Enhancement in neuromorphic NbO2 memristive device switching at cryogenic temperatures

The electrical properties and performance characteristics of niobium dioxide (NbO$_\mathrm{2}$)-based memristive devices are examined at cryogenic temperatures. Sub-stoichiometric Nb$_\mathrm{2}$O$_\mathrm{5}$ was deposited via magnetron sputtering and patterned in microscale (2$\times$2 - 15$\times$15 $\mu$m$^2$) cross-bar Au/Ru/NbO$_\mathrm{x}$/Pt devices and electroformed at 3-5 V to make NbO$_\mathrm{2}$ filaments. At cryogenic temperatures, the threshold voltage ($V_\mathrm{th}$) increased by more than a factor of 3. The hold voltage ($V_\mathrm{h}$) was significantly lower than the threshold voltage for fast voltage sweeps (200 ms per measurement). If the sample is allowed to cool between voltage measurements, the hold voltage increases, but never reaches the threshold voltage, indicating the presence of non-volatile Nb$_\mathrm{2}$O$_\mathrm{5}$ in the filament. The devices have an activation energy of $E_a \approx 1.4$ eV, lower than other NbO$_\mathrm{2}$ devices reported. Our works shows that even nominally ``bad" memristive devices can be improved by reducing the leakage current and increases the sample resistance at cryogenic temperatures.