Optical and electrical switching in nanostructured coatings of VO<inf>2</inf>

Publication Type:
Conference Proceeding
Citation:
Proceedings of SPIE - The International Society for Optical Engineering, 2007, 6647
Issue Date:
2007-12-01
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Vanadium dioxide undergoes a reversible metal-insulator phase transition at about 68°C. Coatings of this compound are reflective in the infrared above this temperature, and transmissive or absorptive below it, while resistivity changes by several orders of magnitude. We present a convenient method for depositing films with nano-size grains, which are then optically and electrically characterised. Emphasis in this study is the impact of aluminum doping and grain structure. The optical hysteresis is presented and its switching range is not altered at different doping levels but the value of transition temperature Tc does shift. In contrast hysteresis in dc resistance does change with a strong correlation between the fall in resistance in the semiconductor state with doping, the drop in Tc and the electrical properties in the metal state. For grain sizes under about 180 nm the conductivity in the metal phase is not linear in temperature but is thermally activated, with activation energies AE dependent on both grain size G and doping level. Simple mathematical relationships are found connecting ΔE with G and with carrier density in the semiconductor state. ΔE ranges in our samples from 0.15 eV in the smallest grain sizes to around 0.06 eV. This anomalous low frequency metal response is linked to excitations that arise in the metal phase associated with transient singlet pairing on neighbouring sites. Such pairing is weakened by doping, and in large grains appears to be present but incoherent.
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