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

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.

Suppression of Strain Relaxation in VO2/TiO2 Multilayered Films

“…Because the very small lattice mismatch (∼0.9%) between (001) R–VO 2 and (001) R–TiO 2 stabilizes the R–VO 2 lattice at room temperature using a (001) TiO 2 substrate significantly reduces the T c of an epitaxial VO 2 film . Additionally, a similar effect has been experimentally demonstrated using a TiO 2 buffer layer grown on another substrate. ,, …”

“…However, the transition temperature ( T c ) of bulk VO 2 (∼68 °C) is still too high for use in practical applications. In the past few decades, many studies have investigated strategies to reduce T c of VO 2 . ,− …”

“…8 Additionally, a similar effect has been experimentally demonstrated using a TiO 2 buffer layer grown on another substrate. 10,15,16 However, one critical issue has yet to be resolved. A high strain condition is needed to stabilize R−VO 2 , but strain relaxation occurs when a critical thickness (t c ∼15 nm) is exceeded.…”

Photothermal Effect and Phase Transition in VO2 Enhanced by Plasmonic Particles

“…The correlation of MIT shift and mechanical stresses in VO 2 /TiO 2 films was reported by several authors. However, a related MIT is typically shifted to a lower temperature [ 38 , 39 ]. Therefore, YX-128° cut LiNbO 3 substrate likely results in mechanically induced MIT shift to a higher temperature.…”

Multiphysical Field Modulated VO2 Device for Information Encryption

“…According to the differential curve in the insert, it can be observed that the phase transition temperature in the heating process is 67.7 °C, and that in the cooling process is 62.3 °C, which is very consistent with the previous report. [40,41] Figure 1d shows the SEM cross-section of the film, where a distinct difference between the VO 2 film and the alumina substrate can be observed. The Surface topography of the VO 2 sample was also examined by SEM and AFM as shown in Figure 1e and Figure S1, Supporting Information.…”