Yijian Ouyang

Room-temperature all-semiconducting sub-10-nm graphene nanoribbon field-effect transistors.

Sub-10 nm wide graphene nanoribbon field-effect transistors (GNRFETs) are studied systematically. All sub-10 nm GNRs afforded semiconducting FETs without exception, with Ion/Ioff ratio up to 10(6) and on-state current density as high as approximately 2000 microA/microm. We estimated carrier mobility approximately 200 cm2/V s and scattering mean free path approximately 10 nm in sub-10 nm GNRs. Scattering mechanisms by edges, acoustic phonon, and defects are discussed. The sub-10 nm GNRFETs are comparable to small diameter (d< or = approximately 1.2 nm) carbon nanotube FETs with Pd contacts in on-state current density and Ion/Ioff ratio, but have the advantage of producing all-semiconducting devices.

Performance Limits of Monolayer Transition Metal Dichalcogenide Transistors

The performance limits of monolayer transition metal dichalcogenide ( MX₂) transistors are examined with a ballistic MOSFET model. Using an ab initio theory, we calculate the band structures of 2-D transition MX₂. We find the lattice structures of monolayer MX₂ remain the same as the bulk MX₂. Within the ballistic regime, the performances of monolayer MX₂ transistors are better compared with those of the silicon transistors if a thin high-κ gate insulator is used. This makes monolayer MX₂ promising 2-D materials for future nanoelectronic device applications.

Controlled Chlorine Plasma Reaction for Noninvasive Graphene Doping

We investigated the chlorine plasma reaction with graphene and graphene nanoribbons and compared it with the hydrogen and fluorine plasma reactions. Unlike the rapid destruction of graphene by hydrogen and fluorine plasmas, much slower reaction kinetics between the chlorine plasma and graphene were observed, allowing for controlled chlorination. Electrical measurements on graphene sheets, graphene nanoribbons, and large graphene films grown by chemical vapor deposition showed p-type doping accompanied by a conductance increase, suggesting nondestructive doping via chlorination. Ab initio simulations were performed to rationalize the differences in fluorine, hydrogen, and chlorine functionalization of graphene.

A theoretical study on thermoelectric properties of graphene nanoribbons

We investigate the thermoelectric properties of graphene nanoribbons (GNRs) by solving atomistic electron and phonon transport equations in the nonequilibrium Green’s function formalism. The dependence of thermopower on temperature and chemical potential is compared to that of graphene, which shows the important role of quasi-one-dimensional geometry in determining the thermoelectric properties of a GNR. The edge roughness and lattice vacancy are found to increase the thermopower but decrease the thermoelectric ZT factor because the decrease in the electronic conductance outweighs the decrease in the thermal conductance and the increase in the thermopower.

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.

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 ballistic monolayer and bilayer graphene field-effect transistors

We examine and compare ballistic performance limits of metal-oxide-semiconductor field-effect transistors with monolayer and bilayer graphene channels. Under low source-drain biases and cryogenic temperatures, the leakage current of the bilayer device is orders of magnitude smaller than that of the monolayer device. The advantage lowers at raised temperatures and source-drain biases. The bilayer device, however, still has qualitatively different and more favorable I-V characteristics. We find the ballistic on-state channel conductance and the minimum channel conductance have distinctly different dependences on the channel length.

Carrier scattering in graphene nanoribbon field-effect transistors

The elastic scattering mean free path (mfp) in a graphene nanoribbon (GNR) is characterized to be short. In the absence of other scattering mechanisms, elastic scattering has a large effect on the source-drain current of a GNR field-effect transistor due to its quasi-one-dimensional channel. In the presence of optical phonon scattering, the effect of elastic scattering is reduced. The coupling of inelastic, short-mfp optical phonon scattering to elastic scattering results in an increase rather than a decrease of the source-drain current. Improving the GNR edge quality promises significant on-current improvement.

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