V O 2 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 VO2 films with strong semiconductor-to-metal phase transition. Sputter-deposited metal precursor V films were oxidized in situ in the deposition chamber for 2.5–7h at 370–415°C in 0.2Torr O2 to form 22–220nm VO2. The strength [resistivity ratio, RR=ρS∕ρM] and sharpness (hysteresis width ΔTC) of T-dependent semiconductor-to-metal hysteretic phase transition in VO2 were our most immediate and relevant quality indicators. In 200-nm-range films, the process was optimized to yield RR=(1–2)×103, ΔTC∼11°C and absolute resistivity in a semiconducting phase ρS=0.4±0.2Ωm, close to resistivity in bulk single crystals of VO2. 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 VO2. We also measured optical reflectivity RT(λ) from 200to1100nm, and Rλ(T) from 20to100°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 VO2. The limited study of these films in terms of stability, aging, lithographic processing, and thermal cycling shows that they can be used in applications.