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

Abstract

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)/NbO_x/Nb(O) devices with respect to deposition and post-annealing conditions of NbO_x layers. The device with NbO_x deposited by reactive sputtering with 12% of O₂ 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 NbO_x deposited in the reduced fraction of 7% of O₂ gas and subsequently annealed at 250 °C in vacuum, thanks to the increase of the amount of conducting phases within the NbO_x 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 NbO_x 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 ∼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)/NbO_x/Nb(O) devices by controlling the contents of conducting phases in the NbO_x layer for the application to selector devices in high-density crossbar memory and synapse array architectures.