the properties of SBT.All of the thin films formed a bismuth layered perovskite structure without other impure phases.The sample with a doping content of 10 mol%shows uniform grain,good film density,the lowest Ti³⁺content,the lowest oxygen vacancy concentration,the large remanent polarization 2Pr=33.5?C/cm²,the lowest leakage current density 1.67×10^-77 A/cm²;a larger dielectric constant??r=795?and lower dielectric loss?tan?=0.03?when the test frequency is 105 Hz.