Emanuele Andrea Casu

A Steep-Slope Transistor Combining Phase-Change and Band-to-Band-Tunneling to Achieve a sub-Unity Body Factor

Steep-slope transistors allow to scale down the supply voltage and the energy per computed bit of information as compared to conventional field-effect transistors (FETs), due to their sub-60 mV/decade subthreshold swing at room temperature. Currently pursued approaches to achieve such a subthermionic subthreshold swing consist in alternative carrier injection mechanisms, like quantum mechanical band-to-band tunneling (BTBT) in Tunnel FETs or abrupt phase-change in metal-insulator transition (MIT) devices. The strengths of the BTBT and MIT have been combined in a hybrid device architecture called phase-change tunnel FET (PC-TFET), in which the abrupt MIT in vanadium dioxide (VO2) lowers the subthreshold swing of strained-silicon nanowire TFETs. In this work, we demonstrate that the principle underlying the low swing in the PC-TFET relates to a sub-unity body factor achieved by an internal differential gate voltage amplification. We study the effect of temperature on the switching ratio and the swing of the PC-TFET, reporting values as low as 4.0 mV/decade at 25 °C, 7.8 mV/decade at 45 °C. We discuss how the unique characteristics of the PC-TFET open new perspectives, beyond FETs and other steep-slope transistors, for low power electronics, analog circuits and neuromorphic computing.

Field-enhanced design of steep-slope VO 2 switches for low actuation voltage

The abrupt metal-insulator transition in vanadium dioxide (VO2) offers novel performance and functionality for beyond CMOS switches, enabling simultaneous high ON current and ultra-steep subthreshold slope with low temperature dependence. We developed a field-enhanced design of 2-terminal VO2 switches that allows decreasing their actuation voltage without affecting their performance and reliability. Exploiting this design, we characterized VO2 switches with extremely abrupt transitions (<; 1 mV/dec) until 60°C and a reduction in actuation voltage up to 38.3% with respect to conventional devices.

Tunable RF Phase Shifters Based on Vanadium Dioxide Metal Insulator Transition

This paper presents the design, fabrication, and electrical characterization of a reconfigurable RF capacitive shunt switch that exploits the electro-thermally triggered vanadium dioxide (VO2) insulator to metal phase transition. The RF switch is further exploited to build wide-band RF true-time delay tunable phase shifters. By triggering the VO2 switch insulator to metal transition (IMT), the total capacitance can be reconfigured from the series of two metal–insulator–metal (MIM) capacitors to a single MIM capacitor. The effect of bias voltage on losses and phase shift is investigated, explained, and compared to the state of the art in the field. We report thermal actuation of the devices by heating the devices above VO2 IMT temperature. By cascading multiple stages a maximum of 40° per dB loss close to 7 GHz were obtained.

Lowering motional resistance by partially HfO 2 gap filling in double-ended tuning fork MEMS resonators

This paper reports a 80nm air gap double-ended tuning fork (DETF) resonator partially filled with 20nm of HfO2. The device operates at 10.7 MHz with a quality Q-factor of 7930 and a motional resistance, Rm, x18 better than for the measured counterparts in air-gaps. An enhancement of the S21 transmission signal of 6dB in addition to the reduction of the Rm have been achieved thanks to the improvement of the electromechanical coupling factor by partially filling the electrode-to-resonator-gap with an atomic layer deposition (ALD) of 20nm of HfO2 high-k dielectric (εr∼19). This result proves an outstanding enhancement in the detection signal while keeping all the desired properties of flexural resonators for future mechanical sensing applications.

MoS 2 /VO 2 vdW heterojunction devices: Tunable rectifiers, photodiodes and field effect transistors

In this work we report a new class of ultra-thin film devices based on n-n van der Waals (vdW) heteroj unctions of MoS<inf>2</inf> and VO<inf>2</inf>, which show remarkable tunable characteristics. The favorable band alignment combined with the sharp and clean vdW interface determines a tunable diode-like characteristic with a rectification ratio larger than 10³. Moreover, the heterojunction can be turned into a Schottky rectifier with higher on-current by triggering the VO2 insulator to metal transition (IMT), by either applying a sufficiently large voltage or increasing the temperature above 68 °C. The proposed devices are photosensitive with linear photoresponse and temperature tunable photoresponsivity values larger than 1 in the 500/650 nm wavelength range. We finally report the first ever field-effect transistor based on gated MoS<inf>2</inf>/VO<inf>2</inf> heterojunctions, which is a true low power FET exploiting a phase change material where the electrostatic doping effect of the gate on the junction results in a subthreshold slope (SS) of 130 mV/dec at room temperature, I<inf>ON</inf>/I<inf>OFF</inf> > 10³ and I<inf>oFF</inf> < 5 pA/μm at Vd = 1.5V.

