Electro-Thermal Characterization of Conductive Filament in Vanadium Dioxide Memristors Using Thermoreflectance Imaging

Abstract Vanadium dioxide (VO2) is attracting growing interest for memristive applications in neuromorphic circuits; however, its electro-thermal behavior, critical for device reliability, scalability, and thermal management, remains incompletely understood. Here, we present a charge-coupled device (CCD) thermoreflectance-based methodology calibrated for directly characterizing phase change materials such as VO2. Applied to VO2 memristor with a channel area of 3 × 6 μm2, it unambiguously reveals a spatially localized, conductive “hot” filament bridging the electrodes. The VO2 device response to current excitation is investigated across the insulating and metallic states. Near the insulator-to-metal transition, a non-polynomial thermal profile emerges, indicating Joule-induced filament percolation. In the metallic state, the filament width scales linearly with current, increasing from 0.8 to 11.3 μm, providing insight into the current and heat spreading. The maximal filament temperature evolves modestly from 63 to 70 °C despite raising the current from 2 to 20 mA. This slight increase of the filament temperature appears counterintuitive from a conventional Joule heating perspective. This behavior is explained with a 2D simulation of the sheet current density being uniform in the metallic filament with a clear plateau during the filament’s growing regime. We explain these behaviors through the complex interplay between dissipated electrical power and effective heat-exchange area, using power density as a figure of merit. By directly visualizing the current-induced thermal filament and its evolution, this work establishes thermoreflectance as a powerful tool for assessing the electro-thermal relationship in VO2-based memristive devices, contributing to their reliable design, modeling, and scalable integration.