Threshold switching stabilization of NbO₂ films via nanoscale devices

The stabilization of the threshold switching characteristics of memristive $NbOx$ is examined as a function of sample growth and device characteristics. Sub-stoichiometric $Nb2O5$ was deposited via magnetron sputtering and patterned in nanoscale (⁠$50×50$–$170×170nm2$⁠) W/Ir/$NbOx$/TiN devices and microscale (⁠$2×2$–$15×15μm2$⁠) crossbar Au/Ru/$NbOx$/Pt devices. Annealing the nanoscale devices at 700 $°$C removed the need for electroforming the devices. The smallest nanoscale devices showed a large asymmetry in the IV curves for positive and negative bias that switched to symmetric behavior for the larger and microscale devices. Electroforming the microscale crossbar devices created conducting $NbO2$ filaments with symmetric IV curves whose behavior did not change as the device area increased. The smallest devices showed the largest threshold voltages and most stable threshold switching. As the nanoscale device area increased, the resistance of the devices scaled with the area as $R∝A−1$⁠, indicating a crystallized bulk $NbO2$ device. When the nanoscale device size was comparable to the size of the filaments, the annealed nanoscale devices showed similar electrical responses as the electroformed microscale crossbar devices, indicating filament-like behavior in even annealed devices without electroforming. Finally, the addition of up to 1.8% Ti dopant into the films did not improve or stabilize the threshold switching in the microscale crossbar devices.