Complementary black phosphorous FETs by workfunction engineering of pre-patterned Au and Ag embedded electrodes

We propose and experimentally demonstrate top-gated complementary n- and p-type black phosphorous FETs by engineering the workfunction of pre-patterned electrodes embedded in a SiO2 layer. Pre-patterned electrodes offer the possibility of reducing the exposure time of exfoliated flakes to oxidant agents with respect to top-contacted devices and maximize the accessible area for sensing applications. The devices are realized by exfoliating multilayer black phosphorous flakes on top of pre-patterned embedded source and drain contacts. A capping layer consisting of 15 nm thick Al2O3 is used to prevent flakes degradation and serves as top gate dielectric. We deposited both Au and Ag contacts to investigate the impact of the electrode workfunctions on BP FETs polarity. Au contacted devices showed p-type conduction with ON/OFF current ratio 140 and holes mobility up to 40 cm2V−1s−1. Devices with Ag contacts showed prevalent n-type conduction with ON/OFF ratio 1700 and electron mobility 2 cm2V−1s−1. The reported results represent a substantial improvement with respect to reported alternative implementations of black phosphorous FETs with pre-patterned, non-embedded electrodes. Moreover, we demonstrate that Ag is a promising metal for electron injection in black phosphorous FETs.

Shunt capacitive switches based on VO2 metal insulator transition for RF phase shifter applications

This paper presents a wide-band RF shunt capacitive switch reconfigurable by means of electrically triggered Vanadium Oxide (VO2) phase transition to build a true-time delay (TTD) phase shifter. The concept of VO2-based reconfigurable shunt switch has been explained and experimentally demonstrated by designing, fabricating and characterizing an 819 μm long unit cell. The effect of bias voltage on losses and phase shift has been studied and explained. By triggering the VO2 switch insulator to metal transition (IMT) the total capacitance can be reconfigured from the series of two metal-insulator-metal (MIM) capacitors to a single MIM capacitor. Higher bias voltages are more effective in this reconfiguration and give a higher phase shift. The optimal achievable performance has been shown heating the devices above VO2 IMT temperature. A maximum of 16° per dB loss has been obtained near the design frequency (10 GHz).

Van der Waals MoS2/VO2 heterostructure junction with tunable rectifier behavior and efficient photoresponse

Junctions between n-type semiconductors of different electron affinity show rectification if the junction is abrupt enough. With the advent of 2D materials, we are able to realize thin van der Waals (vdW) heterostructures based on a large diversity of materials. In parallel, strongly correlated functional oxides have emerged, having the ability to show reversible insulator-to-metal (IMT) phase transition by collapsing their electronic bandgap under a certain external stimulus. Here, we report for the first time the electronic and optoelectronic characterization of ultra-thin n-n heterojunctions fabricated using deterministic assembly of multilayer molybdenum disulphide (MoS2) on a phase transition material, vanadium dioxide (VO2). The vdW MoS2/VO2 heterojunction combines the excellent blocking capability of an n-n junction with a high conductivity in on-state, and it can be turned into a Schottky rectifier at high applied voltage or at temperatures higher than 68 °C, exploiting the metal state of VO2. We report tunable diode-like current rectification with a good diode ideality factor of 1.75 and excellent conductance swing of 120 mV/dec. Finally, we demonstrate unique tunable photosensitivity and excellent junction photoresponse in the 500/650 nm wavelength range.

Solid-gap resonators based on PVDF-TrFE

High frequency wine-glass mode bulk MEMS resonators actuated by means of capacitive transduction have been fabricated by using solid-gaps based on polyvinylidenefluoride-trifluoroethylene (PVDF-TrFE). The fabrication process flow of patterned PVDF-TrFE gaps applied in RF MEMS is presented for the first time. Measurements of the polarization of the material and the frequency and voltage dependence of the capacitance in the actuation gaps of the resonators are provided.

Design and fabrication of high-k filled sub-100 nm gap resonators with embedded dielectric field effect transistor for ultra high frequency applications

A novel fabrication process for the integration of Field Effect Transistors in electrostatically actuated bulk acoustic resonators is demonstrated. ALD-deposited HfO2 is used as a high-k dielectric for the FET and as an etch-stop layer during the release of the resonator structure as well, enabling the creation of sub-100 nm air-gap resonators with FET amplification enhancement.