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

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Abstract
Since the optoelectronic properties of VO2 are modulated around the critical temperature (Tc) of insulator‐to‐metal transition, VO2 is a promising candidate material for smart window. However, the Tc of bulk VO2 is rather high 341 K (68 °C) and therefore, needs to be decreased down to near temperature for practical applications. Although Tc can be reduced to room temperature if rutile TiO2 crystal is used as the substrate, it is not technologically viable for large‐scale applications because of the size limitation of available TiO2 crystal. Here, this work shows that the Tc can be reduced to near room temperature using M‐plane sapphire as the substrate. VO2/TiO2 bilayer films are fabricated with varied thicknesses on M‐plane sapphire [(100) α‐Al2O3] substrates. The 5.5‐nm‐thick VO2 film on 200‐nm‐thick TiO2 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 VO2 will be useful for the practical application of VO2 as an active material of smart window.
Funding Information
  • Japan Society for the Promotion of Science

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