Kai-Tak Lam

Bilayer graphene nanoribbon nanoelectromechanical system device: A computational study

A bilayer graphene nanoribbon nanoelectromechanical device is investigated via first-principle simulations. The output characteristics as a function of interlayer distance are calculated, with the proposed device acting as a displacement and a force sensor. The operating mechanism of a bistable switch based on this device structure is also explored, and in the present floating gate design, a switching gate bias of 5.6 V is required, resulting in an ON-OFF current ratio of 3 orders at a device bias of 20 mV. This minuscule bistable device could potentially be implemented in future semiconductor memory devices and radio frequency communication circuitry.

An ab initio study on energy gap of bilayer graphene nanoribbons with armchair edges

Dependency of energy bandgap (Eg) of bilayer armchair graphene nanoribbons (AGNRB) on their widths, interlayer distance (D), and edge doping concentration of boron/nitrogen was investigated using local density approximation and compare to the results of monolayer graphene nanoribbons (AGNRM). Although Eg of AGNRB, in general, is smaller than that of AGNRM, of AGNRB exhibits two distinct groups, metal and semiconductor, while AGNRM displays purely semiconducting behavior. Moreover, Eg of AGNRB is highly sensitive to D, indicating a possible application in tuning Eg by varying D. Finally, edge doping of both AGNR systems reduces Eg by 11%–17%/4%–10% for AGNRM∕AGNRB, respectively.

A Simulation Study of Graphene-Nanoribbon Tunneling FET With Heterojunction Channel

The device physics and performance of heterojunction (HJ) graphene-nanoribbon (GNR) tunneling field-effect transistors (TFETs) with different designs are investigated in this letter. Due to the width-dependent energy bandgap (EG), a single GNR with spatially dependent width naturally yields an HJ structure to improve the device performance of a GNR TFET. By adding a small-EG region in the channel near the source and a large-EG region in the middle of the channel, the ON- and OFF-state currents (ION and IOFF, respectively) can be tuned. Last, we have studied the effect of channel length scaling on an HJ GNR TFET, and it has been observed that an ION/IOFF ratio of four orders of magnitude can be achieved with a channel length of 10 nm and a drain bias of 0.6 V.

Device Physics and Characteristics of Graphene Nanoribbon Tunneling FETs

We present a detailed simulation study on the current-voltage characteristics of ballistic graphene nanoribbon (GNR) tunneling FETs of different widths with varying temperatures and channel length. Our model uses the self-consistent nonequilibrium Greens function and the quasi-2-D Poisson solver with the material details of the GNRs modeled by the uncoupled mode space Dirac equation. We find that, in general, the GNR tunneling FETs from the 3p + 1 family have better ION/IOFF characteristics than those from the 3p family due to smaller effective masses of the former. A lower drain doping concentration relative to that of the source enhances the ION/IOFF. Most significantly, we find that a higher doping concentration at the source enhances ION but degrades the subthreshold swing (SS). As a function of temperature, the SS shows highly nonlinear behaviors. In terms of intrinsic delay and power-delay product, the GNR tunneling FETs show very promising scaling behaviors and can be optimized to meet the International Technology Roadmap for Semiconductors roadmap requirements through adjustment in doping concentrations and other parameters.

Shape effects in graphene nanoribbon resonant tunneling diodes: A computational study

The possibility of using graphene nanoribbons (GNRs) as the material for resonant tunneling diodes (RTDs) was investigated using a device simulator based on the nonequilibrium Green’s function with the π-orbital tight-binding approach. The double-barrier quantum well (DBQW) requirements of a RTD can be implemented by adjusting the width of a GNR to derive a negative differential resistance (NDR). The implementation of such a device is demonstrated in this paper and its mechanism was also found to be robust regardless of the eventual shape of the GNR patterned. Furthermore, the effects of the shape of the patterned GNR and the operating temperature on the performance of the device were explored by looking at the real space current flux of the device and the temperature dependency of the peak-valley ratio (PVR), respectively. Although the different shapes of GNR RTDs had a similar DBQW structure, their PVRs were different due to their conduction mechanisms which were dependent on the different geometrical s...

