High-resolution full-field structural microscopy of the voltage-induced filament formation in VO2-based neuromorphic devices

Abstract

Neuromorphic functions in memristive devices, linked to electrically induced resistive switching to form conductive pathways, could be made more efficient by insights into filament formation at micro- and mesoscopic levels. Despite extensive research on VO2, a key material for its filament formation, local operando structural measurements are highly challenging and often involve destructive specimen preparation and long rastering time, which greatly limits the scope of experimental studies. Utilizing dark-field X-ray microscopy (DFXM), a novel full-field X-ray imaging technique, we explored structural signatures of the filament formation process operando in VO2 devices. Our DFXM experiments display that rutile filaments contain isolated monoclinic clusters, indicating structural non-uniformity of the filament. We also show that the formation of the rutile phase beneath device electrodes precedes filament development, with filament paths then guided by nucleation sites within the device. Finally, we observed a new medium-term memory mechanism in VO2 mediated by sites that tend to switch at significantly lower voltages after electrical cycling, a tendency that persists through brief cooling. High spatial resolution, large field-of-view, structure selectivity, and fast signal acquisition of DFXM enabled first-of-their-kind measurements providing unprecedented insight into subtle structural features of the filamentary channel and surrounding regions during voltage cycling.