Nicolas Émond

Highly tunable-emittance radiator based on semiconductor-metal transition of VO2 thin films

This paper describes a VO2-based smart structure with an emittance that increases with the temperature. A large tunability of the spectral emittance, which can be as high as 0.90, was achieved. The transition of the total emittance with the temperature was fully reversible according to a hysteresis cycle, with a transition temperature of 66.5 °C. The total emittance of the device was found to be 0.22 and 0.71 at 25 °C and 100 °C, respectively. This emittance performance and the structure simplicity are promising for the next generation of energy-efficient cost-effective passive thermal control systems of spacecrafts.

Low resistivity WxV1−xO2-based multilayer structure with high temperature coefficient of resistance for microbolometer applications

Materials that exhibit semiconductor-to-metal phase transition (SMT) are commonly used as sensing layers for the fabrication of uncooled microbolometers. The development of highly responsive microbolometers would benefit from using a sensing material that possesses a large thermal coefficient of resistance (TCR) close to room temperature and a resistivity low enough to compromise between noise reduction and high TCR, while it should also satisfies the requirements of current CMOS technology. Moreover, a TCR that remains constant when the IR camera surrounding temperature varies would contribute to achieve reliable temperature measurements without additional corrections steps for TCR temperature dependence. In this paper, the characteristics of the SMT occurring in undoped and tungsten-doped vanadium dioxide thin films deposited on LaAlO3 (100) substrates are investigated. They are further exploited to fabricate a WxV1−xO2 (0 ≤ x ≤ 2.5) multilayer structure exhibiting a bottom-up gradient of tungsten conte...

Modulated scattering technique in the terahertz domain enabled by current actuated vanadium dioxide switches

The modulated scattering technique is based on the use of reconfigurable electromagnetic scatterers, structures able to scatter and modulate an impinging electromagnetic field in function of a control signal. The modulated scattering technique is used in a wide range of frequencies up to millimeter waves for various applications, such as field mapping of circuits or antennas, radio-frequency identification devices and imaging applications. However, its implementation in the terahertz domain remains challenging. Here, we describe the design and experimental demonstration of the modulated scattering technique at terahertz frequencies. We characterize a modulated scatterer consisting in a bowtie antenna loaded with a vanadium dioxide switch, actuated using a continuous current. The modulated scatterer behavior is demonstrated using a time domain terahertz spectroscopy setup and shows significant signal strength well above 0.5 THz, which makes this device a promising candidate for the development of fast and energy-efficient THz communication devices and imaging systems. Moreover, our experiments allowed us to verify the operation of a single micro-meter sized VO2 switch at terahertz frequencies, thanks to the coupling provided by the antenna.

Theoretical and Experimental Investigation of Thermo-Tunable Metal–Insulator–Vanadium Dioxide Coplanar Waveguide Structure

This paper presents a thermo-tunable metal–insulator–vanadium dioxide (MIV) coplanar waveguide structure (CPW). An analytical quasi-TEM model is used to thoroughly study the structural and material-dependent characteristic parameters of the device, while the temperature-dependent broadband permittivity and conductivity of the deposited vanadium dioxide (VO2) thin film at microwave frequencies are experimentally extracted. The designed thermo-tunable MIV-CPW, focused on its slow-wave properties, shows a drastic increase in slow-wave factor when the temperature of the VO2 thin film is increased over its transition temperature. This phase change allows converting this composite structure into the metal–insulator–semiconductor coplanar slow-wave structure. This demonstrates the potential of such MIV-CPW for the design of tunable and reconfigurable integrated microwave passive components with a significantly reduced size.

Impact of tungsten doping on the dynamics of the photo-induced insulator-metal phase transition in VO2 thin film investigated by optical pump-terahertz probe spectroscopy

The influence of tungsten (W) doping on the ultrafast dynamics of the photo-induced insulator-metal phase transition (IMT) is investigated at room temperature in epitaxially grown vanadium dioxide (VO2) thin films by means of optical pump-terahertz (THz) probe spectroscopy. It is observed that the THz transmission variation of the films across the IMT follows a bi-exponential decrease characterized by two time constants, one corresponding to a fast process and the other to a slower process. W-doping (i) reduces the photo-excitation fluence threshold required for triggering the IMT, (ii) accelerates the slow process, and (iii) increases the THz transient transmission variation for corresponding fluences. From the Drude-Smith model, it is deduced that a strong carrier confinement and an enhancement of the transient conductivity occur across the IMT. The IMT is also accompanied by an increase in the carrier concentration in the films, which is enhanced by W-doping. Our results suggest that W-doped VO2 could ...

Field-enhanced design of steep-slope VO 2 switches for low actuation voltage

The abrupt metal-insulator transition in vanadium dioxide (VO2) offers novel performance and functionality for beyond CMOS switches, enabling simultaneous high ON current and ultra-steep subthreshold slope with low temperature dependence. We developed a field-enhanced design of 2-terminal VO2 switches that allows decreasing their actuation voltage without affecting their performance and reliability. Exploiting this design, we characterized VO2 switches with extremely abrupt transitions (<; 1 mV/dec) until 60°C and a reduction in actuation voltage up to 38.3% with respect to conventional devices.

Broadband temperature-dependent dielectric properties of polycrystalline vanadium dioxide thin films

Broadband dielectric properties of VO2 polycrystalline thin films deposited on fused quartz substrate by reactive pulsed laser deposition are measured and characterized at GHz frequencies. The relative dielectric constant is extracted using the platform of coplanar waveguide via a multi-line TRL method, combined with either a conformal mapping model or a full-wave calculation process. It is found in this work that increasing the temperature of the studied VO2 thin film over its transition temperature results in a dramatic increase of both relative permittivity and conductivity by more than 3 orders of magnitude.

Natural and induced growth of VO 2 (M) on VO 2 (B) ultrathin films

This work examines the synthesis of single phase VO2 (B) thin films on LaAlO3 (100) substrates, and the naturally-occurring and induced subsequent growth of VO2 (M) phase on VO2 (B) films. First, the thickness (t) dependence of structural, morphological and electrical properties of VO2 films is investigated, evidencing that the growth of VO2 (B) phase is progressively replaced by that of VO2 (M) when t > ~11 nm. This change originates from the relaxation of the substrate-induced strain in the VO2 (B) films, as corroborated by the simultaneous increase of surface roughness and decrease of the c-axis lattice parameter towards that of bulk VO2 (B) for such films, yielding a complex mixed-phase structure composed of VO2 (B)/VO2 (M) phases, accompanied by the emergence of the VO2 (M) insulator-to-metal phase transition. Second, the possibility of inducing this phase conversion, through a proper surface modification of the VO2 (B) films via plasma treatment, is demonstrated. These natural and induced VO2 (M) growths not only provide substantial insights into the competing nature of phases in the complex VO2 polymorphs system, but can also be further exploited to synthesize VO2 (M)/VO2 (B) heterostructures at the micro/nanoscale for advanced electronics and energy applications.