Preparation and Characterization of Epitaxial VO2 Films on Sapphire Using Postepitaxial Topotaxy Route via Epitaxial V2O3 Films

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Epitaxial VO2 films were prepared on the $C$-planes of $\alpha$-Al2O3 substrates by a metal organic deposition (MOD) process. It was difficult to obtain the single phase of (010)M-oriented VO2 films, in which the subscript M refers to the monoclinic indices, by the heat treatment of amorphous precursor films in the VO2-stable region after the pyrolysis of the coating solution. The product films consisted of discontinuous circular grains of 1–2 μm size on the substrate surface. Therefore, we prepared the (010)M-oriented epitaxial VO2 films using postepitaxial topotaxy (PET), that is, topotactic oxidation of (0001)-oriented epitaxial V2O3 films. First, epitaxial V2O3(0001) films were obtained by MOD starting with a vanadium naphthenate solution. Second, the epitaxial V2O3(0001) films were topotactically oxidized at 500 °C in an Ar–O2 gas mixture with $ p\text{O$_{2}$} = 10^{-4}$ atm to obtain (010)M-oriented epitaxial VO2 films. The epitaxial relationships were VO2(010)M $\parallel$ $\alpha$-Al2O3(0001) and VO2[100]M $\parallel$ $\alpha$-Al2O3[$01\bar{1}0$], [$\bar{1}010$], [$1\bar{1}00$]. The VO2(010)M films exhibited metal–semiconductor transitions with hysteresis loops at 60–80 °C. The resistivity change before and after the transition of the VO2(010)M film oxidized for 6 h was three orders of magnitude.
2008-02-25