Session F56: Devices from 2D Materials: Function, Fabrication and Characterization - II

Live

Atomically thin two-dimensional (2D) crystals are promising channel materials for extremely scaled field-effect transistors (FETs) for the 2030 era [1]. In the quest of ultra-scaled transistors, both channel length (distance from source to drain contacts) and contact length (distance that the contacts overlap the 2D channel) must be scaled. However, contacting 2D materials at scaled contact lengths (L_c < 30 nm) has rarely been pursued or studied in-depth. In this work, we experimentally scaled contact length for Ni-contacted MoS₂ FETs and use asymmetrical contact measurements (ACM) as a new approach for characterizing the devices. We found that, contrary to most previous reports, top contacts can be scaled down to ~30 nm without noticeable degradation in contact resistance. Surprisingly, we also observed significant self-heating in scaled contacts in the saturation regime. While the first observation is promising for extremely scaled FET technologies, the second illustrates that current crowding in metal-2D contacts is a challenge toward the development for future scaled devices.
[1] IEEE International Roadmap for Devices and Systems. https://irds.ieee.org/ (2020).   

Live

We report the creation of accumulation-type ohmic contacts between an intrinsic 2D semiconductor and a degenerately doped 2D semiconductor acting as a 2D metal. Our back-gated WSe₂ field-effect transistors (FETs) with bottom-contacts exhibit excellent device performance, including linear output characteristics, a high on/off ratio of 10⁸, and two-terminal extrinsic field-effect mobility approaching the phonon-limited intrinsic mobility. As the temperature decreases, the drain-source current increases while the output characteristics remain linear. Because the back-gate does not electrostatically modulate the intrinsic WSe₂ in the drain/source contact regions due to screening by the bottom-contacts, the observed excellent device performance and ohmic behavior can be attributed to the formation of accumulation-type ohmic contacts that are free of a Schottky barrier. The accumulation-type ohmic contacts are further confirmed by dual-gated measurements, where a positive top-gate voltage is applied to deliberately deplete the holes in WSe₂ at the drain/source contacts.   

Live

Transistor scaling has been the main driver for a giant boom in productivity and for the development of society and technology as we know them. However, silicon transistor scaling is becoming increasingly difficult and potentially approaches its physical limits. 2D materials could enable this scaling beyond what could be achieved with Si. We will discuss here how a roadmap for 2D materials adoption in transistor scaling can be built and potential technology nodes where these materials can be adopted. Challenges for the technical implementation abound, and we will outline our work to solve them. We will also cover design-technology co-optimization aspects and explain how model calibration can speed-up technological adoption. In this context, we will outline stacked transistor architectures where 2D materials could serve as channel and detail our work for integrated these in flows on 300mm wafers in imec’s CMOS fab.   

Live

Establishing ohmic contact to van der Waals semiconductors such as MoS₂ is crucial to unlocking their full potential in next-generation electronic devices. Encapsulation of few layer MoS₂ with hBN preserves the material’s electronic properties but makes electrical contacts more challenging. Progress toward high quality edge contact to encapsulated MoS₂ has been recently reported. We evaluate an edge contact methodology using sputtered MoRe, a Type II superconductor with a relatively high critical field and temperature. While the resulting contact is not ohmic, this work has ramifications for future contact recipes and may hold promise for application to hybrid structures.   

Live

We demonstrate the integration of TiO2 nanophotonics and monolayer transition metal dichalcogenides (TMDCs) by fabricating a TiO2 waveguide on top of hBN-encapsulated monolayer tungsten diselenide (WSe2). The evanescent field of the propagating modes in the waveguide carries transverse optical spins, which can selectively couple to the WSe2 excitonic states in the two opposite valleys. The coupling allows us to generate a flux of spatially separated valley-polarized excitonic states when we pump from the waveguide. This study sheds light on future valleytronic applications of nanophotonic-TMDC interfaces.   

Live

Layered Van der Waals (VdW) superconductors are a great promise for high frequency devices such as qubits, due to their highly crystalline nature and potentially defect free interfaces. A major obstacle toward this goal is the requirement for extremely high-performance contacts, which is challenging due to the air-sensitivity of common VdW superconductors such as NbSe₂ .We have developed and tested a reliable lift-off recipe for making superconducting contact between multi-layer crystals of NbSe₂ and aluminum films, using argon ion milling to strip away degraded layers of NbSe₂ before in situ aluminum deposition. Because performance at the level required for quantum applications cannot be tested by low-frequency techniques, we have fabricated microwave resonators with embedded NbSe₂ flakes and characterized these at cryogenic temperature. We find that the contacts achieved have low enough losses to be used in high-quality quantum devices.   

