Jonathan Leroy

Voltage and current-activated metal-insulator transition in VO2-based electrical switches : a lifetime operation analysis

Abstract Vanadium dioxide is an intensively studied material that undergoes a temperature-induced metal–insulator phase transition accompanied by a large change in electrical resistivity. Electrical switches based on this material show promising properties in terms of speed and broadband operation. The exploration of the failure behavior and reliability of such devices is very important in view of their integration in practical electronic circuits. We performed systematic lifetime investigations of two-terminal switches based on the electrical activation of the metal–insulator transition in VO2 thin films. The devices were integrated in coplanar microwave waveguides (CPWs) in series configuration. We detected the evolution of a 10 GHz microwave signal transmitted through the CPW, modulated by the activation of the VO2 switches in both voltage- and current-controlled modes. We demonstrated enhanced lifetime operation of current-controlled VO2-based switching (more than 260 million cycles without failure) compared with the voltage-activated mode (breakdown at around 16 million activation cycles). The evolution of the electrical self-oscillations of a VO2-based switch induced in the current-operated mode is a subtle indicator of the material properties modification and can be used to monitor its behavior under various external stresses in sensor applications.

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

Current-induced electrical self-oscillations across out-of-plane threshold switches based on VO2 layers integrated in crossbars geometry

Electrically activated metal-insulator transition (MIT) in vanadium dioxide (VO2) is widely studied from both fundamental and practical points of view. It can give valuable insights on the currently controversial phase transition mechanism in this material and, at the same time, allows the development of original MIT-based electronic devices. Electrically triggered insulator-metal transitions are demonstrated in novel out-of-plane, metal-oxide-metal type devices integrating a VO2 thin film, upon applying moderate threshold voltages. It is shown that the current-voltage characteristics of such devices present clear negative differential resistance effects supporting the onset of continuous, current-driven phase oscillations across the vanadium dioxide material. The frequencies of these self-sustained oscillations are ranging from 90 to 300 kHz and they may be tuned by adjusting the injected current. A phenomenological model of the device and its command circuit is developed, and allows to extract the analytical expressions of the oscillation frequencies and to simulate the electrical oscillatory phenomena developed across the VO2 material. Such out-of-plane devices may further contribute to the general understanding of the driving mechanism in metal-insulator transition materials and devices, a prerequisite to promising applications in high speed/high frequency networks of oscillatory or resistive memories circuits.

Electrical and optical properties of vanadium dioxide containing gold nanoparticles deposited by pulsed laser deposition

Nanostructured vanadium dioxide is one of the most interesting and studied member of the vanadates family performing a reversible transition from an insulating state to a metallic state associated with a structural transition when heated above a temperature of 68 C. On the other hand, noble metal nanoparticles (NPs) support localized surface plasmon resonance which causes selective absorption bands in the visible and near-IR regions. The purpose of this letter is to study structural, optical, and electrical properties of vanadium dioxide thin films containing gold nanoparticles synthetized using pulsed laser deposition process. Thus, we have performed x-ray diffraction, optical transmission, and four point probe electrical measurements to investigate the nanocomposite properties versus its temperature. Interestingly, we have observed switching behavior for VO2 film containing gold NPs with a resistivity contrast of four orders of magnitude and a decrease of its transition temperature

Generation of electrical self-oscillations in two-terminal switching devices based on the insulator-to-metal phase transition of V02 thin films

We present the non-linear electrical properties of simple two-terminal switching devices based on vanadium dioxide (VO2) thin films. The current-voltage characteristics of such devices present negative differential resistance (NDR) regions allowing generating electrical self-oscillations across the investigated devices, with frequencies ranging from several kHz up to 1 MHz. We investigate and compare the factors determining the onset of oscillatory phenomenon in both voltage- and current-activated oscillations and explain its origin. For both activation modes, we will correlate the properties of electrical oscillations (amplitude and frequency) with the amplitude of the continuous excitation signal, the physical geometry of the devices or ambient temperature. We conclude by mentioning several possible applications for the oscillation generation in the radiofrequency (RF)/microwave domains (inverters, integrated a.c. signal generators, pressure and temperature sensors, etc.).

