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,causing BiFeO3 films difficult to be polarized.The implementation of self-polarization presents a viable solution.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.After directional distribution of defects within the film being carefully modulated,the BiFeO3 films are self-polarization when induced by build-in electric field.Piezoresponse force microscopy show that the up-graded and down-graded self-polarization behavior can be modulated by gradient direction of Ca in BiFeO3 thin films.Moreover,diode-like current-voltage signature verifies 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.
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