Live

Recently, non-volatile switching has been observed in various monolayer and multilayer 2D films[1]. In this contribution, we will show that wafer-scale 2D-materials such as MoS2 and hBN can be used for low-power non-volatile switches with applications in communications systems for 5G and 6G. The 2D-based RF switch is a metal-insulator-metal vertical structure made of two electrodes separated by a thin 2D film. It is promising for high-frequency operation because of the favorable scaling compared to other emerging technologies[2].
Owing to the ultimate thickness of the 2D dielectrics, cut-off frequency and switching time figures of merit are superior to those of existing solid state devices. From high-frequency electrical characterization (up to 500GHz, in the THz band) we determine a cut-off frequency figure of merit of 129THz. Furthermore, the switches are 50 times more efficient than other non-volatile switches in terms of a d.c. energy-consumption metric, which is an important consideration for ubiquitous mobile systems. To illustrate the potential of the devices in a real-life application scenario, we operate a switch to route a 125Gbps QAM-32 datastream at 300GHz.

References:
[1] R. Ge et al., Atomristor: Nonvolatile Resistance Switching in Atomic Sheets of Transition Metal Dichalcogenides, Nano Lett. 2018, 18, 434−441
[2] M. Kim et al., Analogue switches made from boron nitride monolayers for application in 5G and terahertz communication systems, Nat. Elec.,2020, 3,479–485   

Live

Fabry-Pérot interferometry of fractional quantum Hall edge states has long been known to provide an effective experimental probe of non-abelian statistics, for instance in even denominator fractional quantum Hall states. Recent work in high quality bilayer graphene van der Waals heterostructures has revealed even-denominator states with large energy gaps, bringing such experiments within reach. However, fabrication of the requisite gate geometries in a van der Waals heterostructure is challenging due to the absence of a band gap and the disorder typically introduced by standard additive as well as subtractive nanofabrication processes. In this talk, we will present progress towards improved gate-defined Fabry-Pérot interferometers using AFM oxidation lithography. We show that sub-100 nm features can be patterned in graphite flakes, and these patterns transferred into arbitrary gate layers within a van der Waals heterostructure. Besides applications in quantum Hall effects, our results suggest a versatile new architecture for local electrostatic control of ultra-high mobility two dimensional electron systems.   

Live

The momentum-relaxing length scale (due to electron-phonon, electron-defect scattering) in two-dimensional carrier systems can be as large as microns, leading to a breakdown of diffusive transport in micron-scale devices. Depending on the strength of momentum-conserving electron-electron scattering, transport in the device can either be ballistic or hydrodynamic. The ballistic regime is characterized by electrons scattering predominantly against the device boundaries, whereas the hydrodynamic regime sets in when electron-electron scattering is sufficiently strong. Both regimes have distinct nonlocal current-voltage relationships, which have been used in several experiments (e.g., in graphene, GaAs/AlGaAs) to detect the departure from diffusive transport. We now exploit the nonlocality inherent in both regimes to design a current amplifier. The amplification is determined by the device geometry, and scales linearly with the appropriate device-scale. We showcase the striking non-diffusive current flow in the amplifier. The operational principle, based on nonlocality, is distinct from previous amplification methods based on nonlinear current-voltage relations (such as the Venturi effect).   

On Demand

Using a computational approach based on the driven diffusion equation for a dipolariton wave packet, we simulate the diffusive dynamics of dipolaritons in a patterned optical microcavity embedded with a transition metal dichalcogenide (TMDC) heterogeneous bilayer. By considering exciton-dipolaritons, which are a three way superposition of direct excitons, indirect excitons and cavity photons; we are able to drive the dipolaritons in our system by the use of an electric voltage and investigate their diffusive properties. More precisely, we study the propagation of dipolaritons present in a MoSe₂-WS₂ heterostructure, where the dipolariton propagation is guided by Y-shaped and Ψ-shaped channel guides. We also consider the propagation of dipolaritons in the presence of a buffer in the Ψ-shaped channel branches and study the resulting changes in efficiency. By our considerations of geometrically novel dipolariton channel guides, we are able to replicate the dipolariton redistribution efficiencies of previously proposed polaritronic applications and propose novel designs for optical transistors at room temperature.   

Not Participating

Two-dimensional heterostructure field-effect transistors (2D-HFETs) with one-dimensional electrical contacts to two-dimensional channels have recently shown improved device performance. While a number of low-frequency noise studies exist on bare graphene devices supported on silicon dioxide gate insulators with surface contacts, such studies in heterostructure devices comprised of epitaxial graphene on hexagonal boron nitride (hBN) with channel side contacts are extremely limited. In this talk, we present a systematic, temperature-dependent study of electrical transport and low-frequency noise in edge-contacted high mobility HFET with a single atomic layer graphene channel encapsulated by hexagonal boron nitride and demonstrate ultra-low noise with Hooge parameter ~ 0.00001. We combine measurements and modeling based on underlying microscopic scattering mechanisms caused by charge carriers and phonons and finally correlate the high-performance, temperature-dependent transport of the device with its noise characteristics. Our study provides a pathway towards engineering low noise graphene-based high-performance 2D-FETs with one-dimensional edge contacts for applications such as digital electronics and chemical/biological sensing.