V. Hung Nguyen

Electronic transport and spin-polarization effects of relativisticlike particles in mesoscopic graphene structures

Motivated by recent studies on the use of graphene for new concepts of electronic/spintronic devices, the authors develop an efficient calculation method based on the nonequilibrium Green’s function to solve the quantum relativisticlike Dirac’s equation that governs the low-energy excited states in graphene. The approach is then applied to investigate the electronic transport and the spin polarization in a single-graphene barrier structure. The obtained results are presented and analyzed in detail aiming to highlight typical properties of the considered graphene structure as well as the efficiency of the developed approach that both may be helpful for further development in electronic devices and in spintronics.

Controllable spin-dependent transport in armchair graphene nanoribbon structures

Using the nonequilibrium Green’s functions formalism in a tight binding model, the spin-dependent transport in armchair graphene nanoribbons controlled by a ferromagnetic gate is investigated. Beyond the oscillatory behavior of conductance and spin polarization with respect to the barrier height, which can be tuned by the gate voltage, we especially analyze the effects of width-dependent band gap and of the nature of contacts. The oscillation of spin polarization in graphene nanoribbons with a large band gap is strong in comparison with that in infinite graphene sheets. Very high spin polarization (close to 100%) is observed in normal-conductor/graphene/normal-conductor junctions. Moreover, we find that the difference in electronic structure between normal conductor and graphene generates confined states which have a strong influence on the transport properties of the device. This study suggests that the device should be carefully designed to obtain a high controllability of spin-polarized current.

Resonant tunnelling diodes based on graphene/h-BN heterostructure

In this work, we propose a resonant tunnelling diode (RTD) based on a double-barrier graphene/boron nitride (BN) heterostructure as a device suitable to take advantage of the elaboration of atomic sheets containing different domains of BN and C phases within a hexagonal lattice. The device operation and performance are investigated by means of a self-consistent solution within the non-equilibrium Greens function formalism on a tight-binding Hamiltonian. This RTD exhibits a negative differential conductance effect, which involves the resonant tunnelling through both the electron and hole bound states in the graphene quantum well. It is shown that the peak-to-valley ratio reaches a value of ?4 at room temperature even for zero bandgap and can be higher than 10 when a finite gap opens in the graphene channel.

Graphene nanomesh-based devices exhibiting a strong negative differential conductance effect

Using atomistic quantum simulation based on a tight binding model, we have investigated the transport characteristics of graphene nanomesh-based devices and evaluated the possibilities of observing negative differential conductance. It is shown that by taking advantage of bandgap opening in the graphene nanomesh lattice, a strong negative differential conductance effect can be achieved at room temperature in pn junctions and n-doped structures. Remarkably, the effect is improved very significantly (with a peak-to-valley current ratio of a few hundred) and appears to be weakly sensitive to the transition length in graphene nanomesh pn hetero-junctions when inserting a pristine (gapless) graphene section in the transition region between n and p zones. The study therefore suggests new design strategies for graphene electronic devices which may offer strong advantages in terms of performance and processing over the devices studied previously.

Resonant tunneling structures based on epitaxial graphene on SiC

Recently some experiments have suggested that graphene epitaxially grown on SiC can exhibit an energy bandgap of 260  meV, which enhances the potential of this material for electronic applications. On this basis, we propose to use spatial doping to generate graphene-on-SiC double-barrier structures. The non-equilibrium Greens function technique for solving the massive Dirac model is applied to highlight typical transport phenomena such as the electron confinement and the resonant tunneling effects. The I–V characteristics of graphene resonant tunneling diodes were then investigated and the effect of different device parameters was discussed. It is finally shown that this kind of double-barrier junction provides an efficient way to confine the charge carriers in graphene and to design graphene resonant tunneling structures.

Giant effect of negative differential conductance in graphene nanoribbon p-n hetero-junctions

The I-V characteristics of graphene nanoribbon (GNR) p-n junctions have been investigated using atomistic quantum simulation. On the basis of results obtained for simple armchair GNR structures with large bandgap, it is suggested to improve significantly the device operation by inserting a small-bandgap section in the transition region between n and p zones. A giant peak-to-valley ratio (PVR) of negative differential conductance (higher than 103) can be achieved in such hetero-junctions. Additionally, the PVR is proved to be weakly sensitive to the transition length and not strongly degraded by the edge disorder, which is an important feature regarding applications.

A Klein-tunneling transistor with ballistic graphene

Today, the availability of high mobility graphene up to room temperature makes ballistic transport in nanodevices achievable. In particular, p-n-p transistors in the ballistic regime give access to Klein tunneling physics and allow the realization of devices exploiting the optics-like behavior of Dirac Fermions (DFs) as in the Veselago lens or the Fabry–Perot cavity. Here we propose a Klein tunneling transistor based on the geometrical optics of DFs. We consider the case of a prismatic active region delimited by a triangular gate, where total internal reflection may occur, which leads to the tunable suppression of transistor transmission. We calculate the transmission and the current by means of scattering theory and the finite bias properties using non-equilibrium Greenʼs function (NEGF) simulation.

Disorder effects on electronic bandgap and transport in graphene-nanomesh-based structures

Using atomistic quantum simulation based on a tight binding model, we investigate the formation of electronic bandgap Eg of graphene nanomesh (GNM) lattices and the transport characteristics of GNM-based electronic devices (single potential barrier structure and p-n junction) including the atomic edge disorder of holes. We find that the sensitivity of Eg to the lattice symmetry (i.e., the lattice orientation and the hole shape) is significantly suppressed in the presence of disorder. In the case of strong disorder, the dependence of Eg on the neck width fits well with the scaling rule observed in experiments [Liang et al., Nano Lett. 10, 2454 (2010)]. Considering the transport characteristics of GNM-based structures, we demonstrate that the use of finite GNM sections in the devices can efficiently improve their electrical performance (i.e., high ON/OFF current ratio, good current saturation, and negative differential conductance behaviors). Additionally, if the length of GNM sections is suitably chosen, t...

Resonant tunneling and negative transconductance in single barrier bilayer graphene structure

Using the nonequilibrium Green’s function method, the electronic transport in a gate-induced barrier bilayer graphene structure is investigated. Strong resonant effects are shown to result in high amplitude oscillation of conductance as a function of Fermi energy and barrier height. Beyond a small effect of negative differential conductance (with peak to valley ratio less than 2), strong oscillations of transconductance are achieved. The amplitude of such oscillations between positive and negative values may exceed 5 mS/μm. This effect might be helpful for further development of graphene-based nanoelectronics.

Large peak-to-valley ratio of negative-differential-conductance in graphene p-n junctions

We investigate the transport characteristics of monolayer graphene p-n junctions by means of the nonequilibrium Green’s function technique. It is shown that due to the high interband tunneling of chiral fermions and a finite bandgap opening when the inversion symmetry of the graphene plane is broken, a strong negative-differential-conductance behavior with a peak-to-valley ratio as large as a few tens can be achieved even at room temperature. The dependence of this behavior on the device parameters such as the Fermi energy, the barrier height, and the transition length is then discussed.