Tunable terahertz metamaterials based on metal-insulator phase transition of VO2 layers

We designed a tunable metamaterial in the terahertz frequency domain (0.1 – 1 THz) based on periodical arrays of metallic resonators on top of vanadium dioxide thin films deposited on a sapphire substrate. We simulate and show experimentally that the frequency response of the fabricated metamaterial is drastically changing as the vanadium dioxide under layer performs a reversible temperature-driven phase transition from an insulating to a metallic state. The hybrid metamaterial is designed also for the electrical activation of the underneath vanadium dioxide material in order to realize faster dynamical switches and modulation functions for the terahertz waves.

High frequency microfluidic biosensors for intracellular dielectric spectroscopy

This paper deals with the development and characterization of a high frequency (HF) label-free microfluidic biosensor for the non-invasive analysis of cell intracellular properties. The presented microfluidic biosensor is based on a band pass filter architecture made of thick gold electrodes designed to ensure a high sensitivity to cells flowing in the microfluidic channel. In a first step, to prove the feasibility of the proposed approach, HF measurements have been successfully achieved on polystyrene beads. Then, combining HF measurements with dielectrophoresis forces, to trap cells in the sensitive area, it has been possible to characterize cell dielectric properties without any denaturation. We demonstrate here the proof of concept of using high frequency impedance spectroscopy to analyze single cells in a microfluidic environment.

Tunable THz metamaterials based on phase-changed materials (VO2) triggered by thermal and electrical stimuli

One of the most peculiar characteristics of the insulator-to-metal transition (MIT) in vanadium dioxide (VO2) material is its broadband response, manifested by drastic electrical and dielectric properties changes between the insulator and metallic states on a very large frequency spectrum. We are presenting the characterization of the MIT in VO2 films over a wide range of the electromagnetic spectrum (75-110GHz, 0.1-1.4THz) and illustrate the materials’ capabilities for manipulating the electromagnetic radiation in the millimeter-waves and THz domains. We demonstrate the possibility of realizing tunable THz devices by introducing this phase transition material as localized patterns in the structure of THz planar metamaterials. We designed, simulated and fabricated tunable VO2-based THz metamaterials devices which show significant variations in their THz transmission under the effect of thermal stimuli but also by applying an electrical voltage across the devices.

Pulsed power operation of power limiters integrating a phase transition material

We present the operation of microwave power limiters based on reversible phase transition of vanadium dioxide thin films integrated on coplanar waveguides submitted to incident pulsed peak power at 10 GHz. During the pulse on-time, the fabricated devices can be reversibly driven by the incident signal from a high transmission state to a low-transmission level on a relatively short timescale (response times as low as 4 μs for incident peak powers of 35 dBm). The broadband operation and the simplicity of fabrication and integration of these devices, their potential for faster response times, make them attractive as protective devices against continuous or transient RF power.

Métamatériaux accordables dans le domaine Térahertz à base des matériaux à transition isolant/métal

Being a Mott type oxide, at a temperature of 68°C and ambient pressure, stoichiometric VO2 undergoes a first order metal-insulator transition, which is accompanied by a structural transition from a high-temperature rutile phase to a low-temperature monoclinic phase. The latter result causes an abrupt change in the resistivity over several orders of magnitude induced by the band gap opening. From optical point of view, this metal-insulator transition is accompanied by a significant and reversible variation of the refractive index under a thermal stimuli. Hence, VO2 based coatings have been attracting considerable interest for fundamental reasons, and certainly for technological applications in the solar energy sector and ultrafast linear and nonlinear photonics. In this contribution, the photonic multi-functionality of nano-structured VO2 based coatings are presented. This includes applications such as (i) smart window for solar heat modulation, (ii) active coating for heat management in satellites, (iii) ultrafast opto-electronic gating, (iv) ultrafast optical limiting and (iv) femtosecond tunable nano-plasmonics.