Events

séminaire CRMQ

Date : 17 April 2018 12:30

Type : CRMQ

Location : D3-2037

Invited talk

Nicolas Emond, INRS-Énergie, Matériaux et Télécommunications, 1650 Boulevard Lionel Boulet, Varennes, Québec J3X 1S2, Canada
Email: emondn@emt.inrs.ca

Title:
"Synthesis and characterization of VO2 and WxV1-xO2 thin films for infrared and terahertz applications"

Abstract:

      Vanadium dioxide (VO2) is a particularly interesting strongly correlated material which undergoes an insulator-metal transition (IMT) at a transition temperature TIMT ≈ 68 °C. This material thermo/photo-induced IMT is accompanied by significant changes in its electrical and optical properties, and both these properties and the TIMT of VO2 greatly depend on stoichiometry, strain and crystallinity. Indeed, doping VO2 with W represents the most efficient method to bring its TIMT close to room temperature, which is beneficial with regards to the integration of VO2 thin films in tunable devices. Simplicity, versatility, high deposition rate and the possibility of doping are among the numerous advantages that make pulsed laser deposition (PLD) a prominent growth technique to synthesize high-quality VO2 films. Herein, the effect of doping and crystalline quality on the structural, morphological, electrical, infrared (IR) and terahertz (THz) properties of pulsed laser deposited VO2 and WxV1−xO2 films grown on various substrates was investigated, and the results of this fundamental study were then exploited to demonstrate the potential of VO2 thin films for various technological applications.

      First, the thickness (t) dependence of the structural, morphological and electrical properties of epitaxial single phase VO2 (B) thin films on LaAlO3 (100) substrates was studied. It evidenced that the growth of VO2 (B) phase is progressively replaced by that of VO2 (M) when t > ~ 11 nm, yielding a complex VO2 (B)/VO2 (M) mixed-phase structure. Furthermore, the possibility of inducing this phase conversion, through a proper surface modification of the VO2 (B) films via plasma treatment, was demonstrated. Such capability could be fruitfully exploited to synthesize VO2 (M)/VO2 (B) heterostructures at the micro/nanoscale for advanced electronics and energy applications.

      It was then demonstrated that the control of the oxygen pressure (PO2) during deposition is one of the key parameter to tune the IMT features of polycrystalline VO2 thin films for specific applications in the THz frequency range. For instance, at low PO2, VO2 films with large THz transmission modulation, sharp transition and narrow hysteresis width, suitable for sensor-type devices, were obtained. On the other hand, the VO2 films synthesized at high PO2 exhibit large hysteresis width, so that they are well-suited for memory-type devices.

      The THz complex conductivity of epitaxially-grown WxV1−xO2 films on Al2O3 (1-102) substrates was thereafter measured across the IMT. The modelling of this conductivity provided clear insights about the gradual nucleation and coalescence of VO2 metallic domains among an insulating host through the IMT. It also evidenced that WxV1−xO2 films behave as a disordered metamaterial in the vicinity of the IMT as a result of a change in the films electronic response from capacitive to inductive. Such a behavior could be exploited to significantly simplify the design of tunable metamaterials for optoelectronic applications.

     Finally, the influence of W doping on the ultrafast dynamics of the photo-induced IMT of WxV1−xO2 films was investigated by means of optical pump-THz probe spectroscopy. It revealed that W-doping (i) accelerates the THz transmission variation across the IMT (ii) reduces the fluence threshold required for triggering the IMT, and (iii) increases the THz transient transmission variation for corresponding fluences. These results suggest that W-doped VO2 could be advantageously exploited for ultrafast THz switching/modulation applications.

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