Lost in Transition: VO₂’s Wild Ride

The semiconductor-to-metal transition (SMT), in addition to being an interesting phenomenon in its own right, is a highly useful effect that can be leveraged in switchable opto-electronic devices. Here we study the SMT exhibited by polycrystalline VO₂ thin films deposited by sputtering a vanadium target in an O₂+Ar ambient followed by annealing at 400°C in N₂. We use wide spectral range ellipsometry spanning the infrared to ultraviolet to characterize the complex optical response of VO₂ thin films in situ over a range of temperatures crossing the SMT. The parametric optical properties fully describe the modification of spectral features as the temperature changes. Ultimately, these results are used to effectively track switching between semiconducting and metallic states in situ, in real time, as the film is thermally cycled across the SMT.