Showing papers by "Jing Guo published in 2013"

One-dimensional electrical contact to a two-dimensional material.

Abstract: Heterostructures based on layering of two-dimensional (2D) materials such as graphene and hexagonal boron nitride represent a new class of electronic devices. Realizing this potential, however, depends critically on the ability to make high-quality electrical contact. Here, we report a contact geometry in which we metalize only the 1D edge of a 2D graphene layer. In addition to outperforming conventional surface contacts, the edge-contact geometry allows a complete separation of the layer assembly and contact metallization processes. In graphene heterostructures, this enables high electronic performance, including low-temperature ballistic transport over distances longer than 15 micrometers, and room-temperature mobility comparable to the theoretical phonon-scattering limit. The edge-contact geometry provides new design possibilities for multilayered structures of complimentary 2D materials.

Degenerate n-doping of few-layer transition metal dichalcogenides by potassium.

Abstract: We report here the first degenerate n-doping of few-layer MoS2 and WSe2 semiconductors by surface charge transfer using potassium. High-electron sheet densities of ~1.0 × 10(13) cm(-2) and 2.5 × 10(12) cm(-2) for MoS2 and WSe2 are obtained, respectively. In addition, top-gated WSe2 and MoS2 n-FETs with selective K doping at the metal source/drain contacts are fabricated and shown to exhibit low contact resistances. Uniquely, WSe2 n-FETs are reported for the first time, exhibiting an electron mobility of ~110 cm(2)/V·s, which is comparable to the hole mobility of previously reported p-FETs using the same material. Ab initio simulations were performed to understand K doping of MoS2 and WSe2 in comparison with graphene. The results here demonstrate the need of degenerate doping of few-layer chalcogenides to improve the contact resistances and further realize high performance and complementary channel electronics.

Ballistic InAs nanowire transistors.

Abstract: Ballistic transport of electrons at room temperature in top-gated InAs nanowire (NW) transistors is experimentally observed and theoretically examined. From length dependent studies, the low-field mean free path is directly extracted as ∼150 nm. The mean free path is found to be independent of temperature due to the dominant role of surface roughness scattering. The mean free path was also theoretically assessed by a method that combines Fermi’s golden rule and a numerical Schrodinger–Poisson simulation to determine the surface scattering potential with the theoretical calculations being consistent with experiments. Near ballistic transport (∼80% of the ballistic limit) is demonstrated experimentally for transistors with a channel length of ∼60 nm, owing to the long mean free path of electrons in InAs NWs.

On Monolayer ${\rm MoS}_{2}$ Field-Effect Transistors at the Scaling Limit

Abstract: The ultimate scaling limit of double-gate molybdenum disulfide (MoS2) field-effect transistors (FETs) with a monolayer thin body is examined and compared with ultrathin-body Si FETs by self-consistent quantum transport simulation in the presence of phonon scattering. Modeling of phonon scattering, quantum mechanical effects, and self-consistent electrostatics allows us to accurately assess the performance potential of monolayer MoS2 FETs. The results revealed that monolayer MoS2 FETs show 52% smaller drain-induced barrier lowering (DIBL) and 13% smaller subthreshold swing (SS) than 3-nm-thick-body Si FETs at a channel length of 10 nm with the same gating. With a requirement of DIBL , the scaling limit of monolayer MoS2 FETs is assessed to be 8 nm, comparing with 10 nm of the ultrathin-body Si counterparts due to the monolayer thin body and higher effective mass, which reduces direct source-to-drain tunneling. By comparing with the international technology roadmap for semiconductor (ITRS) target for high performance logic devices of 2023; double-gate monolayer MoS2 FETs can fulfill the ITRS requirements.

Device Performance of Heterojunction Tunneling Field-Effect Transistors Based on Transition Metal Dichalcogenide Monolayer

Abstract: The ballistic device performances of monolayer transition metal dichalcogenide (MX2) tunneling field-effect transistors (TFETs) and the drive current enhancement via heterojunction are investigated in this letter via the nonequilibrium Green's function formulism. The ultrathin 2-D body and the direct and designable bandgap by choosing a proper MX2 material are advantageous for the performance of TFETs. Through introducing a common- X heterojunction at the source-channel interface, the ION at the same ION/IOFF ratio can be further enhanced by an order for both n- and p-type TFETs.

