Preparation And Doping Of Bismuth Ferrite Thin Films By Sol-Gel Process And Study On The Electric Properties

BiFeO₃is one of the few single-phase multiferroic materials, with distortedperovskite structure （R3c） and eight structural phases transition. The ideal cubicstructure of the perovskite structure was stretched along （111） direction and thestructure transformed into oblique hexagonal structure. The ferroelectric orderand anti-ferromagnetic order both existed at room temperature. Due to the highermagnetic phase transition temperature （Tc=1103K） and the ferroelectric phasetransition temperature （TN=643K）, BiFeO₃has the wide application, such as inmagnetoelectric sensors, self-spinning electronic devices and random accessmemory. It has attracted the increasing attention.In this paper, ethylene glycol monomethyl and glacial acetic acid were usedas solvents. Pure phase BiFeO₃films were prepared on the FTO substrate bysol-gel method with different solvent ratios, annealing temperature and differentthickness in the same solvent. A series of Bi1_-xSm_xFeO₃, Bi1_-xNd_xFeO₃film andBi1_-xTb_xFeO₃films were prepared on the FTO substrates after process parameterswere determined. Effects of solvent ratio, annealing temperature, buffer layer,and Sm, Nd and Tb-doping on phase composition, surface morphology,ferroelectric and dielectric properties of the films were analyzed. The mainresults were as follows:The morphology of different solution ratios had a great difference. Uniformand dense films were obtained with the solvent ratios of2:1and4:1. Theremanent polarization and coercive applied field of BiFeO₃films were0.81μC/cm²and80KV/cm respectively with the solvent ratio of4:1. Thedielectric constant was very stable. It remained about at125. Under the testingelectric field of100KV/cm, the leakage current step remained10-^-5A/cm². Withincreasing annealing temperature, the surface roughness of the films graduallydecreased, but at the same time the surface undulations increased. It resultedfrom the increase of internal residual stress in BiFeO₃films. At the frequencyrange of1kHz to1MHz the dielectric constant remained above125. The best stability was observed in the films of BiFeO₃films annealed at580℃. Minimumdielectric loss at1MHz was obtained in the films of BiFeO₃films annealed at550℃, indicating that the best interface and fewest defects near the interfacelayer.Increasing thickness could enhance the dielectric properties of thin films.Hysteresis loops gradually were deformed to a certain degree and symmetry ofhysteresis loops decreased with the increase of the test voltage. Many dipolesdefects existed near the interface between the films and the substrates. So thedomains could be hardly switched. The BiFeO₃films and SnO2were contactedas semiconductor heterojunction and the barrier led to non-return-to-zero leakagecurrent. Sn⁴⁺obtained the electron was translated into Sn²⁺in the annealingprocess. As a result the oxygen vacancy increased. The phenomenon wasdeceased at high reverse bias voltage. A large number of the defects of donor andacceptor defects combined and defect pairs appeared. Thus leakage densityincreased slowly.Sm3+substitution resulted in structure transition in BiFeO₃films. Themorphotropic phase boundary appeared. Morphotropic phase boundary appearedin9%at Sm³⁺doped BiFeO₃films, near which the best ferroelectric propertiesand saturated hysteresis loops were shown, with the largest Pr of70μC/cm². Thedielectric constant of9%at Sm3+doped BiFeO₃films was the maximum. It was200at10kHz. The morphotropic phase boundary appeared in9%at Sm3+dopedBiFeO₃films, near which structural changes caused a large number of structuraldefects and resulted in large leakage density. The leakage density was increasedto10^-2A/cm²at200kV/cm. Anti-ferromagnetic suddenly increased, and Mrreached the maximum of1.31emu/cm3. Ms was7.8emu/cm3. The aging in6%atSm3+doped BiFeO₃films led to the decrease of dielectric and ferroelectricproperties as well as the dielectric loss and leakage current. In this paper, thedoping amount leading to morphotropic phase boundary differed from the resultsreported in the literature. It indicated that the lattice mismatch and stress betweenthe substrates and the films affected phase change, the structural transition andthe morphotropic phase boundary.The similar phenomenon also appeared in the Bi1_-xNd_xFeO₃andBi1_-xTb_xFeO₃films. In the XRD patterns of15%at Nd³⁺and10%at Tb³⁺doped BiFeO₃films abnormal inflection point also appeared, and structure transitioncould be observed. The appearance of morphotropic phase boundary also led tothe best ferroelectric properties and the largest dielectric constant. Pr were64μC/cm²and56.1μC/cm²respectively. In the15%at Nd³⁺and10%at Tb³⁺doped BiFeO₃films, maximum dielectric constants were180and170at10kHzrespectively. The phenomena of increasing the leakage density were notobserved in10%at Tb³⁺doped BiFeO₃films, which resulted from structuredefects near the morphotropic phase boundary. It might refer to the differentstress divergent mechanisms.In summary, good dielectric and ferroelectric properties were obtained throughthe adjustment of process parameters and doping in this paper. The electricalproperties of BiFeO₃films were enhanced and the emergence of themorphotropic phase boundary was observed. It laid the foundation for thein-depth research.