Contracting current paths in vanadium dioxide thin films

Vanadium dioxide (VO₂) has been extensively investigated due to its thermal-induced metal-insulator transition (MIT) at about 67 °C; which can be lowered by doping. During the transition from semiconductor to metallic phase electrical conductivity can increase by up to 3-4 orders of magnitude, while optical reflectance can drop by almost 50 %. Possible applications include thermally controlled electrical and optical switches. As the thin film is conductive optical changes can be induced electrically by Joule-heating. In this paper we present electro-optical measurements carried out on 100 nm thick VO₂ thin films deposited on sapphire substrates. Phase transitions were electrically induced in contacted films and observed using an optical microscope. Strong spatial inhomogeneity has been detected: a few micron wide dark path connected the two contacts. The visible channel is a metallic area within the semiconducting film as the domain undergoes a MIT because of the Joule-heat of the flowing current. The optically observable fine structure found suggests that VO₂ thin films could be used to visualize isotherms with a resolution of a few microns, similar to that of liquid crystals. Possible benefits include the simple and reliable use of the once deposited solid phase precision films and a higher resolution compared to liquid crystal thermography.