Electroforming-free threshold switching of NbOx-based selector devices by controlling conducting phases in the NbOx layer for the application to crossbar array architectures

Bipolar threshold switching characteristics, featuring volatile transition between the high-resistance state (HRS) at lower voltage than threshold voltage (V-th) and the low-resistance state (LRS) at higher voltage irrespective of the voltage polarity, are investigated in the Nb(O)/NbOx/Nb(O) devices with respect to deposition and post-annealing conditions of NbOx layers. The device with NbOx deposited by reactive sputtering with 12% of O-2 gas mixed in Ar shows threshold switching behaviors after electroforming operation at around +4 V of forming voltage (V-f). On the other hand, electroforming-free threshold switching is achieved from the device with NbOx deposited in the reduced fraction of 7% of O-2 gas and subsequently annealed at 250 degrees C in vacuum, thanks to the increase of the amount of conducting phases within the NbOx layer. Threshold switching is thought to be driven by the formation of a temporally percolated filament composed of conducting NbO and NbO2 phases in the NbOx layer, which were formed as a result of the interaction with Nb electrodes such as oxygen ion migration either by annealing or electrical biasing. The presence of a substantial amount of oxygen in the Nb electrodes up to similar to 40 at%, named Nb(O) herein, would alleviate excessive migration of oxygen and consequent overgrowth of the filament during operation, thus enabling reliable threshold switching. These results demonstrate a viable route to realize electroforming-free threshold switching in the Nb(O)/NbOx/Nb(O) devices by controlling the contents of conducting phases in the NbOx layer for the application to selector devices in high-density crossbar memory and synapse array architectures