Mechanism Study On The Effect Of Substrate On The Properties Of Sm-doped BiFeO₃ Thin Films

The coupling between different properties of multiferroic materials will produce new properties such as magnetoelectric effect,which has great development potential in miniaturization and multifunctional devices.Multiferroic materials are one of the most valuable multifunctional materials in the future,and have a good application prospect in the field of multiferroic devices.Polyferroic materials include single-phase materials and composite materials,but currently found single-phase polyferroic materials are relatively few,and the Curie temperature is usually low.Among them,single-phase Bi Fe O₃（BFO）has attracted extensive attention from materials scholars due to its high Curie temperature（T_C=1103K）and Niel temperature（T_N=647K）,and exhibits ferroelectric and G-type antiferromagnetism at room temperature,and has become a hot spot in the deep exploration of polyferric materials.However,Fe ion variation and Bi ion volatilization in BFO seriously restrict the practical application of BFO.In this study,Bi_1-xSm_xFe_0.98Mn_0.02O₃（x=0,0.02,0.04,0.06）(BSFM_x=0-0.06)thin films with different Sm contents were prepared by ion doping method,and the optimal content was selected through performance comparison.Secondly,this experiment takes the BSFM_x=0.04 as the research object to explore the reasons for the differences in the performance of thin films with oxide electrodes such as ITO/glass（ITO）,FTO/glass（FTO）,noble metal composite electrodes such as Pt/Ti/Si O₂/Si（Pt）and common metal sheets such as Al sheet and stainless steel sheet as substrate materials.It is explained from the aspects of phase structure,grain size,oxygen vacancy content,lattice mismatch and microstrain.The Bi_1-xSm_xFe_0.98Mn_0.02O₃（x=0,0.02,0.04,0.06）（BSFM）thin films were prepared on ITO/glass substrate by sol-gel method.Through XRD and Raman spectrum analysis,it is concluded that the sample has both R3c phase and Pnma phase.Further XRD fitting of the film and detailed analysis of the two-phase content of the sample,it is found that the Pnma phase content of BSFMx=0 film is the smallest.XPS analysis shows that BSFM_x=0.04 film has the smallest oxygen vacancy content,which is conducive to the decrease of leakage current density and the improvement of dielectric properties.At the same time,the（110）diffraction peak has the maximum intensity when the content is 4%,and shows the minimum leakage current density and the larger residual polarization intensity（2P_r=91.859μC/cm²）at room temperature.In conclusion,by doping proper amount of Sm into BSFM films,the leakage property of BSFM films can be reduced,the dielectric property can be improved,and the aging process can be delayed.Moreover,the properties of BSFM films can be further explained from different aspects such as phase content and oxygen vacancy content.The Bi_0.96Sm_0.04Fe_0.98Mn_0.02O₃（BSFM）thin films were prepared by sol-gel method with ITO/glass（ITO）,FTO/glass（FTO）,Pt/Ti/Si O₂/Si（Pt）as the base electrode and Au as the top electrode.XRD results show that BSFM films with different bottom electrodes have different crystallinity,and all films contain two phases,R3c and Pnma.The results of SEM and TEM analysis show that the grain of BSFM film deposited on ITO is well grown and the surface structure of the film is dense.The sectional map shows that the thickness of the prepared film is about 445nm.XPS analysis results show that the bottom of the ITO electrodes on the membrane of oxygen vacancy content is minimum.Ferroelectric test shows that the film deposited on ITO has the highest residual polarization intensity（2P_r=92.35μC/cm²）.The leakage current test shows that the leakage current density of the film on the bottom electrode of ITO is the smallest.Under the electric field of 200k V/cm,the leakage current density is7.47×10^-5 A/cm²,which is about 2 orders of magnitude lower than that of the Pt bottom electrode(1.16×10^-3 A/cm²).Meanwhile,it was found in the light absorption performance test that the band gap of BSFM film could be narrowed（2.60 e V）by using ITO as the bottomelectrode.In addition,the films deposited on ITO also show excellent ferroelectric and dielectric stability.The BSFM films with aluminum Al sheet and stainless steel SS sheet as substrates and as bottom electrodes（BSFM/Al,BSFM/SS）were prepared respectively to explore the reasons for the differences in the performance of the two capacitorstructures.The experimental results show that compared with BSFM/Al thin film,BSFM/SS thin film exhibits larger lattice mismatch and microstrain,compact surface structure and smaller oxygen vacancy content.At the same time,SS electrode can reduce the leakage current density(3.90×10^-5 A/cm²),increase the residual polarization intensity（2P_r=54.21μC/cm²）,increase the dielectric constant（ε=162）,and delay the ferroelectric aging process(D_2Pr=33.12%).In addition,ferroelectric flexible films have been successfully prepared,that is,BSFM/Al films still show excellent ferroelectric properties after mechanical bending.