Study On Preparation And Propertie Of Ce And Sc Co-doping In BiFeO₃ Thin Films

Bismuth ferrite(BiFeO3,BFO)has attracted widespread attention due to its advantages of simultaneously exhibiting ferroelectricity and ferromagnetism at room temperature,so it has great potential for development in storage devices,magnetoelectric coupling sensors,and other fields,which has attracted the attention of semiconductor researchers.However,some urgent problems have been discovered during its research,such as the easy formation of impurities during preparation,large leakage current density,and much lower ferroelectric and magnetic properties than the theoretical values.These issues greatly limit the practical application of bismuth ferrite.To address the above issues,this study used sol-gel method to prepare singledoped and co-doped BFO thin films with lanthanide element cerium(Ce)and transition metal element scandium(Sc)on single crystal Si substrates,and studied the structure,ion valence state,surface morphology,magnetic,and ferroelectric properties of the films.Based on this,Ce and Sc co-doped BFO epitaxial thin films were prepared on single crystal LaAlO3(00l)substrate,and the structure,thin film orientation,and magnetoelectric properties of the epitaxial thin films were studied.The main conclusions are as follows:(1)Bi1-xCexFeO3 films(x=0～0.12)and BiFe1-yScyO3 films(y=0～0.04)were prepared using LaNiO3 as the bottom electrode on Si substrate.The results show that the introduction of Ce and Sc does not change the structure of BFO.Compared with pure BFO films,the ratio of Fe3+/Fe2+in the films increased after ion doping,and the ferroelectric polarization strength of the films was enhanced.At the same time,the number of grains below 62 nm increased after ion doping,which would lead to the disruption of the periodic spiral magnetic structure in the BFO structure and thus improve the magnetic properties of the films.In the case of Ce doping,the film obtained the maximum remanent polarizatio n strength of 32.5 μC/cm2,the lowest leakage current density of 1.02×10-3 A/cm2,the maximum saturation magnetization strength of 5.32 emu/cm3 when x=0.09.In the case of Sc doping,the film obtained the maximum remanent polarization strength of 8.1μC/cm2,the minimum leakage current density of 1.06×10-3 A/cm2,the maximum saturation magnetization strength of 4.88 emu/cm3 when y=0.03.Compared with pure BFO films,the remanent polarization strength of the films increased by more than 1000%after single doping with Ce and Sc,the leakage current density decreased by three orders of magnitude,and the saturation magnetization strength increased by more than 35%.(2)Bi0.91Ce0.09Fe1-zSczO3 thin films(z=0～0.04)were prepared on Si substrate with LaNiO3 as the bottom electrode to investigate the combined effects of Ce and Sc ion doping on the structure,morphology,and properties of the BFO thin film when the Ce ion introduction amount was 0.09.The results showed that with an increase in Sc ion doping,the structure of the film remains unchanged,but the diffraction peak position shifted towards lower angles due to the substitution of Sc ions.The residual polarization and saturation magnetization of the film increased first and then decreased with an increase in Sc ion doping,and the surface grain size showed a trend of first increasing and then decreasing.The long bond angle of Fe-O-Fe bond is changed because of co-doping,so the properties of the films are improved.When z=0.03,the film exhibited the best ferroelectric and magnetic properties,with a minimum leakage current density of 1.46×10-5 A/cm2,residual polarization of 37.1 μC/cm2,and saturation magnetization of 9.57 emu/cm3.Compared with single doping of Ce and Sc(x=0.09 and y=0.03),on the basis of reducing the volatilization of Bi elements,the co-doped film further intensifies the deformation degree of BFO structure,resulting in the residual polarization of the film increased by more than 14%,the leakage current density decreased by two orders of magnitude,and the saturation magnetization increased by more than 30%.(3)Epitaxial thin films of Bi0.91Ce0.09Fe1-zSczO3(z=0～0.04)were prepared with single crystal LaAlO3(00l)as substrate and LaNiO3 as the bottom electrode.The tested showed that the growth direction of LaNiO3 and Bi0.91Ce0.099Fe1-zSc2O3 are consistent with the direction of the substrate,with good in-plane and out-of-plane orientation.The stress and doping of substrate aggravate the deformation of Fe-O-Fe,so the properties of epitaxial films are further improved.At z=0.03,the thin film had a residual polarization intensity of 75.2μC/cm2,the leakage current density reached the lowest of 5.44×10-7A/cm2,the maximum saturation magnetization of the film is 10.05 emu/cm3.Compared to the polycrystalline thin film of Bi0.91Ce0.09Fe0.97Sc0.03O3,the residual polarization intensity was increased by 102%,the leakage current density was reduced by two orders of magnitude,and the saturation magnetization was increased by over 5%.