Internal Electric Field and Polarization Backswitching Induced by Nb Doping in BiFeO₃ Thin Films

We report an effective internal electric field induced by Nb doping in BiFeO3 (BFO) thin films and the consequent effects on polarization backswitching and conduction behaviors. Bi(Fe1–xNbx)O3 (x = 0, 0.3, 1, 3%) thin films were epitaxially grown on SrRuO3-buffered SrTiO3 substrates. The obtained square-like polarization hysteresis loops show good ferroelectricity for all the thin films, but with increasing Nb content, the polarization loops shift gradually along the positive voltage axis (i.e., an imprint-like behavior). This manifests an effective internal electric field in the films induced by the Nb donor doping. Furthermore, it is demonstrated that the electrically switched downward domains in the 3% Nb-doped BFO thin films can switch back rapidly to upward when the applied voltage is removed. The internal electric field may drive the polarization backswitching, resulting in distinct evolutions in the ferroelectric switchable diode and resistive switching behaviors. The internal electric field can be well ascribed to the oriented defect dipoles consisting of charged donor ions and cation vacancies. This study has direct implications for engineering charged defects to manipulate the functional properties of ferroelectric thin films.