Effect Of Seed Layer On Microstructure And Porperties Of BiFeO₃ Thin Film

Multiferroic materials, which simultanenously exhibit ferroelectricity, ferromagnetism and ferroelasticity. As one of a few single phase magnetoelectric materials, BiFeO₃ manifestes both ferroelectric and antiferromagnetic polarization at room temperature, thus offers significantly potential applications in FeRAM, which have been studied intensively. BiFeO₃ has excellent ferroelectric properties, but BiFeO₃ had been proved to have the poor ferroelectric properties because of high leakage current density. In this work,we added SrTiO₃ seed layer in between the bottom electrode and BiFeO₃ thin film in aiming to not only reduce the leakage current density but also improve the crystallinity of BiFeO₃ film.In this work, a low-cost sol-gel deposition method was used to prepare the BiFeO₃ thin films. The Bi(NO₃)₃·5H₂O, Fe(NO₃)₃·9H₂O, Sr(NO₃)2, Ti(OC₄H₉)₄ were used as precursors and 2-methoxyethanol was used as solvent. Excessive 5% mol Bi (NO₃)₃·5H₂O was added to compensate for bismuth volatility during post-annealing. The SrTiO₃ seed layer was also prepared using sol-gel method. The as-deposited thin films were annealed at 650 oC for 5 min. The XRD, AFM and XPS were used to characterize the microstructure and chemical valence as a function of seed layer thickness. We use ferroelectric testing system to evaluate the effect of seeding layer on electric conduction mechanism and ferroelectric properties.XRD and AFM results revealed that the seed layer did not cause any change of phase composition. Namely, the addition of seed layer does not result in seconrday phase in BiFeO₃ thin films. However, the seed layer induced the texture. As the seed layer thickness increases, the preferred orientation changes from (010) to (110). In addition, XPS results show that SrTiO₃ seed layer also changed the chemical valence of Fe. The Fe²⁺ concentration decreases as the seed layer thickness increases, which means that oxygen vacancy concentration decreases.The introduction of SrTiO₃ seed layer decreases the leakage current destity, conduction mechanism and the ferroelectric properties of the BiFeO₃ thin film. With the SrTiO₃ seed layer thickness increasing, the leakage current density decreases and then increases, peaking at the seed layer thickness of 4 nm. Morever, the conduction mechanism of BiFeO₃ thin films also changed a lot accordingly. Meanwhile, the SrTiO₃ seed layer also caused changes in polarization behavior of BiFeO₃ thin film. As the seed layer thickness increases, the remnant polarization definitely increases. However, there is an optimized seed layer thickness for ferroelectric property due to the leakage current density.