Microscopic physical origin of polarization induced large tunneling electroresistance in tetragonal-phase BiFeO₃

Ferroelectric tunnel junctions have attracted intensive research interest due to the fundamental physics and potential applications in high-density data storage and neuromorphic computation. However, the intrinsic physical origin, especially at atomic scale, of polarization-controllable tunneling current is still controversial due to the degradation of ferroelectric polarization and the extrinsic conduction induced by defects or oxygen vacancies. Here, a large tunneling electroresistance effect of over 10,000% in a thick (∼15 nm) tetragonal-phase BiFeO₃ thin film is observed, where a nanoscale point-contact geometry is delicately designed to reduce the extrinsic defect effects. By combining transmission electron microscopy and first-principles calculations, the atomic and electronic structures of BiFeO₃ tunneling layer are investigated. The corresponding results indicate the different charge transfer occurs at the top and bottom interface, which induces distinct tunneling barrier asymmetry when the polarization direction is opposite.