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

Since the optoelectronic properties of VO2 are modulated around the critical temperature (T-c) of insulator-to-metal transition, VO2 is a promising candidate material for smart window. However, the T-c of bulk VO2 is rather high 341 K (68 degrees 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 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 T-c 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 [(101 over bar 0) alpha-Al2O3] substrates. The 5.5-nm-thick VO2 film on 200-nm-thick TiO2 buffered sapphire substrate shows clear insulator-to-metal transition at approximate to 305 K (32 degrees 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.