Room Temperature Insulator‐to‐Metal Transition of VO2/TiO2 Epitaxial Bilayer Films Grown on M‐plane Sapphire Substrates

Abstract Since the optoelectronic properties of VO 2 are modulated around the critical temperature ( T c ) of insulator‐to‐metal transition, VO 2 is a promising candidate material for smart window. However, the T c of bulk VO 2 is rather high 341 K (68 °C) and therefore, needs to be decreased down to near temperature for practical applications. Although T c can be reduced to room temperature if rutile TiO 2 crystal is used as the substrate, it is not technologically viable for large‐scale applications because of the size limitation of available TiO 2 crystal. Here, this work shows that the T c can be reduced to near room temperature using M‐plane sapphire as the substrate. VO 2 /TiO 2 bilayer films are fabricated with varied thicknesses on M‐plane sapphire [(100) α‐Al 2 O 3 ] substrates. The 5.5‐nm‐thick VO 2 film on 200‐nm‐thick TiO 2 buffered sapphire substrate shows clear insulator‐to‐metal transition at ≈305 K (32 °C). It is also found that the insulator‐to‐metal transition is sensitive to the in‐plane lattice distortion, which induces carrier generation. The systematic study on the effect of in‐plane lattice distortion on the insulator‐to‐metal transition of VO 2 will be useful for the practical application of VO 2 as an active material of smart window.