Session E15: 2D Devices: Mobility and Energy Relaxation

Graphene based GHz detectors.

Graphene demonstrates great promise as a detector over a wide spectral range especially in the GHz range. This is because absorption is enhanced due to the Drude contribution. In the GHz range there are viable detection mechanisms for graphene devices. With this in mind, two types of GHz detectors are fabricated on epitaxial graphene using a lift off resist-based clean lithography process to produce low contact resistance.[1] ~Both device types use asymmetry for detection, consistent with recent thoughts of the photothermoelectric effect (PTE) mechanism. The first is an antenna coupled device. It utilizes two dissimilar contact metals and the work function difference produces the asymmetry. The other device is a field effect transistor constructed with an asymmetric top gate that~creates a PN junction and facilitates tuning the photovoltaic response.~ The response of both device types, tested from 100GHz to 170GHz, are reported. 1. Nath Anindya et al Applied Physics Letters 104, 224102 (2014)   

Optimization of thermoelectric power factor in ion-gated ultrathin WSe2 single crystals

We report an electric field tuning of the thermopower in ultrathin WSe2 single crystals over a wide range of carrier concentration by using electric double-layer (EDL) technique. We fabricated a micro-sized EDL transistor with on-chip heaters and thermometers for an ultrathin flake of WSe2. We succeeded in the optimization of power factor not only in the hole but also in the electron side, which has never been chemically accessed. The maximized values of power factor are one-order larger than that obtained by changing chemical composition, reflecting the clean nature of electrostatic carrier doping.   

Quantum lifetime in BN-encapsulated graphene devices

Encapsulating monolayer graphene in BN has lead to vastly enhanced device quality, leading to significantly increased mobility and quantum lifetimes on the order of picoseconds. However, magnetoresistance measurements in the quantum Hall regime reveal remnant disorder that continues to inhibit transport measurement. Here we report a study of the Shubnikov-de Haas oscillations in very high mobility devices. Comparison of the mean scattering and quantum lifetimes suggest that remote impurities remain the dominant scattering mechanism. The source of this remnant disorder, and the consequence for mobility enhancement in BN-supported 2D materials beyond graphene is discussed.   

Velocity Saturation of Hot Carriers in Two-Dimensional Transistors

Two-dimensional (2D) materials, including graphene and transition-metal dichalcogenides, have emerged in recent years as possible ``channel-replacement'' materials for use in future generations of post-CMOS devices. Realizing the full potential of these materials requires strategies to maximize their current-carrying capacity, while minimizing Joule losses to its environment. A major source of dissipation for hot carriers in any semiconductor is spontaneous optical-phonon emission, resulting in saturation of the drift velocity. In this presentation, I discuss the results of studies of velocity saturation in both graphene and molybdenum-disulphide transistors, emphasizing how this phenomenon impacts resulting transistor operation. While in graphene the large intrinsic optical-phonon energies promise high saturation velocities, experiments to date have revealed a significant degradation of the drift velocity that arises from the loss of energy from hot carriers to the underlying substrate. I discuss here how this problem can be overcome by implementing a strategy of nanosecond electrical pulsing [H. Ramamoorthy et al., Nano Lett., under review], as a means to drive graphene's hot carriers much faster than substrate heating can occur. In this way we achieve saturation velocities that approach the Fermi velocity near the Dirac point, and which exceed those reported for suspended graphene and for devices fabricated on boron nitride substrates. Corresponding current densities reach those found in carbon nanotubes, and in graphene-on-diamond transistors. In this sense we are able to ``free'' graphene from the influence of its substrate, revealing a pathway to achieve the superior electrical performance promised by this material. Velocity saturation is also found to be important for the operation of monolayer molybdenum-disulphide transistors, where it limits the drain current observed in saturation [G. He et al., Nano Lett. \textbf{15}, 5052 (2015)]. The implications of these results for 2D transistor performance will be explored in my presentation.   

Electrical Transport in Ultra-Short Atomically Thin Devices

Ultra-short nanoscale devices that incorporate atomically-thin materials have the potential to be the smallest electronics. These materials represent the ultimate size-scaling in the vertical dimension and could be ideal as channel, electrode, and dielectric materials for a variety of applications -- especially for ultrafast electronics. Such extremely-scaled devices can show unique transport characteristics that depended sensitively on their atomic-scale configurations. Here we report several atomically-thin ultra-short device schemes we have been developing which includes those consisting of single and bilayer graphene channels. Electrical transport measurements show very unique characteristics between these ultra-short devices that are highly sensitive to the atomic layer number. This sensitivity suggests that these ultra-short devices are strongly dependent on the unique chiral nature of the charge carriers in these atomically-thin channel materials.   

Chemical assembly of atomically thin transistors and circuits in a large scale

Next-generation electronics calls for new materials beyond silicon for increased functionality, performance, and scaling in integrated circuits. 2D gapless graphene and semiconducting TMDCs have emerged as promising electronic materials due to their atomic thickness, chemical stability and scalability. However, difficulties in the assembly of 2D electronic structures arise in the precise spatial control over the conducting and semiconducting crystals, typically relying on physically transferring them. Ultimately, this renders them unsuitable for an industrial scale and impedes the maturity of integrating atomic elements in modern electronics. Here, we report the large-scale spatially controlled synthesis of the single-layer MoS$_{\mathrm{2}}$ laterally in electrical contact with graphene using a seeded growth method. TEM studies reveal that the single-layer MoS$_{\mathrm{2}}$ nucleates at the edge of the graphene, creating a lateral van der Waals heterostructure. The graphene allows for electrical injection into MoS$_{\mathrm{2}}$, creating 2D atomic transistors with high transconductance, on-off ratios, and mobility. In addition, we assemble 2D logic circuits, such as a heterostructure NMOS inverter with a high voltage gain, up to 70.   

