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Phase-Change Materials and Switches for Enabling Beyond-CMOS Energy Efficient Applications

Periodic Reporting for period 1 - PHASE-CHANGE SWITCH (Phase-Change Materials and Switches for Enabling Beyond-CMOS Energy Efficient Applications)

Reporting period: 2017-01-01 to 2018-12-31

The project PHASE‐CHANGE SWITCH addresses the need for combined energy efficiency and extended functionality with the engineering of new classes of solid‐state Beyond CMOS switches exploiting the abrupt phase‐change (Metal‐Insulator‐Transition ‐ MIT) in materials and at
temperatures that make them interesting for electronic circuits and systems by their performance and scalability. The project includes disruptive research contributions on the whole value chain, from novel phase‐change materials to new device and
circuit architectures together with their scaling and integration on silicon and GaN platforms. Materials alloying and straining techniques in phase‐change systems are used for the engineering of the transition temperature and the ON and OFF band gaps (conductivity) of VO2.

The project focuses on smart design and exploitation of the unique properties of the phasechange VO2 beyond CMOS switches, by targeting with the same technology platform:
(i) von‐ Neumann steep‐slope logic devices and circuits, to extend CMOS with novel functionality and energy efficiency,
(ii) uniquely reconfigurable energy efficient radio‐frequency (RF) circuit functions from 1 to 100GHz,
(iii) unconventional scalable neuristors exploiting the hysteretic RC switching behaviour for neuromorphic computation, and,
(iv) disruptive classes of solid‐state devices for neuromorphic computation, exploiting non‐volatile memory effects.

The project is expected to create new applications and markets and reinforce the leadership of European industrial players in the field of energy efficient Internet‐of‐Things and high frequency communications.
WP1 Figures of Merit (FoMs) at phase change material level have been derived and target values for these FoMs have defined (according to the targeted device functions), these were continuously updated in the second period.
Furthermore, Raman spectroscopy as a fast and effective characterization technique for VO2 layers has been successfully established. Among main achievements of the reporting period, we cite: (i) demonstration of ultra-thin VO2 films by ALD (UCAM) with very promising properties for phase switching, (ii) realization of very first Ge-doped VO2 films by sputtering and PLD (EPFL), with the unique property of increasing the phase-change temperature close to 80-90°C, without deteriorating the ON/OFF conductivity ratio, which opens new avenue for RF electronic applications and industrial take-up.
WP2: two-terminal VO2 switches were optimized for steep-slope switching and for oscillating neural networks. VO2-based oscillators have been fabricated and their operational characteristics were analysed. The oscillation frequency was controlled with an additional external capacitor. 3T hybrid VO2 transistors, with the VO2 switch connected to the drain and the gate of transistors (in order to provide a more abrupt switches at the phase transition pint), consisting of a nanowire tunnel-FET (TFET) and a 2T VO2 switch, were fabricated and tested. Steep-slope (with values lower than 60mV//dec at room temperature) have been demonstrated.
WP3: Optimized VO2 switches were fabricated in order to be implemented for reconfigurable RF functions in GaN monolithic microwave integrated circuits (MMIC) technology or in high resistivity Si RF CMOS technologies. The RF VO2 switch technology on GaN was demonstrated with improved RF performances, high isolation better than 20 dB and low losses better than 0.6 dB, which is an advance in the state of the art.
On silicon technology, VO2 RF switches have been implemented for reconfigurable defected ground plane Ka band filters exhibiting with 19% tunning range ( and lowest reported footprint), reconfigurable inductors (with 55% tunning range and 2 reconfigurable states for S and C bands) and phase shifters were fabricated and reported in 3 journal publications, advancing the state of the art. A successful CMOS compatible fabrication has been achieved, however due to the limited values of VO2 conductivity in the metallic state (around 20kS/m-30kS/m), split ring filters for metamterial applications were developed with higher attenuation levels and tuning ranges are obtained (23.3%). Peano inductors with higher quality factors in the on state, higher tuning range with 3 reconfigurable states were also fabricated.
WP4 is connecting the VO2 switching properties to its use in neuromorphic applications. We have investigated: (i) VO2-based relaxation oscillators and (ii) VO2 non-volatile memory elements. The relaxation oscillators were fabricated on 4” Si/SiO2 wafers with polycrystalline VO2 films deposited in EPFL. Such films showed an insulator-to-metal transition around 340 K with ~15 K hysteresis. Relaxation oscillators were obtained by using external circuitry connected through a probe card.
A circuit model of these devices was validated to demonstrate hebbian learning capabilities of networks of coupled oscillators.
Non-volatile memory switching was demonstrated for VO2 films grown by pulsed laser deposition (PLD). X-ray diffraction (XRD) and atomic force microscopy (AFM) were carried out to characterize the film quality.
WP5 explored by simulation how to optimize the properties of VO2 material. The actual transition temperature Tc is not easy to vary, which would make it even more valuable. WP5 tries to predict based on atomistic models how Tc would vary by alloying (doping) with different oxides. First models have been developed and, currently, a study of the physical understanding of how to engineer the level (%) of Ge and metal doping and their impact on TC and VO2 bandgap in OFF state. The models of WP5 are expected to also help the fundamental performance limits of VO2 switches.
Phase-Change Switch project made significant progress in the reporting period, achieving all the milestones and the majority of deliverables (with the exception of the single one that is not impacting the project progress).
Essentially, the material production of VO2 with optimized properties by three methods (sputtering, PLD and ALD) made strong progress, including specific films for energy efficiency and RF functions and with increased phase change temperature by Ge doping.The Consortium produced multiple types of devices that are discussed in the corresponding WPs: steep-slope switches, VO2 oscillators for neuromorphic applications and novel reconfigurable RF electronic functions. In the field of RF devices, some of the RF performances achieved in both VO2 on GaN and Si technology outperform the state of the art. This part of the project was also particularly successful in terms of publications. The understanding of the physics of VO2 and its modeling is also in great progress but the predictive modeling has to be validated against the experimental results in the coming period.
4 journal publications, 2 conference papers and one accepted paper in press were published.