Phase transformation and semiconductor-metal transition in thin films of VO2 deposited by low-pressure metalorganic chemical vapor deposition

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Abstract
Thin films of the semiconducting, monoclinic vanadium dioxide, VO 2 (M) have been prepared on ordinary glass by two methods: directly by low-pressure metalorganic chemical vapor deposition(MOCVD), and by argon-annealing films of the VO 2 (B) phase deposited by MOCVD. The composition and microstructure of the films have been examined by x-ray diffraction, scanning electron microscopy, and transmission electron microscopy. Films made predominantly of either the B or the M phase, as deposited, can only be obtained over a narrow range of deposition temperatures. At the lower end of this temperature range, the as-deposited films are strongly oriented, although the substrate is glass. This can be understood from the drive to minimize surface energy. Films of the B phase have a platelet morphology, which leads to an unusual microstructure at the lower-deposition temperatures. Those grown at ∼370 °C convert to the metallic, rutile (R) phase when annealed at 550 °C, whereas those deposited at 420 °C transform to the R phase only at 580 °C. (When cooled to room temperature, the annealedfilms convert reversibly from the R phase to the M phase.) Electron microscopy shows that annealing leads to disintegration of the single crystalline VO 2 (B) platelets into small crystallites of VO 2 (R), although the platelet morphology is retained. When the annealing temperature is relatively low, these crystallites are nanometer sized. At a higher-annealing temperature, the transformation leads to well-connected and similarly oriented large grains of VO 2 (R), enveloped in the original platelet. The semiconductor-metal transition near 68 °C leads to a large jump in resistivity in all the VO 2 (M) films, nearly as large as in epitaxial films on single-crystal substrates. When the annealedfilms contain well-connected large grains, the transition is very sharp. Even when preferred orientation is present, the transition is not as sharp in as-deposited VO 2 (M), because the crystallites are not densely packed as in annealed VO 2 (B). However, the high degree of orientation in these films leads to a narrow temperature hysteresis.