Bipolar and unipolar resistive switching in cu-doped SiO₂

Scalable nonvolatile memory devices that operate at low voltage and current, exhibit multilevel cell capability, and can be read nondestructively using simple circuitry, are highly sought after. Such devices are of particular interest if they are compatible with back-end-of-line processing for CMOS integrated circuits. A variety of resistance-change technologies show promise in this respect, but a new approach that is based on switching in copper-doped silicon dioxide may be the simplest and least expensive to integrate. This paper describes the characteristics of W-(Cu/SiO₂)-Cu programmable metallization cell (PMC) devices formed by the thermal diffusion of Cu into deposited SiO₂. PMC devices operate by the electrochemical control of metallic pathways in solid electrolytes. Both unipolar and bipolar resistive switching could be attained in these devices. Bipolar switching, which is identical to that seen in PMC devices based on other solid electrolytes, was observed for low bias (a few tenths of volts) and programming currents in the microampere range. The resistance ratio between high and low states was on the order of 10³, and a multibit storage is considered possible via the strong dependence of ON-state resistance on programming current. The low and high resistance states were stable for more than 5 × 10⁴ s. The devices could be made to exhibit unipolar switching using a negative bias on the order of -1 V combined with erase currents of hundreds of microampere to a few milliampere. In this case, the OFF/ON ratio was 10⁶.