Post-deposition annealing of thin RF magnetron sputter-deposited VO2 films above the melting point

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Vanadium dioxide (VO2) is a promising material for electronics and optics due to abrupt change in conductivity of VO2 that occurs at ∼68 °C, which is the metal-insulator transition temperature. The abruptness and magnitude of this transition is governed by the crystal structure of VO2. The growth of thin VO2 films therefore requires substrate heating during deposition and, as well as the occasional need for additional post-deposition annealing. The processes that occur during annealing depend upon the deposition technique. However, the published reports related to these studies are rarely systematic and lack consistency. The annealing of thin nanocrystalline VO2+x films at temperatures above the melting point is reported here. Thin films were deposited onto a polycrystalline corundum substrate by means of radio-frequency reactive magnetron sputtering, with the thin film composition fine-tuned by applying negative substrate bias during deposition and proceeding with ex-situ control of the vanadium oxidation state. Post-deposition annealing causes recrystallization, which is in accordance with the VO phase diagram. No epitaxial or other substrate-induced processes were detected. However, the sharpest transition was attributed to the film possessing the most uniform grain size, as opposed to the film with the largest VO2 crystallites. The temperature dependency of the microwave radiation transmittance was investigated over the 8–18 GHz range, with the samples measured in free-space conditions. This was intended to reveal any frequency-dependent features, which can possibly appear as benefits or obstacles during the development of designs in the microwave range that are based on the VO2 films being studied.