Fatemeh Qaderi Rahaqi

Millimeter-wave to near-terahertz sensors based on reversible insulator-to-metal transition in VO2

Mihai Adrian Ionescu, Fatemeh Qaderi Rahaqi, Teodor Rosca

In the quest for low power bio-inspired spiking sensors, functional oxides like vanadium dioxide are expected to enable future energy efficient sensing. Here, we report uncooled millimeter-wave spiking detectors based on the sensitivity of insulator-to-metal transition threshold voltage to the incident wave. The detection concept is demonstrated through actuation of biased VO2 switches encapsulated in a pair of coupled antennas by interrupting coplanar waveguides for broadband measurements, on silicon substrates. Ultimately, we propose an electromagnetic-wave-sensitive voltage-controlled spike generator based on VO2 switches in an astable spiking circuit. The fabricated sensors show responsivities of around 66.3 MHz.W-1 at 1 mu W, with a low noise equivalent power of 5 nW.Hz(-0.5) at room temperature, for a footprint of 2.5 x 10(-5) mm(2). The responsivity in static characterizations is 76 kV.W-1. Based on experimental statistical data measured on robust fabricated devices, we discuss stochastic behavior and noise limits of VO2 -based spiking sensors applicable for wave power sensing in mm-wave and sub-terahertz range. Vanadium dioxide is a strongly correlated material interesting for its ultra-fast resistive switching controlled by an electric-field-driven insulator-metal transition. Here, VO2 stochastic oscillator power sensors for mm-wave to sub-THz radiation are demonstrated, displaying high responsivities, low noise, and a small scalable footprint.

SPRINGERNATURE2023

Millimeter-wave to near-terahertz sensors based on reversible insulator-to-metal phase transition in Vanadium dioxide

Fatemeh Qaderi Rahaqi

Millimeter-wave (mm-wave) and terahertz (THz) detection has opened a significant research direction in imaging, sensing, spectroscopy, and ultrafast wireless communication.The sensitivity of the phase transition to electromagnetic waves in Vanadium dioxide (VO2) offers solutions to highly-sensitive radio frequency (RF), mm-wave, and THz detectors. We present the concept of uncooled mm-wave detection based on the sensitivity of field-induced insulator-to-metal transition (IMT) threshold voltage to electromagnetic waves in VO2 thin film devices. After exploring potential principles for implementation of a VO2-based detector, we present our experimental results on two main approaches: first, threshold voltage shifting in VO2 two-terminal switches along with its readout methods, and second, utilizing non-linearity in VO2 electrical characteristics close to the transition point by electrically biased devices.Based on an accumulative effect, the static threshold voltage is down-shifted in presence of different types of excitation, such as elevated temperatures or electromagnetic waves. We first demonstrate the detection concept through actuation of biased VO2 two-terminal switches encapsulated in a pair of coupled antennas on a Si/SiO2 substrate. We also study the behavior of VO2 switches interrupting coplanar waveguide (CPW)s for broadband measurement purposes. Then we propose a mm-wave-sensitive voltage-controlled spike generator based on the two-terminal VO2 devices in an astable circuit. By the shift in the threshold voltage, one reads out the impact on the frequency of oscillations in the astable output. Based on experimental statistical data, we report our approach to harness the stochastic behavior and noise limits of VO2-based spiking sensors, which are expected to form a new class of energy efficient transducers.The fast dynamics of the non-linear impedance characteristics of VO2 provides capability of large-bandwidth (BW), broadband detection of electromagnetic waves. We scale down the threshold voltage and the hysteresis width by implementation of miniaturised vertical switches. The film operates in sub-threshold regime, exploiting local nonlinearity and partial switching. The implementation is by growing VO2 on top of a metallic surface and overlapping a top electrode to realize the switching through the thickness of the VO2 film. The two-terminal vertical devices can detect the envelop of an intensity modulated sub-THz signal.They are encapsulated seamlessly inside CPWs (for broadband measurements) as well as bowtie antennas (for free-space experiments). The optimum responsivity can be achieved at a certain bias point on the devices close to the IMT, where the nonlinearity reaches its highest level.The antennas are optimized to operate at sub-THz carrier frequencies according to the availability of sources, however the design can adapt for higher frequencies, as VO2 shows reception for carrier frequencies up to the visible range. The responsivity of the proposed devices to mm-wave radiation along with its large BW characteristics makes it a viable detector for a wide range of high-data-rate applications.

