Superconducting Filaments Formed During Nonvolatile Resistance Switching in Electrodeposited δ-Bi₂O₃

We show that electrodeposited films of δ-Bi2O3 in a Pt/δ-Bi2O3/Au cell exhibit unipolar resistance switching. After being formed at a large electric field of 40 MV/m, the cell can be reversibly switched between a low resistance state (156 Ω) and a high resistance state (1.2 GΩ) by simply cycling between SET and RESET voltages of the same polarity. Because the high and low resistance states are persistent, the cell is a candidate for nonvolatile resistance random access memory (RRAM). A Bi nanofilament forms at the SET voltage, and it ruptures to form a 50 nm gap during the RESET step at a current density of 2 × 10(7) A/cm(2). The diameter of the Bi filament is a function of the compliance current, and can be tuned from 140 to 260 nm, but the current density in the RESET step is independent of the Bi diameter. An electromigration rupture mechanism is proposed. The Bi nanofilaments in the low resistance state are superconducting, with a Tc of 5.8 K and an Hc of 5 kOe. This is an unexpected result, because bulk Bi is not a superconductor.