VO2 films with strong semiconductor to metal phase transition prepared by the precursor oxidation process

Abstract

We describe a relatively simple, reliable, and reproducible preparation technique, the precursor oxidation process, for making V O2 films with strong semiconductor-to-metal phase transition. Sputter-deposited metal precursor V films were oxidized in situ in the deposition chamber for 2.5-7 h at 370-415 °C in 0.2 Torr O2 to form 22-220 nm V O2. The strength [resistivity ratio, RR= ρ S ρ M] and sharpness (hysteresis width Δ TC) of T -dependent semiconductor-to-metal hysteretic phase transition in V O2 were our most immediate and relevant quality indicators. In 200-nm -range films, the process was optimized to yield RR= (1-2) × 103, Δ TC ∼11 °C and absolute resistivity in a semiconducting phase ρ S =0.4±0.2 m, close to resistivity in bulk single crystals of V O2. Films were characterized by scanning electron microscopy, atomic force microscopy, grazing-incidence x-ray diffraction, and Raman spectroscopy, and found to be polycrystalline single-phase V O2. We also measured optical reflectivity RT (λ) from 200 to 1100 nm, and Rλ (T) from 20 to 100 °C. RT (λ) measured in thin-film interference structures allowed us to calculate the index of refraction in the two phases, which agrees well with the published data and, together with structural measurements, confirms that our films are essentially pure V O2. The limited study of these films in terms of stability, aging, lithographic processing, and thermal cycling shows that they can be used in applications. © 2007 American Institute of Physics.