EPFL2022

Subthreshold VO2 vertical switches for large-bandwidth millimeter-wave and sub-terahertz detection

Ali Gilani, Mihai Adrian Ionescu, Daesung Park, Fatemeh Qaderi Rahaqi

n-linear impedance characteristics of VO2 offer solutions to highly sensitive radio-frequency (RF), millimeterwave (mm-wave), and terahertz (THz) detectors. We demonstrate high-speed broadband sensibility of miniaturized VO2 switches by utilizing the nonlinearity close to the abrupt insulator-to-metal transition (IMT). We apply an intensity modulated sub-THz signal to the 2-terminal devices inside a coplanar waveguide (CPW) and detect the down-mixed component at the intermediate frequency (IF). The maximum output IF power selectivity over noise reaches 45 dB with a-3 dB bandwidth of 12 GHz. Its responsivity to subTHz radiation and high-speed switching makes our design a potential detector for a wide range of high-data-rate wireless applications.

IEEE2022

High Tuning Range Spiking 1R-1T VO2 Voltage-Controlled Oscillator for Integrated RF and Optical Sensing

Mihai Adrian Ionescu, Fatemeh Qaderi Rahaqi, Teodor Rosca

In this work we propose and experimentally validate a relaxation spiking oscillator architecture with ultra-high tuning range (higher than 400%, from 5KHz to more than 25kHz when the control voltage is varied from 2.5 to 5V) that exploits the reversible metal-insulator transition in 2-terminal Vanadium Dioxide (VO2) thin film devices loaded to a MOSFET common source amplifier. We propose and validate an analytical model that connects key output signal metrics (frequency and amplitude) to the intrinsic properties of the phase-change VO2 device employed (switching thresholds, hysteresis and electrical resistance in on- and off- states). We show that the proposed analytical model of the switching dynamics enables us to accurately simulate and predict the oscillation waveform based solely on VO2 DC electrical characteristics. Finally, we report two experiments of sensing RF and optical power with VO2 devices using the spiking oscillator as a readout circuit, showing highly linear responses: (a) for the RF power sensing we report a sensitivity of 4.64 Hz/dBm in the GHz range, and, (b) for the optical power sensing, we report sensitivities as high as 4.23 Hz/mW in the range of 500 to 700 nm (visible optical spectrum).

IEEE2021

Radio-Frequency Characteristics of Ge-doped Vanadium Dioxide Thin Films with Increased Transition Temperature

Riyaz Mohammed Abdul Khadar, Matteo Cavalieri, Mihai Adrian Ionescu, Anna Krammer, Andrei Müller, Fatemeh Qaderi Rahaqi, Teodor Rosca, Andreas Schueler, Igor Stolichnov

This work investigates and reports on the radio-frequency (RF) behavior in the frequency range of 5 – 35 GHz of germanium-doped (Ge-doped VO2) vanadium dioxide thin films, deposited on silicon substrates via sputtering and pulsed laser deposition (PLD) with estimated Ge concentrations of 5 % and 5.5 %. Both films exhibit critical transition temperatures (Tc) of 76.2 and 72 °C, respectively, which are higher compared to that of the undoped VO2 which undergoes reversible insulator-to-metal phase transition at 68°C. Both types of Ge-doped films show low hysteresis (< 5 °C) in their conductivity versus temperature characteristics and preserve an high off-state DC-conductivities (corresponding to the insulating state of the phase change material) of 13 S/m, for the sputtered, and, 55 S/m, for the PLD deposited film, respectively. The DC on-state (corresponding to the conductive state of the phase change material) conductivity reaches 145,000 S/m in the case of the PLD film, which represents a significant increase compared to the state-of-the art values measured for undoped VO2 thin films deposited on identical substrates. In order to further understand the off-state dissimilarities and RF behavior of the deposited Ge-doped VO2 films, we propose an original methodology for the experimental extraction of the dielectric constant (ɛr) in the GHz range of the films below 60 °C. This is achieved by exploiting the frequency shift of resonant filters. For this purpose we have fabricated coplanar waveguide (CPW) structures incorporating ultra-compact Peano space filling curves, each resonating at a different frequency between 5 and 35 GHz on two types of substrates, one with the Ge-doped VO2 thin films and another one using only SiO2 to serve as reference. The reported results and analysis contribute to the advancement of the field of metal-insulator-transition material technology with high Tc for RF industrial applications.

2020

Millimeter-wave to near-terahertz sensors based on reversible insulator-to-metal phase transition in Vanadium dioxide

Fatemeh Qaderi Rahaqi

EPFL2022

Radio-Frequency Characteristics of Ge-doped Vanadium Dioxide Thin Films with Increased Transition Temperature

Riyaz Mohammed Abdul Khadar, Matteo Cavalieri, Mihai Adrian Ionescu, Anna Krammer, Andrei Müller, Fatemeh Qaderi Rahaqi, Teodor Rosca, Andreas Schueler, Igor Stolichnov

2020