Tuning the Resistance of a VO2 Junction by Focused Laser Beam and Atomic Force Microscopy

Vanadium Dioxide (VO₂) is a material that exhibits a phase transition from an insulating state to a metallic state at ≈68 °C. During a temperature cycle consisting of warming followed by cooling, the resistivity of VO₂ changes by several orders of magnitude over the course of the hysteresis loop. Using a focused laser beam (λ = 532 nm), it is shown that it is possible to optically generate micron-sized metallic patterns within the insulating phase of a VO₂ planar junction which can be used to tune, on demand, the resistance of the VO₂ junction. A resistor network simulation is used to characterize the resulting resistance drops in the devices. These patterns persist while the base temperature is held constant within the hysteretic region while being easily removed totally by simply lowering the base temperature. Surprisingly, it is also observed that the pattern can be partially erased using an atomic force microscope (AFM) tip on the submicron scale. This erasing process can be qualitatively explained by the temperature difference between the VO₂ surface and the tip which acts as a local cooler. This optical and AFM resistive fine-tuning offers the possibility of creating controllable synaptic weights between room-temperature VO₂ neuristors.