Preparation Of Sol-gel-processed BiFeO₃ Thin Films And Their Piezoelectric Property

With the development of electronic devices towards miniaturization and multi-functional,the application of ferroelectric and piezoelectric thin films has been receving increasing attention.Among the ferroelectric materials,BiFeO₃ has been a hotspot in the past decade due to its rich and excellent physical properties,including room-temperature multiferroic,high Curie temperature,large remanent polarization,domain wall conductivity and photocatalytic property etc.Besides,BiFeO₃ is also a promising lead-free material in the field of piezoelectric application.In this work,we prepared BiFeO₃ thin films via sol-gel method.The texture films,randomly oriented films and epitaxial films were obtained by optimizing the process parameters.Then,the effect of different factors on the piezoelectric properties of the films was analyzed by combining the phase structure analysis and characterization of electric properties.By introducing a PbO seeding layer,?001?-textured BiFeO₃ thin films were obtained on platinized Si substrates.A high degree of?001?orientation up to 98.3%was achieved in 200 nm film via a layer-by-layer annealing process.The textured structure was due to the large c/a ratio of PbO and the later layer could inherit the orientation during the layer-by-layer annealing process.Significant enhancements of dielectric,ferroelectric and piezoelectric properties were achieved in textured samples.A group of Sm-doped BiFeO₃ thin films with different Sm contents and thicknesses were fabricated on platinized silicon substrates,and their effects on phase structure and piezoelectricity were investigated.Co-existence of rhombohderal and orthorhombic phases was found around 10%Sm doping compositional vicinity,and piezoelectric performance was enhanced in this region.At 15%Sm doping composition,the phase turned to centrosymmetric Pnma and piezoelectric behavior disappeared.Based on the structure analysis and piezoelectric characterization results,a phase diagram of thickness versus composition was proposed,in which the phase boundary shifted towards Sm-rich side with increasing thickness.The leakage current of BiFeO₃ thin films was reduced by Sm or Ti doping,and it can be further reduced by Sm and Ti codoping,which can be ascribed to the decrement of Fe²⁺ions and oxygen vacancies.Leakage mechanisms of pristine and doped BiFeO₃ films were studied and it revealed that Ti could stabilize the trapped carriers,while Sm might decrease the trap energy and contrarily benefit the excitation of trapped carriers.The codoping would reduce the remenant polairzaiton but enhance the electric breakdown property.Pristine and Sm-doped BiFeO₃ epitaxial thin films were prepared on Nb-doped SrTiO₃?100?single crystal substrates by optimizing the annealing process.The epitaxy was verified by RSM,PFM and TEM.In-situ PFM characterization was employed to investigate domain behavior and piezoelectric properties from 20?to 200?.It revealed that domains became active from 110?to 170?and domain configurations changed obviously,corresponding to the phase transition temperature.A significant enhancement of piezoelectric response was obtained at the phase boundary around 170?,which could be ascribed to the extrinsic contribution.Besides,the film exhibited typical antiferroelctric behavior at 200?,implying that the film turned into the Pbam phase and domain configurations disappeared.