Youngki Yoon

N-Doping of Graphene Through Electrothermal Reactions with Ammonia

Negatively Doped Graphene Nanoribbons The potential applications in electronic devices of graphene (single atom, thick layers of graphite) would be even greater if it can be accessed in both p- and n-doped forms. Graphene nanoribbons (long strips only tens of nanometers in width) are readily p-doped by adsorbates from the ambient atmosphere. Wang et al. (p. 768) show that when graphene nano-ribbons are electrically heated in an ammonia atmosphere, nitrogen is incorporated mainly at the edges of the ribbon and creates an n-type material. Field-effect transistors that operate at room temperature can be made from this material. The edges of graphene nanoribbons incorporate nitrogen atoms after heating in an atmosphere of ammonia. Graphene is readily p-doped by adsorbates, but for device applications, it would be useful to access the n-doped material. Individual graphene nanoribbons were covalently functionalized by nitrogen species through high-power electrical joule heating in ammonia gas, leading to n-type electronic doping consistent with theory. The formation of the carbon-nitrogen bond should occur mostly at the edges of graphene where chemical reactivity is high. X-ray photoelectron spectroscopy and nanometer-scale secondary ion mass spectroscopy confirm the carbon-nitrogen species in graphene thermally annealed in ammonia. We fabricated an n-type graphene field-effect transistor that operates at room temperature.

How Good Can Monolayer MoS2 Transistors Be

Monolayer molybdenum disulfide (MoS(2)), unlike its bulk form, is a direct band gap semiconductor with a band gap of 1.8 eV. Recently, field-effect transistors have been demonstrated experimentally using a mechanically exfoliated MoS(2) monolayer, showing promising potential for next generation electronics. Here we project the ultimate performance limit of MoS(2) transistors by using nonequilibrium Greens function based quantum transport simulations. Our simulation results show that the strength of MoS(2) transistors lies in large ON-OFF current ratio (>10(10)), immunity to short channel effects (drain-induced barrier lowering ∼10 mV/V), and abrupt switching (subthreshold swing as low as 60 mV/decade). Our comparison of monolayer MoS(2) transistors to the state-of-the-art III-V materials based transistors, reveals that while MoS(2) transistors may not be ideal for high-performance applications due to heavier electron effective mass (m = 0.45 m(0)) and a lower mobility, they can be an attractive alternative for low power applications thanks to the large band gap and the excellent electrostatic integrity inherent in a two-dimensional system.

Effect of edge roughness in graphene nanoribbon transistors

The effects of edge irregularity and mixed edge shapes on the characteristics of graphene nanoribbon transistors are examined by self-consistent atomistic simulations based on the non-equilibrium Greens function formalism. The minimal leakage current increases due to the localized states induced in the band gap, and the on-current decreases due to smaller quantum transmission and the self-consistent electrostatic effect in general. Although the ratio between the on-current and minimal leakage current decreases, the transistor still switches even in the presence of edge roughness. The variation between devices, however, can be large, especially for a short channel length.

Scaling Behaviors of Graphene Nanoribbon FETs: A Three-Dimensional Quantum Simulation Study

The scaling behaviors of graphene nanoribbon (GNR) Schottky barrier field-effect transistors (SBFETs) are studied by self-consistently solving the nonequilibrium Greens function transport equation in an atomistic basis set with a 3-D Poisson equation. The armchair edge GNR channel shares similarities with a zigzag carbon nanotube; however, it has a different geometry and quantum confinement boundary condition in the transverse direction. The results indicate that the I-V characteristics are ambipolar and strongly depend on the GNR width because the bandgap of the GNR is approximately inversely proportional to its width, which agrees with recent experiments. A multiple gate geometry improves immunity to short channel effects; however, it offers smaller improvement than it does for Si MOSFETs in terms of the on-current and transconductance. Reducing the oxide thickness is more useful for improving transistor performance than using a high-k gate insulator. Significant increase of the minimal leakage current is observed when the channel length is scaled below 10 nm because the small effective mass facilitates strong source-drain tunneling. The GNRFET, therefore, does not promise to extend the ultimate scaling limit of Si MOSFETs. The intrinsic switching speed of a GNR SBFET, however, is several times faster than that of Si MOSFETs, which could lead to promising high-speed electronics applications, where the large leakage of GNR SBFETs is of less concern.

