Imaging current filaments created by the metal-insulator transition in vanadium dioxide

Vanadium dioxide (VO₂) undergoes a reversible metal-insulator transition at 67 °C, changing from a semiconductor at low temperatures to a metal at high temperatures. The crystal structure changes from monoclinic to rutile and this structural change is sufficient to modify the bandstructure. Current biasing a thin film of VO₂ shows high resistivity Ohmic behavior at low currents, and at higher currents the resistance suddenly drops as the Joule heating is sufficient to trigger the metal - insulator transition. At this point the heating decreases allowing sections of the material to revert to a semiconductor, concentrating the current within a metallic filament. At intermediate currents the metallic filament forms but the low resistance path does not dissipate enough heat to maintain it, and thermal oscillations are observed where the filament repeatedly forms and collapses. In this work we image a metallic filament within a current biased thin film VO₂ sample using a laser scanning microscope. We observe the evolution of the filament in time when the device is oscillating, following the formation and collapse of the filament, and measure the time scales for each process.