Low-temperature growth of environmentally stable epitaxial VO2 thin films

Abstract

The requirement of substantially higher growth temperature and post-deposition annealing for epitaxial growth of vanadium oxide (VO2) films has always remained a bottleneck for their practical applicability in various thermochromic smart devices. Here, we report epitaxial growth of vanadium oxide (VO2) thin films on sapphire substrates using the reactive gas pulsing technique (RGPT) in conjunction with intense plasma activation facilitated by asymmetric bipolar pulse DC (ABPDC) sputtering. The deposition methodology followed in this work utilizes growth temperature as low as 275 °C as opposed to the substantially higher temperatures (>500 °C) reported in previous works. The process also avoids any post-processing of the films including annealing. The films deposited in RGPT mode show good environmental stability over a long time range using a simple deposition methodology without placing any top barrier layers to stop ageing. Investigation of the semiconductor-to-metal transition (SMT) behavior reveals a very sharp reversible first-order phase transition with sharpness ∼2 °C (very close to bulk single crystals) and switching ratio of ∼1 × 104 achieved with the RGPT mode. The epitaxial relationship of [010]VO2 ǁ [0001]Al2O3 (out-of-plane) and [001]VO2 ǁ [11-20]Al2O3 (in-plane) is established through the measurement of in-plane and out-of-plane microstructural characterization. Overall, this work presents a reliable method for the low temperature growth of epitaxial VO2 thin films with good environmental stability and excellent semiconductor–metal switching performance.