Françoise Cosset

High-speed metal-insulator transition in vanadium dioxide films induced by an electrical pulsed voltage over nano-gap electrodes

We report the fabrication of VO2 -based two terminal devices with 125-nm gaps between the two electrodes, using a simple, cost-effective method employing optical lithography and shadow evaporation. Current-voltage characteristics of the obtained devices show a main abrupt metal-insulator transition (MIT) in the VO2 film with voltage threshold values of several volts, followed by secondary MIT steps due to the nanostructured morphology of the layer. By applying to the two-terminal device a pulsed voltage over the MIT threshold, the measured switching time was as low as 4.5 ns and its value does not significantly change with device temperature, supporting the evidence of an electronically driven

Structural and electrical properties of large area epitaxial VO2 films grown by electron beam evaporation

Large area (up to 4 squared inches) epitaxial VO 2 films, with a uniform thickness and exhibiting an abrupt metal-insulator transition with a resistivity ratio as high as 2.85 × 10 4 have been grown on (001)-oriented sapphire substrates by electron beam evaporation. The lattice distortions (mosaicity) and the level of strain in the films have been assessed by X-ray diffraction. It is demonstrated that the films grow in a domain-matching mode where distortions are confined close to the interface which allows to grow high-quality materials despite the high film-substrate lattice mismatch. It is further shown that a post-deposition high-temperature oxygen annealing step is crucial to ensure the correct film stoichiometry and provide the best structural and electrical properties. Alternatively, it is possible to obtain high quality films with an RF discharge during deposition, which hence do not require the additional annealing step. Such films exhibit similar electrical properties and only slightly degraded structural properties.

Electric field-assisted metal insulator transition in vanadium dioxide (VO2) thin films: optical switching behavior and anomalous far-infrared emissivity variation

We present the vanadium dioxide (VO2) thin films deposition using e-beam evaporation of a vanadium target under oxygen atmosphere on different substrates (sapphire, Si, SiO2/Si…) and we focus on their electrical and optical properties variations as the material undergoes a metal-insulator transition under thermal and electrical stimuli. The phase transition induces extremely abrupt changes in the electronic and optical properties of the material: the electrical resistivity increases up to 5 orders of magnitude while the optical properties (transmission, reflection, refractive index) are drastically modified. We present the integration of these films in simple planar optical devices and we demonstrate electrical-activated optical modulators for visible-infrared signals with high discrimination between the two states. We will highlight a peculiar behavior of the VO2 material in the infrared and far infrared regions (2- 20 μm), namely its anomalous emissivity change under thermal- end electrical activation (negative differential emittance phenomenon) with potential applications in active coatings for thermal regulation, optical limiting or camouflage coatings.

High cut-off frequency RF switches integrating a metal-insulator transition material

We present the design and the RF performances of two-terminals in-plane RF switches based on dioxide vanadium (VO2)-phase transition material. The VO2 material undergoes a thermally-driven metal-insulator transition showing a resistivity variation up to five orders of magnitude as the material changes between its insulator and metallic states. We realized inline RF switches integrating VO2 patterns of different lengths (3 to 20 μm) and investigated their switching properties for both thermal and electrical actuation. For a thermally actuated 3-μm length VO2 pattern switch we measured an off-state capacitance of 7.7 Ff and an on-state resistance of ~2 Ω, with a cut-off frequency higher than 10 THz.