Thermochromic VO₂ Films Deposited by RF Magnetron Sputtering Using V₂O₃ or V₂O₅ Targets

Thermochromic monoclinic-tetragonal VO₂ films were successfully deposited on glass substrates with high reproducibility by rf magnetron sputtering using V₂O₃ or V₂O₅ targets. In the case of reactive sputtering using a V-metal target, the VO₂ films could be obtained only under the very narrow deposition conditions of the "transition region" where the deposition rate decreases drastically with increasing oxygen gas flow rate. In the case of a V₂O₃ target, polycrystalline VO₂ films with a thickness of 400 to 500 nm were obtained by the introduction of oxygen gas [O₂/(Ar+O₂)=1–1.5%], whereas hydrogen gas [H₂/(Ar+H₂)=2.5–10%] was introduced in the case of a V₂O₅ target. Furthermore, the VO₂ films were successfully grown heteroepitaxially on a single-crystal sapphire [α-Al₂O₃(001)] substrate, where the epitaxial relationship was confirmed to be VO₂(010)[100]∥Al₂O₃(001)[100], [010], [110] by an X-ray diffraction pole figure measurement. The resistivity ratio between semiconductor and metal phases for the heteroepitaxial VO₂ films was much larger than the ratio of the polycrystalline films on glass substrates under the same deposition conditions.