There are so many interesting materials which can be applied for energy-efficient windows. (1) Vanadium dioxide (VO₂) is one of the most attractive thermochromic materials, which shows large changes in optical and electrical properties at around 68°C, nearly room temperature. This thermochromic behavior has been explained in terms of the Mott-Hubbard transition from a high-temperature rutile structure (metal phase) to a low-temperature monoclinic structure (semiconductor phase). We already reported that r.f. magnetron sputtering using V₂O₃ or V₂O₅ targets enable us to deposit polycrystalline thermochromic VO₂ films with high reproducibility by introduction of oxygen gas (O₂/Ar+O₂=1～1.5%) or hydrogen gas (H₂/Ar+H₂=2.5～10%), respectively, as reactive gases. (2) However, the thickness of the VO₂ films should be larger than 400 nm because stoichiometry x of the VO_x film deposited on glass substrate was higher than 2 (slightly over-oxidation state) in the early stages of the film growth. In this study, ZnO polycrystalline films were deposited as a buffer layer between the VO₂ film and glass substrate also by r.f. magnetron sputtering, which have been known to exhibit <001> preferred orientation in the wide range of the deposition conditions. Very thin thermochromic VO₂ films with thickness of 70 nm were successfully deposited on the ZnO coated glass substrate because of the heteroepitaxial relationship of VO₂(010)[100]//ZnO(001)[100],[010],[ī ī 0]. (3) Such thin VO₂ films could be applied for the “smart windows” of the high total energy efficiency in various architectures or automobiles.

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