Searches for heavy long-lived sleptons and R-hadrons with the ATLAS detector in pp collisions at √s = 7 TeV

Abstract: A search for long-lived particles is performed using a data sample of 4.7 fb−1 from proton–proton collisions at a centre-of-mass energy √s =7 TeV collected by the ATLAS detector at the LHC. No excess is observed above the estimated background and lower limits, at 95% confidence level, are set on the mass of the long-lived particles in different scenarios, based on their possible interactions in the inner detector, the calorimeters and the muon spectrometer. Long-lived staus in gauge-mediated SUSY-breaking models are excluded up to a mass of 300 GeV for tanβ =5–20. Directly produced long-lived sleptons are excluded up to a mass of 278 GeV. R-hadrons, composites of gluino (stop, sbottom) and light quarks, are excluded up to a mass of 985 GeV (683 GeV, 612 GeV) when using a generic interaction model. Additionally two sets of limits on R-hadrons are obtained that are less sensitive to the interaction model for R-hadrons. One set of limits is obtained using only the inner detector and calorimeter observables, and a second set of limits is obtained based on the inner detector alone.

Quantum simulation of topological insulator based spin transfer torque device

Abstract: We developed a quantum transport model to simulate transport properties of topological insulator (TI)-based spintronic memory devices. The model captures the effects of spin-momentum locking, Klein tunneling, and coupled spin dynamics. Based on the model, we present a design of spin-transfer torque (STT) device, which consists of a thin layer TI coupled to a top ferromagnetic film. The device removes the requirement of spin-polarized contacts and magnetic tunnel junctions in conventional STT memory cells by exploiting intrinsic spin-momentum locking of the TI surface states. The analysis shows that by introducing partial perpendicular magnetic anisotropy, both fast switching and low switching current can be achieved.

Computational study of graphene-based vertical field effect transistor

Abstract: Poisson and drift-diffusion equations are solved in a three-dimensional device structure to simulate graphene-based vertical field effect transistors (GVFETs). Operation mechanisms of the GVFET with and without punched holes in the graphene source contact are presented and compared. The graphene-channel Schottky barrier can be modulated by gate electric field due to graphene's low density of states. For the graphene contact with punched holes, the contact barrier thinning and lowering around punched hole edge allow orders of magnitude higher tunneling current compared to the region away from the punched hole edge, which is responsible for significant performance improvement as already verified by experiments. Small hole size is preferred due to less electrostatic screening from channel inversion layer, which gives large electric field around the punched hole edge, thus, leading to a thinner and lower barrier. Bilayer and trilayer graphenes as the source contact degrade the performance improvement because stronger electrostatic screening leads to smaller contact barrier lowering and thinning. High punched hole area percentage improves current performance by allowing more gate electric field to modulate the graphene-channel barrier. Low effective mass channel material gives better on-off current ratio.

Modeling and simulation of carbon nanotube-semiconductor heterojunction vertical field effect transistors

Abstract: The scaling behavior of carbon nanotube (CNT)-organic semiconductor heterojunction enabled vertical field effect transistors are comprehensively examined by two-dimensional consistent device simulations. Tunneling current is modeled by introducing tunneling induced carrier generation into the current continuity equation. Modulation of both the CNT-semiconductor Shottky barrier height and thickness are examined. The tunneling current and thermionic current dominate at on-state and off-state, respectively. Barrier height modulation plays an important role and improves the on-off current ratio and sub-threshold swing considerably. Small diameter CNT is preferred for enhancing the gate control on the CNT-channel barrier height. Reducing the effective gate oxide thickness by either a thin oxide or a high-κ gate insulator gives improvement of device performance, but the former one works more efficiently. The channel length and CNT spacing should be carefully engineered due to the trade-off between device characteristics in the sub-threshold and above-threshold region.

Coupled Electro–Thermal Simulation for Self-Heating Effects in Graphene Transistors

Abstract: The effects of self-heating on graphene transistor characteristics are examined by self-consistently solving the coupled nonequilibrium Green's function transport equation with the thermal transport equation. The results indicate that the substrate and gate insulator have significant effects on the temperature rise, decrease of on-current, and lowering of transconductance through surface polar phonon scattering, which causes self-heating effects. For graphene transistors with the channel length of a few hundred nanometers, the temperature rise of about 110 K and on-current lowering of about 17% are predicted because of self-heating. This indicates the importance of a coupled electro-thermal simulation for predicting graphene device characteristics and of thermal engineering on device performance. The dependence of the self-heating effects on the transistor channel length and phonon energy are also examined.

