Vanadium dioxide is an interesting material that reversibly switches between a semiconductor and a metal at a temperature of 68 C.  In the semiconducting state it is a cubic crystal, and in the metallic state it is a hexagonal crystal. The switching produces large changes in its resistivity, optical constants and stress. Most of the commercially available uncooled thermal cameras contain a variant of VO2. The sensing element is VOx, which is a mixture of several different vanadium oxides. Instead of rapidly switching at 68 C, it exhibits a slow switching, which is better suited as a temperature sensing element.

Our interest in VO2 is for its applications in optical limiters and switchable polarizers. Optical limiters are front-end components that protect sensitive electronics and cameras from high energy laser threats. The temperature rise in the VO2 film can make the film switch from a highly transparent state to a highly reflective state. But VO2 is not easy to make. The process window is narrow because vanadium has different stable oxidation states besides VO2. Deposition is typically done at substrate temperatures of 500 C and a narrow range of oxygen partial pressure. We are using a novel method where metallic vanadium is thermally oxidized in a vacuum furnace.  Compared to conventional physical vapor deposition methods, our method makes it easier to pattern the vanadium film, and also produce these films in large batch quantities.

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