Coexistence of unipolar and bipolar resistive switching in BiFeO₃ and Bi_0.8Ca_0.2FeO₃ films

Ferroelectric BiFeO₃ and paraelectric Bi_0.8Ca_0.2FeO₃ polycrystalline films were prepared to study the dependence of resistive switch on defect density. With defect density and the corresponding leakage current increasing, current-voltage loops allow four different types, i.e., overlapping, hysteresis without memory effect, bipolar resistive switch, and unipolar resistive switch. The first three types can transform to the last one, once electroforming introduces enough charged defects to films and the resistance monotonically increases to a certain value. Unipolar resistive switch is due to conductive filamentary type and can be treated as an especial electroforming process. Furthermore, its high resistance status allows the second or third type at low voltage region.