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 VO 2 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 O 2 to form 22-220 nm VO 2. The strength ͓resistivity ratio, RR= S / M ͔ and sharpness ͑hysteresis width ⌬T C ͒ of T-dependent semiconductor-to-metal hysteretic phase transition in VO 2 were our most immediate and relevant quality indicators. In 200-nm-range films, the process was optimized to yield RR= ͑1-2͒ ϫ 10 3 , ⌬T C ϳ 11°C and absolute resistivity in a semiconducting phase S = 0.4± 0.2 ⍀ m, close to resistivity in bulk single crystals of VO 2. 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 VO 2. We also measured optical reflectivity R T ͑͒ from 200 to 1100 nm, and R ͑T͒ from 20 to 100°C. R T ͑͒ 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 VO 2. The limited study of these films in terms of stability, aging, lithographic processing, and thermal cycling shows that they can be used in applications.

Tuning the conductivity of vanadium dioxide films on silicon by swift heavy ion irradiation

“…37,57,58 Depending on the film quality, the conductivity increases by 3 -4 orders of magnitude when heating the samples above the MIT temperature. Adopting a resistivity of 1 cm for VO 2 at room temperature and 1 m cm at …”

Nanothermochromics: Calculations for VO2 nanoparticles in dielectric hosts show much improved luminous transmittance and solar energy transmittance modulation

“…120 From a practical point of view, VO 2 may be awkward to prepare since the vanadium ions are in states with intermediate oxidation and it may be more facile to fabricate materials under nonoxidizing ͑metallic V͒ or fully oxidizing ͑in-sulating V 2 O 5 ͒ conditions. The limitations are not as stringent as they may seem, though, and it should be observed that metallic vanadium can be transformed to VO 2 by controlled oxidation [121][122][123][124] and that insulating V 2 O 5 van be transformed to VO 2 by controlled reduction in films. [125][126][127] There is a tendency that limited dimensions decreases c and widens the hysteresis in nanocrystalline films 46,48,[128][129][130][131][132][133][134][135] and nanorods.…”

Spinodally decomposed nanostructures in a TiO2–VO2 crystal

“…We are interested in this system because of distinct differences in their chemical and physical properties: TiO 2 without d electrons is a wide band-gap insulator and shows a photocatalytic activity, 14,15 while VO 2 with one d electron is a metal at high temperatures and becomes an insulator at low temperatures; a metal-insulator (MI) transition takes place at T MI = 342 K upon heating. 16 VO 2 is potentially important for applications as ultrafast resistive or optical switching devices, 17,18 because conductivity and optical absorption change dramatically across the transition. These large differences in properties between the two constituents make the TiO 2 -VO 2 system exceptional; in other SD systems, two constituents possess quite similar properties in all aspects.…”