Low temperature synthesis of VO 2 and hysteresis free VO x thin films with high temperature coefficient of resistance for bolometer applications

Vanadium dioxide (VO 2 ) is an insulator to metal phase transition material, which finds many electronic and optical applications due to its reversible switching while VO x -based bolometers are widely used in commercial uncooled infrared detectors due to their high-temperature coefficient of resistance and low resistivity. Atmospheric pressure thermal oxidation (APTO) of Vanadium (V) is a simple process used to synthesize VO 2 films without the need for precise control of oxygen partial pressure unlike other synthesis methods. In this work, we report the effect of reducing the V oxidation temperature to 250–450 °C during APTO and synthesize films with different oxidation durations. For an optimized oxidation duration at each oxidation temperature, VO 2 films are obtained with more than two orders of resistance switching. This optimum oxidation duration is found to decrease exponentially with increase in oxidation temperature with an activation energy of 1.05 eV. The phase transition temperature of these VO 2 films is found to decrease monotonically with decrease in oxidation temperature. We also observe that partially oxidized (oxidation time less than optimized oxidation duration to form VO 2 ) VO x films show broad resistance switching from room temperature up to 70 °C instead of steep switching at the phase transition temperature. The partially oxidized film shows a temperature coefficient of resistance (TCR) as high as ∼10%/K, which is 3–4 times greater than TCR of commercial VO x thin films used in bolometers while showing similar resistivity. The resistivity of these films also exhibits almost zero hysteresis with temperature, an important requirement for bolometer applications. This study will widen the usefulness of the APTO process for fabricating VO 2 -based devices and high TCR VO x films for bolometer applications. The demonstrated low temperature process will especially be suitable to integrate VO 2 /VO x on substrates which can sustain only low temperatures such as some of the flexible substrates.