Publications by Teodor Rosca | EPFL Graph Search

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

Energy Efficient Sensing using Steep Slope Devices

Teodor Rosca

Today, we are witnessing the Internet of Things (IoT) revolution, which facilitates and improves ourlives in many aspects, but comes with several challenges related to the technology deployment atlarge scales. Handling ever growing amounts of information that needs to be sensed, stored,transmitted and processed requires severe improvements in energy efficiency and smart distributionof computational power spanning from Cloud systems (handling Big Data in a massively parallelfashion) all the way to Edge devices (interfaces to the real world), where co-integration of sensingand computation plays a big role. Innovations in this field require the development of new deviceprinciples in existing technology platforms and/or new abundant and non-toxic materials that canenable electronic functions beyond the classical semiconductors, such as the field of oxideelectronics that holds promise for both classical electronics functions and for future neuromorphicimplementations. In this thesis, we explore both these aspects, having as a common denominatorsteep slope devices, which have the merit of offering a path for improved energy efficiency viavoltage scaling. We particularly focus our work on their ability to serve energy efficient sensingfunctions that can be integrated with the computational platforms.The first part of the thesis focuses on Tunnel Field Effect Transistors (TFETs) and how they can beused to perform similar tasks to Single Electron Transistors for qubit readout and also for serving asinterfacing electronics. Such applications rely on cryogenic operation where conventional CMOStechnology shows performance degradation due to low temperature effects such as dopantdeactivation and carrier freeze-out. Our study shows that state-of-the-art heterostructure nanowireTFET arrays maintain excellent figures of merit over wide temperature ranges, down to the Kelvinregime, while simultaneously showing reduced temperature dependence once Trap AssistedTunnelling mechanisms are removed below 150K. Leveraging such properties, we suggest thatTFETs are promising candidates as charge sensing devices for qubit readout architectures with highsensitivity to single or few elementary charges.In the second part of the thesis we focus towards sensing architectures more suitable forEdge-of-Cloud (EoC) applications, by exploring phase-transition materials such as VanadiumDioxide (VO2). In this context, we explore the optimization of a Pulsed Laser Deposition (PLD)process in order to achieve high quality VO2thin films grown on CMOS compatible substrates,followed by electrical characterization of fabricated VO2two-terminal devices, which providesvaluable data that aid us in developing compact SPICE-compatible device models. Built on top ofthe VO2 resistor elements, we propose a novel Spiking Voltage-Controlled Oscillator (VCO)architecture that exhibits low device count (1 Transistor 1 Resistor - 1T1R) while at the same timeproviding frequency tuning capabilities in excess of 400% in the 10s of kHz range. Weexperimentally validate that the VCO cell can be used as a power-to-frequency transducer in a widespectrum, ranging from near-UV, throughout the entirety of the visible domain, and as far as theMid-Infrared and mmWave ranges, suggesting a new class of sensors capable of responding to abroad range of stimuli.

EPFL2022

Spike-Based Sensing and Communication for Highly Energy-Efficient Sensor Edge Nodes

Mihai Adrian Ionescu, Teodor Rosca

Highly energy-efficient wireless sensor nodes are a prerequisite for a sustainable operation of the Internet of things. Therefore, classical approaches for system design based on digital signal processing are not a viable solution, but system design has to follow entirely new paradigms. In this regard, we present a sensory system with analog spike-based signal processing for sensing and communication, encoding the sensory information in the pulse repetition frequency (PRF), getting rid of energy hungry A/D and D/A conversion. Our spiking sensory system can generate spikes from any conventional analog output sensor using a compact, highly tunable voltage-controlled oscillator based on vanadium dioxide, and an analog differentiator circuit performing the transmit pulse shaping. The sole conversion from analog to digital takes place at the base station followed by the estimation of the PRF, for which we compare a conventional receiver design consisting of an analog-to-digital converter (ADC) with the use of an integrate-and-fire time encoding machine (IFTEM). Results show the successful communication of sensory information from the edge node over an additive white Gaussian noise channel to the base station, with the IF-TEM outperforming the conventional ADC for a signal-to-noise ratio above 0 dB.

IEEE2022

Gate energy efficiency and negative capacitance in ferroelectric 2D/2D TFET from cryogenic to high temperatures

Luca Capua, Matteo Cavalieri, Carlotta Gastaldi, Mihai Adrian Ionescu, Sadegh Kamaei Bahmaei, Teodor Rosca, Ali Saeidi

We report the fabrication process and performance characterization of a fully integrated ferroelectric gate stack in a WSe2/SnSe2 Tunnel FETs (TFETs). The energy behavior of the gate stack during charging and discharging, together with the energy loss of a switching cycle and gate energy efficiency factor are experimentally extracted over a broad range of temperatures, from cryogenic temperature (77 K) up to 100 degrees C. The obtained results confirm that the linear polarizability is maintained over all the investigated range of temperature, being inversely proportional to the temperature T of the ferroelectric stack. We show that a lower-hysteresis behavior is a sine-qua-non condition for an improved energy efficiency, suggesting the high interest in a true NC operation regime. A pulsed measurement technique shows the possibility to achieve a hysteresis-free negative capacitance (NC) effect on ferroelectric 2D/2D TFETs. This enables sub-15 mV dec(-1) point subthreshold slope, 20 mV dec(-1) average swing over two decades of current, I-ON of the order of 100 nA mu m(-2) and I-ON/I-OFF > 10(4) at V-d = 1 V. Moreover, an average swing smaller than 10 mV dec(-1) over 1.5 decades of current is also obtained in a NC TFET with a hysteresis of 1 V. An analog current efficiency factor, up to 50 and 100 V-1, is achieved in hysteresis-free NC-TFETs. The reported results highlight that operating a ferroelectric gate stack steep slope switch in the NC may allow combined switching energy efficiency and low energy loss, in the hysteresis-free regime.

