Vanadium dioxide (VO2) has been extensively investigated due to
its thermal-induced metal-insulator transition (MIT) at about 67
oC; 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.
