Ali Hendaoui

A photoinduced metal-like phase of monoclinic VO2 revealed by ultrafast electron diffraction

How to make vanadium dioxide metallic At about 70°C, the material vanadium dioxide (VO2) switches from being a semiconductor to a metal. The switch happens so fast that it may be useful in electronic devices, but it is not clear whether the switch is primarily caused by enhanced interactions between electrons or by a change in the crystal structure. Morrison et al. shone laser light on a sample of VO2, initially in a semiconducting state. They used electron diffraction to monitor the changes in the materials crystal structure and simultaneously measured its optical properties to monitor the electronic state. For certain laser powers, VO2 switched to a long-lived metallic state even though it preserved its initial crystal structure. Science, this issue p. 445 Simultaneous measurements of structural and optical properties are used to study optically excited vanadium dioxide. The complex interplay among several active degrees of freedom (charge, lattice, orbital, and spin) is thought to determine the electronic properties of many oxides. We report on combined ultrafast electron diffraction and infrared transmissivity experiments in which we directly monitored and separated the lattice and charge density reorganizations that are associated with the optically induced semiconductor-metal transition in vanadium dioxide (VO2). By photoexciting the monoclinic semiconducting phase, we were able to induce a transition to a metastable state that retained the periodic lattice distortion characteristic of the semiconductor but also acquired metal-like mid-infrared optical properties. Our results demonstrate that ultrafast electron diffraction is capable of following details of both lattice and electronic structural dynamics on the ultrafast time scale.

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...

Large Tuneability IR Emittance Thermal Control Coating for Space Applications

Passive thermal control systems are important components to reduce the power budget and cost of space missions and satellites. MPB has developed advanced technologies (US Patent # 7,761,053), for the passive dynamic thermal control of space structures and payloads. It is based on thin film Vanadium dioxide (VO2) doped with Tungsten (W) or Titanium (Ti) to control the transition temperature and its slope. In previous work, based on IR spectroscopy, Raman spectroscopy and XPS measurement, it was demonstrated that the VO2 deposited on Aluminium substrate had an inverse transition of the VO2 commonly deposited on Si, SiO2 and sapphire. The VO2/Al had low emittance (0.1 to 0.3) at low temperature, with VO2 in semiconductor phase, and has high emittance (0.4 to 0.65) at high temperature, with the VO2 in metallic state.

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 ...

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.