Performance Comparison of Graphene Nanoribbon FETs With Schottky Contacts and Doped Reservoirs

We present an atomistic 3-D simulation study of the performance of graphene-nanoribbon (GNR) Schottky-barrier field-effect transistors (SBFETs) and transistors with doped reservoirs (MOSFETs) by means of the self-consistent solution of the Poisson and Schrodinger equations within the nonequilibrium Greens function (NEGF) formalism. Ideal MOSFETs show slightly better electrical performance for both digital and terahertz applications. The impact of nonidealities on device performance has been investigated, taking into account the presence of single vacancy, edge roughness, and ionized impurities along the channel. In general, MOSFETs show more robust characteristics than SBFETs. Edge roughness and single-vacancy defect largely affect the performance of both device types.

Comparison of performance limits for carbon nanoribbon and carbon nanotube transistors

Carbon-based nanostructures promise near ballistic transport and are being intensively explored for device applications. In this letter, the performance limits of carbon nanoribbon (CNR) field-effect transistors (FETs) and carbon nanotube (CNT) FETs are compared. The ballistic channel conductance and the quantum capacitance of the CNRFET are about a factor of 2 smaller than those of the CNTFET because of the different valley degeneracy factors for CNRs and CNTs. The intrinsic speed of the CNRFET is faster due to a larger average carrier injection velocity. The gate capacitance plays an important role in determining which transistor delivers a larger on current.

Gate Electrostatics and Quantum Capacitance of Graphene Nanoribbons

Capacitance-voltage (C-V) characteristics are important for understanding fundamental electronic structures and device applications of nanomaterials. The C-V characteristics of graphene nanoribbons (GNRs) are examined using self-consistent atomistic simulations. The results indicate strong dependence of the GNR C-V characteristics on the edge shape. For zigzag edge GNRs, highly nonuniform charge distribution in the transverse direction due to edge states lowers the gate capacitance considerably, and the self-consistent electrostatic potential significantly alters the band structure and carrier velocity. For an armchair edge GNR, the quantum capacitance is a factor of 2 smaller than its corresponding zigzag carbon nanotube, and a multiple gate geometry is less beneficial for transistor applications. Magnetic field results in pronounced oscillations on C-V characteristics.

Analysis of InAs vertical and lateral band-to-band tunneling transistors: Leveraging vertical tunneling for improved performance

Using self-consistent quantum transport simulation on realistic devices, we show that InAs band-to-band Tunneling Field Effect Transistors (TFET) with a heavily doped pocket in the gate-source overlap region can offer larger ON current and steeper subthreshold swing as compared to conventional tunneling transistors. This is due to an additional tunneling contribution to current stemming from band overlap along the body thickness. However, a critical thickness is necessary to obtain this advantage derived from “vertical” tunneling. In addition, in ultra small InAs TFET devices, the subthreshold swing could be severely affected by direct source-to-drain tunneling through the body.

Metal-semiconductor junction of graphene nanoribbons

Patterned all-graphene circuits, in which semiconducting graphene nanoribbon (GNR) device channels are connected by metallic GNR interconnects, raise possibilities for carbon-based electronics. The properties of GNR metal-semiconductor junctions, which are the key elements in all-graphene circuits, are studied by atomistic simulations. The junction conductance strongly depends on the atomistic features of the access geometry from the metallic GNR to the semiconducting GNR. Highly localized states exist at the junction edges, which can result in sharp metal-induced gap states. A defect of a single lattice vacancy can significantly increase rather than decrease the junction conductance for certain junction geometries.

Effect of phonon scattering on intrinsic delay and cutoff frequency of carbon nanotube FETs

The effect of phonon scattering on the intrinsic delay and cutoff frequency of Schottky-barrier carbon nanotube (CNT) FETs (CNTFETs) is examined. Carriers are mostly scattered by optical and zone-boundary phonons beyond the beginning of the channel. It is shown that the scattering has a small direct effect on the dc on current of the CNTFET, but it results in a significant decrease of intrinsic cutoff frequency and increase of intrinsic delay