Measurement of the differential cross-section of B[superscript +] meson production in pp collisions at √s = 7 TeV at ATLAS

Abstract: The production cross-section of B+ mesons is measured as a function of transverse momentum pT and rapidity y in proton-proton collisions at centre-of-mass energy √ s = 7 TeV, using 2.4 fb−1 of data recorded with the ATLAS detector at the Large Hadron Collider. The differential production cross-sections, determined in the range 9 GeV < pT < 120 GeV and |y| < 2.25, are compared to next-to-leading-order theoretical predictions.

Multi-channel search for squarks and gluinos in \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$\sqrt{s}=7\mbox{ TeV}$\end{document}pp collisions with the ATLAS detector at the LHC

Abstract: A search for supersymmetric particles in final states with zero, one, and two leptons, with and without jets identified as originating from b-quarks, in 4.7 fb−1 of \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$\sqrt{s}=7\mbox{ TeV}$\end{document} pp collisions produced by the Large Hadron Collider and recorded by the ATLAS detector is presented. The search uses a set of variables carrying information on the event kinematics transverse and parallel to the beam line that are sensitive to several topologies expected in supersymmetry. Mutually exclusive final states are defined, allowing a combination of all channels to increase the search sensitivity. No deviation from the Standard Model expectation is observed. Upper limits at 95 % confidence level on visible cross-sections for the production of new particles are extracted. Results are interpreted in the context of the constrained minimal supersymmetric extension to the Standard Model and in supersymmetry-inspired models with diverse, high-multiplicity final states.

Search for microscopic black holes in a like-sign dimuon final state using large track multiplicity with the ATLAS detector

Abstract: A search is presented for microscopic black holes in a like-sign dimuon final state in proton–proton collisions at √s=8 TeV. The data were collected with the ATLAS detector at the Large Hadron Co ...

Dissipative transport in nanoscale monolayer MoS 2 transistors

Abstract: Performances of monolayer MoS2 transistors with phonon scattering are investigated. The on-current of a 20nm long device can be degraded by 40% with phonon scattering compared with the ballistic limit. The sub-threshold swing (SS) and drain induced barrier lowering (DIBL) are found to be insensitive to phonon scattering. SS 5nm and DIBL 8nm for a double gate structure with a 3nm thick high-k gate insulator. Treatment of phonon scattering is important for accurate assessment of intrinsic delay and on/off ratio even at LG ~ 10nm.

Measurement of jet shapes in top-quark pair events at \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$\sqrt{s} = 7 \ \mbox{TeV}$\end{document} using the ATLAS detector

Abstract: A measurement of jet shapes in top-quark pair events using 1.8 fb−1 of \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$\sqrt{s} = 7 \ \mbox{TeV}$\end{document} pp collision data recorded by the ATLAS detector at the LHC is presented. Samples of top-quark pair events are selected in both the single-lepton and dilepton final states. The differential and integrated shapes of the jets initiated by bottom-quarks from the top-quark decays are compared with those of the jets originated by light-quarks from the hadronic W-boson decays \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$W\rightarrow q\bar{q}'$\end{document} in the single-lepton channel. The light-quark jets are found to have a narrower distribution of the momentum flow inside the jet area than b-quark jets.

On pseudomagnetoresistance in graphene junctions

Abstract: Using bilayer-graphene (BG), monolayer-graphene (MG), and hydrogenated-bilayer-graphene (hBG), we study the performance of three different pseudospinvalve (PSV) junctions: (1) BG-BG, (2) MG-BG, and (3) hBG-BG, modulated by vertical electric-field. Although pseudomagnetoresistance (PMR) of nearly 100 % at zero temperature could be obtained for the BG-BG structure, this PMR decreases rapidly as temperature increases, and the decrease is worsened with the increasing lateral distance between the required pairs of the electric gates. On the other hand, the decrease is suppressed by using a MG-BG structure, which is less sensitive to temperature and not affected by the electric-gate distance. Unlike our expectation, our results also show that the hBG-BG has the worst performance.