Preparation And Modification Of Zr-doped BiFeO₃-based Ferroelectric Thin Films

In today's society,against the background of rapid progress in science and technology and the tendency of electronic devices to be miniaturized and integrated,people's requirements for materials are becoming more and more demanding,and the research and development of new green multi-functional materials have become a hot spot for research.Ferroelectric materials have received a lot of attention from researchers due to their rich and excellent physical properties.Among many ferroelectric materials,bismuth ferrite(Bi Fe O₃,BFO)is the only room temperature single-phase multiferroic material found so far,which has broad application potential in microelectronics,integrated optics,and micromechanics.However,the large leakage current due to the volatilization of Bi elements and the valence change of Fe ions in BFO films,and the weak magnetic properties of the material itself limit its further applications.To reduce the leakage current of BFO film and improve its performance,various technological methods were used to modify it in this thesis.BFZr O films with different Zr content(0-12 mol%)were prepared by sol-gel process and layer by layer rapid annealing process.On this basis,Bi Fe_0.91Zr_0.09O₃was used as the research object,the effects of annealing atmosphere(Air,O₂,and N₂),grain orientation(Mg O substrates with different orientation)and internal stress(Mg O and Si substrates with different lattice constants)on the properties of the films were investigated.The structure and properties of all film samples and their internal relationship were systematically studied,and the related mechanism was deeply discussed.Firstly,the effects of high valence Zr doping on the structural phase transition and multiferroic properties of BFO films were studied.Through XRD refinement and the Raman test,it was found that all the films were composed of rhombohedral R3c and orthorhombic Pnma.Further investigation of the effect of structural phase transition on their properties revealed that the decrease of polar R3c phase content in BFZr O films led to the decrease of ferroelectric properties,while the increase of the content of nonpolar Pnma phase leads to the increase of ferromagnetic properties and bandgap width.The XPS tests showed that Zr doping significantly reduced the defect concentration such as Fe²⁺and oxygen vacancies in the BFO films,which also reduced the leakage current density,and explored the leakage mechanism of the BFZr O films.Since the degree of aging is affected by the rearrangement of defective dipoles and domain flipping through the concentration of oxygen vacancies,Zr doping makes the BFO films significantly more resistant to aging.Based on the doping experiment,it is found that the leakage current density of BFO film is the lowest and the multiferroic property is better when the Zr content is 9mol%.Therefore,we studied the effects of different annealing atmospheres(Air,O₂,and N₂)on the structure and properties of BFZr O films.XRD and TEM analysis showed that all samples were composed of R3c and Pnma.Under oxygen annealing atmosphere,BFZr O film has relatively lower leakage currents(4.83×10^-6A/cm~2)and higher remanent polarization strengths(2P_r=70.26?C/cm~2),and its light absorption and photovoltaic properties are also improved.Based on UPS and UV-vis,the influence mechanism of annealing atmosphere on photovoltaic performance is explained.It is proposed that the photovoltaic performance of BFZr O film can be improved by adjusting the polarization intensity,bandgap width,and oxygen vacancy concentration.In addition,annealing in oxygen atmosphere can effectively inhibit the aging of BFZr O films.Further,the structure and properties of BFZr O films are regulated by changing the substrate orientation and stress.BFZr O/LNO/Mg O samples with<111>orientation have a large grain size and low Fe²⁺and oxygen vacancy defect concentration,so the leakage current density is relatively small,and its leakage mechanism is mainly Ohmic conduction.Compared with<100>and<110>samples,<111>samples have the expected large residual polarization value,the best dielectric and aging resistance properties.The XRD diffraction peaks of BFZr O/LNO/Mg O and BFZr O/LNO/Si films on different substrates move to a high angle due to the tensile strain.Mg O substrate samples have relatively low leakage current density and good dielectric and aging resistance properties.The excessive tensile stress provided by Si substrate may lead to the existence of more antiferroelectric O phases in the film samples,resulting in the reduction of its remanent polarization strengths.