Stability and electronic structure of two dimensional Cx(BN)y compound

The thermal stability and electronic structures of two dimensional Cx(BN)y compounds are studied using first-principles calculations based on the density functional theory. Although, from total energy calculations, it was well-established that phase-segregated atomic arrangements had the lowest energy, we found that due to the high activation energy required for phase-segregation process, evenly distributed configurations are stable at room temperature. Furthermore, the energy bandgap (EG) of the evenly distributed Cx(BN)y compounds is dependent on the carbon concentration. By controlling the carbon concentration in the compound, the EG of the compound material can be adjusted for electronic applications.

Single Atomically Sharp Lateral Monolayer p-n Heterojunction Solar Cells with Extraordinarily High Power Conversion Efficiency

The recent development of 2D monolayer lateral semiconductor has created new paradigm to develop p-n heterojunctions. Albeit, the growth methods of these heterostructures typically result in alloy structures at the interface, limiting the development for high-efficiency photovoltaic (PV) devices. Here, the PV properties of sequentially grown alloy-free 2D monolayer WSe2 -MoS2 lateral p-n heterojunction are explores. The PV devices show an extraordinary power conversion efficiency of 2.56% under AM 1.5G illumination. The large surface active area enables the full exposure of the depletion region, leading to excellent omnidirectional light harvesting characteristic with only 5% reduction of efficiency at incident angles up to 75°. Modeling studies demonstrate the PV devices comply with typical principles, increasing the feasibility for further development. Furthermore, the appropriate electrode-spacing design can lead to environment-independent PV properties. These robust PV properties deriving from the atomically sharp lateral p-n interface can help develop the next-generation photovoltaics.

Ambipolar bistable switching effect of graphene

Reproducible current hysteresis is observed in graphene with a back gate structure in a two-terminal configuration. An opposite sequence of switching with different charge carriers, holes, and electrons is found. The charging and discharging effect is proposed to explain this ambipolar bistable hysteretic switching. To confirm this hypothesis, one-level transport model simulations including charging effect are performed and the results are consistent with our experimental data. Methods of improving the on/off ratio of graphene resistive switching are suggested.

Influence of edge roughness on graphene nanoribbon resonant tunnelling diodes

The edge roughness effects of graphene nanoribbons on their application in resonant tunnelling diodes with different geometrical shapes (S, H and W) were investigated. Sixty samples for each 5%, 10% and 15% edge roughness conditions of these differently shaped graphene nanoribbon resonant tunnelling diodes were randomly generated and studied. Firstly, it was observed that edge roughness in the barrier regions decreases the effective barrier height and thickness, which increases the broadening of the quantized states in the quantum well due to the enhanced penetration of the wave-function tail from the electrodes. Secondly, edge roughness increases the effective width of the quantum well and causes the lowering of the quantized states. Furthermore, the shape effects on carrier transport are modified by edge roughness due to different interfacial scattering. Finally, with the effects mentioned above, edge roughness has a considerable impact on the device performance in terms of varying the peak-current positions and degrading the peak-to-valley current ratio.

Quantum transport simulations of graphene nanoribbon devices using Dirac equation calibrated with tight-binding π-bond model

We present an efficient approach to study the carrier transport in graphene nanoribbon (GNR) devices using the non-equilibrium Greens function approach (NEGF) based on the Dirac equation calibrated to the tight-binding π-bond model for graphene. The approach has the advantage of the computational efficiency of the Dirac equation and still captures sufficient quantitative details of the bandstructure from the tight-binding π-bond model for graphene. We demonstrate how the exact self-energies due to the leads can be calculated in the NEGF-Dirac model. We apply our approach to GNR systems of different widths subjecting to different potential profiles to characterize their device physics. Specifically, the validity and accuracy of our approach will be demonstrated by benchmarking the density of states and transmissions characteristics with that of the more expensive transport calculations for the tight-binding π-bond model.