Nonhysteretic Phenomena in the Metal-Semiconductor Phase-Transition Loop of VO2 Films for Bolometric Sensor Applications

Hysteresis observed in the resistive semiconductor-to-metal phase transition in VO2 causes problems in bolometric readout, and thus is an obstacle in utilizing this strong phase transition in bolometric sensor applications. It is possible to avoid the unwanted hysteresis when operating in limited temperature ranges within the hysteresis loop of VO2. Nonhysteretic branches (NHB-s) traced in such limited temperature intervals turned out to have much higher temperature coefficient of resistance (TCR) than VO2 at room temperature: while TCR at 25 degrees C in VO2 is close to 3%, peak TCR values in NHB-s reach 6% in VO2 films on Si/SiO2 substrates and 21% in films on crystalline sapphire substrates. At the same time, the nanoscopic-scale mixture of semiconducting and metallic phases in VO2 within its hysteresis loop provides for partially shunted low resistivity, thus creating an unprecedented combination of record high semiconducting TCR and metal-like low resistance. This combination may benefit the uncooled focal plane array microbolometer IR visualization technology.