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Milli-Volt Switch Technologies for Energy Efficient Computation and Sensing

Periodic Reporting for period 3 - Milli-Tech (Milli-Volt Switch Technologies for Energy Efficient Computation and Sensing)

Reporting period: 2019-06-01 to 2020-11-30

The Milli-Tech proposal aims at a novel technology platform serving both computation and sensing: electronic switch architectures, called steep slope switches, exploiting new device physics and concepts in emerging 2D materials to achieve operation at voltages below 100 millivolts. Such switches will have a subthreshold slope below 10mV/decade, significantly more abrupt than MOSFET thermal limit of 60mV/decade at room temperature and in great advance to any beyond CMOS switches. Such characteristics will dramatically improve both the energy efficiency of logic circuits and the transduction sensitivity for many classes of sensors.

The project will develop a technological platform called ‘millivolt technology’ focusing on low power digital and sensing/ analog electronic functions exploiting steep slopes, with the goal of lowering the energy per useful function (computed and sensed bit of information) by a factor of 100x. Such ultra-low operation voltage will contribute to solving major challenges of nanoelectronics such as power issues and it will enable energy efficient super-sensitive sensors for Internet-of-Everything (IoE). Milli-Tech includes fundamental research on new solid-state steep slope device concepts: heterostructure tunnel FETs in 2D Transition-Metal-Dichalcogenides (TMD), 2D Van der Waals super-lattice energy filter switch and hybrid architectures combining two switching principles: band-to-band-tunneling and metal-insulator-transition or negative capacitance in VO2, used as additive technology boosters.

Milli-Tech plans breakthroughs by precise demonstrators:
(i) energy efficient computation blocks for Von-Neumann ICs at sub-100mV;
(ii) Active Pixel Sensors based on 2D TMD/GeSn tunnel FETs for IR imagers;
(iii) Terahertz detectors based on hybrid 2D VO2/TMD switches (iii) ultra-sensitive 2D steep slope charge detectors for biosensing.

The high-risk ‘millivolt technology’ will bring 2D materials on established advanced CMOS platforms and will be highly rewarding by enabling the energy efficient revolution needed for the Internet of Things.
"MilliTech implemented from Period #1 to Period #3 the following actions:

• Hiring of the PhD students according to the workplan and the scientific objectives of the project.
• Ordering, installing, calibrating and accepting the Pulsed Laser Deposition (PLD) equipment needed for producing the hybrid devices using phase change VO2 material and 2D materials. This equipment is currently able to produce layers of VO2 with thicknesses of less than 50nm in which the MIT switching has been demonstrated.
• Realizing the very first technological proof of concepts in terms of fabricated devices and their characterization followed by the first publications supported in MilliTech.
• Participation of the MilliTech team in international events. remarkable was the participation of three Millitech members (2 PhDs and the PI) in IEEE International Electron Devices Meeting 2017, San Francisco, the leading international conference in micro/nanoelectronics. Nicolo Oliva presented the first hybrid VO2/MOS2 junction transistor, Francesco Bellando reported a demonstration of functionalization of gate stacks on thin SOI FETs for electrolyte sensing (his work being included in the press kit of the conference), and prof. Adrian Ionescu presented a keynote talk centred on the core theme of MilliTech: Energy efficient computing and sensing in the Zettabyte era: from silicon to the cloud. During the reporting period, the project produced 11 conference and journal contributions (and another 2 have been accepted early 2018).
In the 3rd period many works matured and some of the PhD reached the completion of their work:
• Nicolo Oliva graduated with his PhD thesis entitled: ''At the end of scaling: 2D materials for computing and sensing applications'', EPFL. In period #3 he made two major publications in Millitech, demonstrating, according to our initial planning first subthermionic 2D/2D tunnel FEts and even the possibility to hybridize them on same flake with a MOSFET to achieve a dual-transport device
-> WSe 2/SnSe 2 vdW heterojunction Tunnel FET with subthermionic characteristic and MOSFET co-integrated on same WSe 2 flake, N Oliva, J Backman, L Capua, M Cavalieri, M Luisier, AM Ionescu, Nature npj 2D Materials and Applications 4 (1), 1-8, 2020.
-> Co-integrated Subthermionic 2D/2D WSe2/SnSe2 Vertical Tunnel FET and WSe2 MOSFET on same flake: towards a 2D/2D vdW Dual-Transport Steep Slope FET, N Oliva, L Capua, M Cavalieri, AM Ionescu, 2019 IEEE International Electron Devices Meeting (IEDM), 37.2. 1-37.2.4.
• Francesco Bellando graduated with hid PhD entitled: Sweat monitoring with CMOS compatible technology: ISFETS and beyond , EPFL, 2020. In lats period, he made one major publications concerning the use of negative concept principle in biosensing:
-> Subthermionic negative capacitance ion sensitive field-effect transistor, F Bellando, CK Dabhi, A Saeidi, C Gastaldi, YS Chauhan, AM Ionescu, Applied Physics Letters 116 (17), 173503
• The period #3 is the period in which a new raising star PhD make his contributions in the group, Sadegh Kamaei, who is working towards the integration in 2D devices of ferroelectric gate stacks, to achieve negative capacitance 2D/2D devices.His work is continuing the work of Ali Saeidi on negative capacitance concept but with full focus on 2D ferroelectric implementations. Sadegh will target to evaluate in the last reporting period the feasibility of an source energy filter 2D device.
• The work on VO2 continued with applications in THz ( Fatameh Qaderi) and in low energy oscillator IC design (T. Rosca) aiming at electromagnetic power sensing. The work of T. Rosca also included the planned modeling of this type of devices. Their work is currently in revision in Nature Communication and the full publication out in period #4."
In terms of major achievements beyond the state of the art for the period of reporting, these are related to:

(i) the realization of first ever van der Waals devices based on VO2/MOS2 junctions and their use for electronic switching and for light detection;
(ii) the achievement of the first ever Hybrid Phase-Change – Tunnel FET (PC-TFET) Switch with Subthreshold Swing < 10mV/decade and sub-0.1 body factor that has been benchmarked for both digital and analog applications;
(iii) the realization of the functionalization of HfO2 gate stacks to selectively detect electrolytes in sweat that is currently transferred to 2D tunnel FET sensors;
(iii) the optimization of a technological process for the transfer of 2D material flakes with improved control that enabled the fabrication of various classes of 2D devices.
(iv) the realization of complementary 2D FETs (n- and p-type) based on 2D BP material, by engineering the workfunctions of the metal contacts.
(v) the very first co-integration of a subthermionic 2D/2D WSe2/SnSe2 Vertical Tunnel FET and WSe2 MOSFET on same flake.
(vi) the first experimental demonstration of negative capacitance pH ISFET sensor, with improved current sensitivity.
(vii) the demonstration of GHz to THz power detection with stochastic sensors using VO2 thin film devices and oscillators (including full analog modeling of VO2 switches).
2D/2D tunnle FET and MOSFET integrated on same flake of n a WSe2/SnSe2 material system platform.
First MoS2/VO2 fabricated junction and transistor by N. Oliva in Milli-Tech.