NATURE PORTFOLIO2021

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

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HfO2-based ferroelectrics are considered a promising class of materials for logic and memory applications due to their CMOS compatibility and ferroelectric figures of merit. A steep-slope field-effect-transistor (FET) switch is a device for logic applications in which a ferroelectric gate stack exploits a stabilized negative capacitance regime capable to differentially amplify the surface potential in a metal-oxide-semiconductor FET structure, resulting in the improvement of the subthreshold swing and overdrive. In a number of relevant studies of negative capacitance, intrinsic (thermodynamic) switching is assumed, since alternative switching scenarios predict undesirable hysteretic responses in logic devices. However, there is little support from the experimental data showing that the polarization reversal in HfO2-based ferroelectrics is really driven by the intrinsic switching mechanism. In this work, polarization hysteresis loops are measured over wide temperature ranges on polycrystalline Si-doped HfO2 (Si:HfO2) capacitors. The analysis herein, which is based on the classic Landau-Ginzburg-Devonshire theory, yields the temperature-dependent dielectric susceptibility values, which fit the Curie-Weiss law. The extrapolated Curie temperature value is in line with the data obtained for other HfO2-based ferroelectrics using different techniques. The work also illustrates a method to evaluate the ferroelectric equivalent negative capacitance value and range of voltages, aiming at study and optimization of a stabilized negative capacitance FET. This study indicates that the intrinsic switching provides an adequate description of the polarization hysteresis in Si:HfO2 films. This confirms the usability of hafnia-based ferroelectrics for negative capacitance logic devices, and the important role that the intrinsic mechanism plays in the dielectric response of these materials.

AMER INST PHYSICS2021

Nanowire Tunnel FET with Simultaneously Reduced Subthermionic Subthreshold Swing and Off-Current due to Negative Capacitance and Voltage Pinning Effects

Matteo Cavalieri, Mihai Adrian Ionescu, Teodor Rosca, Ali Saeidi, Igor Stolichnov

Nanowire tunnel field-effect transistors (TFETs) have been proposed as the most advanced one-dimensional (1D) devices that break the thermionic 60 mV/decade of the subthreshold swing (SS) of metal oxide semiconductor field-effect transistors (MOSFETs) by using quantum mechanical band-to-band tunneling and excellent electrostatic control. Meanwhile, negative capacitance (NC) of ferroelectrics has been proposed as a promising performance booster of MOSFETs to bypass the aforementioned fundamental limit by exploiting the differential amplification of the gate voltage under certain conditions. We combine these two principles into a single structure, a negative capacitance heterostructure TFET, and experimentally demonstrate a double beneficial effect: (i) a super-steep SS value down to 10 mV/decade and an extended low slope region that is due to the NC effect and, (ii) a remarkable off-current reduction that is experimentally observed and explained for the first time by the effect of the ferroelectric dipoles, which set the surface potential in a slightly negative value and further blocks the source tunneling current in the off-state. State-of-the-art InAs/InGaAsSb/GaSb nanowire TFETs are employed as the baseline transistor and PZT and silicon-doped HfO2 as ferroelectric materials.

AMER CHEMICAL SOC2020

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

Sensing device for sensing minor charge variations

Cem Alper, Mihai Adrian Ionescu, Teodor Rosca

A charge sensing device for sensing charge variations in a charge storage area includes: a TFET having at least one sense gate; and a capacitive coupling for coupling the charge storage area with the sense gate.

2019

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In this work, we experimentally report the figures of merit of state-of-the-art heterostructure Tunnel Field-Effect-Transistor (TFET) arrays from room (300K) down to cryogenic temperature (10K) at supply voltages below 400mV. We demonstrate here, for the first time, that InAs/InGaAsSb/GaSb Nanowire (NW) TFETs are robust enough to maintain excellent figures of merit over a large temperature range even in devices with a large number arrayed nanowires (here, from 4 to 184 nanowires per device), accounting for technological variability. The investigated Tunnel FETs have temperature-independent min and average subthreshold swings of 45mV/dec/67mV/dec in large NW arrays, versus similar to 36/45mV/dec in smaller arrays, once the trap-assisted tunneling is removed (from 150K down to 10K). In all NW arrays we observe improvement of the on-current and of maximum transconductance, g(max), at cryogenic temperatures, with very little dependence of temperature, from 150K to 10K. The paper reports that in the range 150K to 10K only band-to-band-tunneling dominates the analog figures of merit of Tunnel FETs; we measured transconductance efficiencincies higher than 60V(-1) for small arrays (breaking the limit of CMOS at RT) and close to 42V(-1) for large arrays, for supply volrages smaller than 100mV, offering the possibility to design future energy efficient readouts and analog-to-digital converters. In contrast with cryogenic MOSFETs, Tunnel FETs show almost no hysteresis (

IEEE2018