In Situ Nanostructural Analysis of Volatile Threshold Switching and Non-Volatile Bipolar Resistive Switching in Mixed-Phased a-VOx Asymmetric Crossbars

Vanadium oxide is a multi-functional, phase-change material that shows both apolar threshold switching (TS) and bipolar resistive switching (BRS) in a mixed-phase film. However, the origins of its multifunctional behavior and respective switching mechanisms are still unproven. Here, asymmetric cross-point devices are introduced based on an amorphous mixed-phase vanadium oxide film. The devices show condition-dependent volatile TS post-electroforming as well as electroforming-free non-volatile BRS. An interesting two-step volatile TS is also observed post-electroforming in few devices. This work proves that the volatile TS is due to the insulator-to-metal (IMT) transition in local crystal islands of vanadium dioxide based on the electrical in situ transmission electron microscopy and highlights origins of two-step volatile TS based on nanostructural changes. This insight will not only pave a path to adopt vanadium oxide in crossbar arrays and control its functionality with electrical conditions, but also highlights the process of crystallizing vanadium dioxide just with electrical bias without any high temperature annealing. Stimuli-triggered or condition-dependent switching control of memory elements will enable programmable logic and neuromorphic circuits incorporating vanadium oxide.