| In modern society, the ferroelectric materials are used in many applications, in which the most widely used is PZT family, but some lead-containing materials are harmful to humans, with the harmonious development of society, the material applications for environmental protection have become increasingly demanding, so it is very important to develop a new material to replace PZT material, which is harmless to human. Thus, it requires that the new material not only have little difference from the PZT material in performance, but also is a green material to meet the requirements on the basis of actual use, so it presents relatively high requirements to the development of the material.Many advantages of multi-ferrous materials have been demonstrated by researchers. It owns magnetic, and makes multiferroic materials producing magnetic effects and coupling effect when the magnetic and ferroelectric properties will exist together in multi ferroic materials within a certain temperature range. Therefore, based on these characteristics, multiferroic materials have been the focus of materials research community. At the same time, the characteristics of multiferroic properties of the material itself can meet the demand for developing versatile devices.As a kind of multiferroic materials, BiFeO3shows the coexistence of ferroelectric ordering with Curie temperature of850℃and magnetic ordering with Neel temperature of370℃,which causes it to reflect the ferroelectric and relatively weak antiferromagnetic at room temperature. For the various performances, BiFeO3is possible to replace PZT materials and apply to the actual devices, at the same time, BiFeO3itself does not contain hazardous lead elements. Comprehensively considering the practical application and the environmental protection factor, BiFeO3has become the particular interest of practical application and development of new materials.Throughout BiFeO3materials sector on research, we found that although there were are a lot of reports and experiments on BiFeO3, but the results performance reported are not the same, mainly because the preparation process parameters of BiFeO3films have a very large impact on the quality of the films produced.Aiming at these obstacles, BiFeO3thin films were fabricated by sol-gel method in this paper. Further more, we focused on the effects of process parameters of the preparation of BiFeO3film on the phase structure, microstructure, while, on this basis, we have done a bit part A and B site doping modification experiments, by preliminary analysis of the experimental,we discussed the effects of Zr and Sm on properties of the microstructure, leakage current and ferroelectric performance of BiFeO3thin films. The main contents of this paper are as follows:First, the necessity of adopting ITO substrate as the electrode to prepare BiFeO3thin films in this experiment. Overall, we did a reference from the perspective of the film growth mechanism, and discussed the different growth patterns caused by different physical parameters between the ITO substrate and the bismuth ferrite films. XRD results show that ITO is a single crystal substrate with (222) preferred orientation. SEM micrograph shows that the surface of ITO substrate was even and neat enough to facilitate the uniform coating of films and avoid the stress problems. By analyzing the curve of the resistance change with temperature, it is discovered that the resistance of ITO films did not change below600℃, it is possible to avoid the substrate resistance changes which may affect the test results of the ferroelectric properties. Furthermore, the12.49%lattice mismatch of the ITO substrate and BFO makes a semi-coherent interface, in which their thermal expansion coefficients are in the same order of magnitude to produce less stress. In conclusion, BiFeO3thin films fabricated on ITO substrates by sol-gel method is appropriate.Second, the various process parameters of BiFeO3thin films produced by the pecursor solution, which was prepared in our laboratory, are studied. In the experiment of preparing pure phase BiFeO3thin films, we studied the effect of excessive bismuth addition on the thin film with different content. At the same time, the thin films with different layers are prepared and SEM tomography is used to characterize the results. Also, we found that annealing temperature and process have a great effect on crystallization and behavior of the films, and thus we did relative experiments. Finally, we obtained the optimal processing conditions to prepare BFO film with the formula:annealing layer after layer, spin coating speed of4000r/min, pyrolysis temperature of350℃, annealing temperature of550℃, film thickness of about282nm,10%excess bismuth, grain size of about60nm.Third, a series of BiF1-xZrxO3films are fabricated on ITO/glass substrate, with the same process parameters for the preparation of pure phase film, the influences of Zr dopant (0-20%) on the structure and electrical properties of BiFeO3films are investigated. The results demonstrated that pure phase BFeO3film is high (012)-oriented polycrystalline structure, the (012) peak intensity decreased with the increase of the substitution ratio of Zr, the peak intensity of (110) with the increase of increases of the substitution ratio of Zr, the film eventually become high (110)-oriented polycrystalline structure, it can be seen that the Zr-doped cased the (110)-oriented grains and (100)-oriented grains competitive growth. The pyrochlore phase began to be observed, when the film doping ratio is14%, it is thought that an excessive amount of Zr substitution resulted in the structural imbalance. The leakage currents in the BiF1-xZrxO3films, that the doping ratio are16%and20%, are about two or three orders of magnitude lower than that of the film deposited directly on the ITO substrate. This is due to appearing of the pyrochlore phase which has better insulating properties. The leakage currents in the BiF1-xZrxO3films that the doping ratio are10%and14%are higher than that of the pure film, it is may be due to an excess of cationic substitution which caused a large vacancy. The remaining polarization of16%doped film is the maximum, as the oxygen octahedrons deformed by displacement or substitutions of the ion, which causes an increase in the ferroelectricity. The remaining polarization of20%doped film is the minimum, because the presence of the pyrochlore phase reduces the ferroelectric of the film.Forth, a series of Bii-xSmxFeO3films are fabricated on ITO/glass substrate, the influences of Sm dopant (0-8%) on the structure and electrical properties of BiFeO3films are investigated. The results demonstrated that all Bi1-xSmxFe03films have similar polycrystalline structures with a strongest diffraction peak of (110). When the ratio of Sm content changed from0to0.08, the (111) diffraction peaks had tend to move toward large-angle, which means Sm doping caused a change in bismuth ferrite crystal structure. By SEM characterization, it is found that the film grains are clear, and the surface morphology differed with different Sm-doped content. The largest remanent piezoelectric coefficient was observed when x=0.75%, it is about50μC/cm2. The experimental results demonstrated that that the leakage currents in the Bi1-xSmxFe03films were about two or four orders of magnitude lower than that of the film deposited directly on the ITO/Si substrate. The lowest leakage current was observed when x=0.75%, it is about8.42×10-10A. The phenomenon could be explained by the defect theory of acceptor type defects and donor type defects. |
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