Nonlinear Ballistic Transport in Graphene Devices

Through the extreme size scaling of electronic devices, there is great potential to achieve highly efficient and ultrafast electronics. By scaling down the channel length in graphene transistors to the point where the mean free path exceeds the relevant channel length, the electron transport can transition from a diffusive regime to an intrinsic ballistic regime. In such a regime, both quantum tunneling at the electrode-channel interface and the screening length, as determined by electrode-channel barrier width, can have a strong effect on current nonlinearity and asymmetric gate response. Here we discuss our experimental results on nangap electrodes to graphene channels that show quantitative agreement with an intrinsic ballistic model. Moreover, this behavior persists to room temperature and on standard oxide substrates, providing strong evidence for a new regime of nonlinearity in graphene devices that could be of potential use for electronic applications.   

Resonant high harmonic generation in a ballistic graphene transistor with an AC driven gate

We report a theoretical study of time-dependent transport in a ballistic graphene field effect transistor. We develop a model based on Floquet theory describing Dirac electron transmission through a harmonically driven potential barrier. Photon-assisted tunneling results in excitation of quasibound states at the barrier. Under resonance condition, the excitation of the quasibound states leads to promotion of higher-order sidebands and enhanced higher harmonics of the source-drain conductance. The resonances in the main transmission channel are of the Fano form, while they are of the Breit-Wigner form for sidebands. We discuss the possibility of utilizing the resonances in prospective ballistic high-frequency devices, in particular frequency multipliers.   

Substrate dependence of Hall and Field-effect mobilities in few-layer MoS$_{2}$ field-effect transistors

In this work, we systematically study the Hall and field-effect mobilities of few-layer MoS$_{2}$ FETs fabricated on different substrates. Hall bar devices were fabricated on SiO$_{2}$ and hBN to directly measure carrier density. Standard four-probe transport measurement and Hall effect measurement were carried out for a wide temperature range to determine the carrier mobility and understand the scattering mechanisms. By comparing field-effect and Hall mobilities, we demonstrate that the intrinsic drift mobility of multiplayer MoS$_{2}$ in the high carrier density metallic region is independent of substrate and sample thickness. While the optical-phonon scattering remains the dominant scattering mechanism in MoS$_{2}$ devices on h-BN down to \textasciitilde 100 K, extrinsic scattering mechanisms start to degrade the carrier mobility of MoS$_{2}$ on all other substrates below \textasciitilde 200 K.   

What Makes Effective Gating Possible in Two-Dimensional Heterostructures?

Electrostatic gating provides a way to obtain key functionalities in modern electronic devices and to qualitatively alter materials properties. While electrostatic description of such gating gives guidance for related doping effects, inherent quantum properties of gating provide opportunities for intriguing modification of materials and unexplored devices. Using first-principles calculations for Co/bilayer graphene, Co/BN, and Co/benzene, as well as a simple physical model, we show that magnetic heterostructures with two-dimensional layered materials can manifest tunable magnetic proximity effects [1]. van der Waals bonding is identified as a requirement for large electronic structure changes by gating. In particular, the magnitude and sign of spin polarization in physisorbed graphene can be controlled by gating, which is important for spintronic devices [2,3]. [1] P. Lazi\'c, K. D. Belashchenko, and I. \v{Z}uti\'c, arXiv:1510.05404. [2] P. Lazi\'c, G. M. Sipahi, R. K. Kawakami, and I. \v{Z}uti\'c, Phys. Rev. B {\bf 90}, 085429 (2014). [3] H. Dery et al., IEEE Trans. Electron. Dev. {\bf 59}, 259 (2012).   

Highly sensitive hBN/graphene hot electron bolometers with a Johnson noise readout

Graphene has remarkable opto-electronic and thermo-electric properties that make it an exciting functional material for various photo-detection applications. In particular, owed to graphenes unique combination of an exceedingly low electronic heat capacity and a strongly suppressed electron-phonon thermal conductivity G$_{th}$, the electronic and phononic temperatures are highly decoupled allowing an operation principle as a hot electron bolometer (HEB). Here we demonstrate highly sensitive HEBs made of high quality hBN/graphene/hBN stacks and employ a direct electronic temperature read out scheme via Johnson noise thermometry (JNT). We perform combined pump-probe and JNT measurements to demonstrate strongly damped C$_{e}$ and G$_{th}$ in the ultra-low impurity $\sigma_{i}=$10$^{9}$ cm$^{-2}$ hBN/G/hBN stacks, which result in unprecedented photo-detection sensitivity and noise equivalent power for graphene HEBs.   

Design and fabrication of an antenna-coupled graphene terahertz mixer

Graphene has shown promise for tunable terahertz (THz) technology, including detectors, modulators, filters, and emitters. Graphene exhibits a significant change in conductivity when the Fermi energy is altered by applying a gate voltage. Near the Dirac point, graphene field effect transistors (FETs) show a strongly nonlinear response (i.e. a strong change in resistivity with applied voltage) that can be exploited to provide efficient rectification and mixing of THz signals. Although rectification in graphene field-effect transistors has been demonstrated, heterodyne mixing in the THz band has not been explored.~We examine a THz graphene mixer using an antenna-coupled graphene FET configuration. We will discuss the antenna and graphene device design optimized for heterodyne mixing 0.35 THz.~In addition, fabrication and preliminary measurements of a lower frequency prototype will be presented to demonstrate the principle of the operation.