This
work investigates and reports on the radio-frequency (rf)
behavior in the frequency range of 5–35 GHz of germanium-doped
vanadium dioxide (Ge-doped VO2) 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 (T
c)
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 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 (εr) 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 structures incorporating ultracompact
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 the reference. The reported results and analysis contribute
to the advancement of the field of metal–insulator–transition-material
technology with high T
c for rf industrial
applications.
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 μW, with a low noise equivalent power of 5 nW.Hz−0.5 at room temperature, for a footprint of 2.5 × 10−5 mm2. 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.
Sensitivity to low-energy photons in phase change materials enables the development of efficient millimeter-wave (mm-wave) and terahertz (THz) detectors. Here, we present the concept of uncooled mm-wave detection based on the sensitivity of IMT threshold voltage to the incident wave by exploiting the characteristics of reversible insulator-to-metal transition (IMT) in Vanadium dioxide (VO2) thin film devices. The detection concept is demonstrated through actuation of biased VO2 2-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. Ultimately, we propose an electromagnetic wave-sensitive voltage-controlled spike generator based on the VO2 switches in an astable circuit. The fabricated sensors show record figs. of merit, such as responsivities of around 66.3 kHz/mW with a low noise equivalent power (NEP) of 20 nW at room temperature, for a footprint of 2.5×10−5 mm2, which can be easily scaled. This solution gives 3 times better responsivity with only 1/10 footprint of the state of the art. However, the footprint is capable of being scaled down to few hundreds of nanometers. The responsivity in static measurements is 76 kV/W in the same circumstances. Based on experimental statistical data measured on robust fabricated devices, we investigate and report stochastic behavior and noise limits of VO2-based spiking sensors that are expected to form a new class of energy efficient transducers. The results highlight the capability of VO2 phase transition to serve for building electromagnetic power sensors, that can be triggered by low energy photons.
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