Variations in optical reflectivity in the semiconductor–metal phase transition of vanadium dioxide

Crystalline vanadium dioxide (VO2) is a material with a phase transition at a critical temperature of 67 °C [Metal–Insulator Transitions, Taylor & Francis, London, 1974, Nauka, Moscow, 1979; The Metal–Semiconductor Phase Transition and its Applications, Nauka, Leningrad, 1979, p. 183]. However this semiconductor–metal phase transition takes place at various temperatures Tc+ΔT, depending on the grain size in the crystal [Fis. Tverd. Tela (St. Petersburg) 36 (1994) 1643]. This ΔT in VO2 can have different values: 1–2 °C in a single crystal and 20–30 °C in a poly-crystalline layer. The inverse transition, from metal to semiconductor, takes place at Tc−ΔT temperature. As a result, hysteresis of the physical properties exists with a width of 2ΔT, and with various shapes of the ring [Tech. Phys. 47 (9) (2002) 1134]. In this work, we study the hysteresis loops of the optical reflectivity and conductivity of amorphous vanadium dioxide films around the semiconductor-metal phase transition.We demonstrate that a small number (2–3) of thermal cycles suppresses the phase transition, possibly due to the diffusion of oxygen from VO2-clusters into adjacent clusters, consisting of the lower oxides of a Magnelli-series. Results of these studies as well as the AFM-data, indicate the high optical quality of polycrystalline vanadium dioxide films obtained by annealing amorphous VO2-films, that can be used for application in interferometers and optical limiters.