Self-polarization Behavior Of BiFeO₃ Thin Film Prepared By Sol-gel Process

The electric domains in the prepared ferroelectric thin films are randomly distributed in the absence of an applied electric field. When applied with an outer electric field, the ferroelectric material is polarized and the domains are highly-oriented. However, only when there were electrodes conducting the voltage into the material, the material would be polarized. On the other hand, some kinds of ferroelectric materials can not be polarized due to their low resistance, high leakage current and high coercive field at room temperature. Therefore it is so important to fabricate ferroelectric thin films with the characteristics of self-polarization that it attracts great attention of researchers. Self-polarization behavior means that the ferroelectric materials show macro polarization without the help of the outer electric filed nor electrodes. The self built-in field in the self-polarized ferroelectric thin film can separated the electros and holes inspired by the photons of sunlight, so it has great applications in photo catalysis and photovoltaic devices. BiFeO₃ is the only ferroelectric material that shows multi-ferroic phenomenon at room temperature. Therefore, we studied BiFeO₃ thin film and the influence of donor-acceptor co doping on its structure, properties and self-polarization behavior.Pure BiFeO₃ and(Li_0.5Nb_0.5)_xBi_1-xFeO₃, Li⁺-Nb⁵⁺ co-doped BiFeO₃ thin films, were prepared by the sol-gel process and influence of process parameter on microstructure, morphologies, electric properties and the self-polarization behavior was investigated and we mainly discussed about the self-polarization behavior via I-V curve testing and piezoelectric power microscope analysis.The specific preparing process determined by its influence on the properties is as follows: Bi was 5mol% excess,（Bi（NO₃）₃·5H₂O）was dissolved in the solution of ethylene glycol dimethylether and acetic acid with a ratio of 3:10, the stabilizing agent here was glycerol, which made the gel more stable and improved the quality of the thin film. LaNiO₃ seed layer could lead to an obvious（010）orientation with a smaller surface roughness, lower dielectric loss and more obvious rectification phenomenon so that LaNiO₃ was chosen as the seed layer in this paper whose annealing temperature was 650℃.Li⁺-Nb⁵⁺pairs and defect dipoles in the film tend to form a regular arrangement which is controlled by the thermal or other kinds of strains and generate a local inner electric filed. Then BiFeO₃ thin film will be poled by that electric field and self-polarization behavior happens.(Li_0.5Nb_0.5)_xBi_1-xFeO₃ thin film when x=5% had the best properties including well-（010） orientation with smaller grain size, lower dielectric loss, super obvious rectification act, significant domain structure and so a more enhanced self-polarization behavior, meaning a great magnitude of self built-in field inside the film. The more enhanced the self-polarization behavior, the larger the photocurrent will be. Finally, the doped film with x=5% annealed at 700℃ had a more obvious rectification phenomenon so that it had a stronger self built-in field.