Self-polarization Achieved by Compositionally Gradient Doping in BiFeO₃ Thin Films

BiFeO3 is a highly promising lead-free ferroelectric material, surpassing most conventional ferroelectric materials in terms of the polarization and Curie temperature, offering a pathway for potential applications at elevated temperatures.Nevertheless, challenges arise due to strong clamping effect of substrate, large coercive fields, and high leakage currents, making BiFeO3 films difficult to be polarized.The implementation of self-polarization presents a viable solution, wherein the ferroelectric polarization in the film spontaneously arranges without the need for external electric field excitation.In general, self-polarization phenomenon is primarily attributed to the built-in electric field, originating from defects, flexoelectric and interfacial electrode effects.Herein, we prepared BiFeO3, up-graded films (which transition from BiFeO3 to Bi0.80Ca0.20FeO2.90 from the substrate to the film surface), and down-graded films (which transition from Bi0.80Ca0.20FeO2.90 to BiFeO3 from the substrate to the film surface) using the Sol-Gel method on Pt(111)/Ti/SiO2/Si substrates.By carefully modulated the directional distribution of defects within the film, the BiFeO3 films are self-polarization induced by build-in electric field.Piezoresponse force microscopy results show that the upward and downward self-polarization behavior can be modulated by the gradient direction of Ca in BiFeO3 thin films.Moreover, diode-like current-voltage signature verify the composition gradient induced self-polarization.The X-ray photoelectron spectroscopy results indicate that the polarization orientation mechanism may arise from the internal electric field attributed to the gradient distribution of oxygen vacancy.This work provides a new strategy to achieve self-polarization in ferroelectric thin films, as well potential novel application in improving the performance of photovoltaic or photosensitive devices as assisted by internal field via self-aligned ferroelectric polarization.