Mode space approach for tight-binding transport simulations in graphene nanoribbon field-effect transistors including phonon scattering

Abstract

In this paper, we present a mode space method for atomistic non-equilibrium Green's function simulations of armchair graphene nanoribbon FETs that includes electron-phonon scattering. With reference to both conventional and tunnel FET structures, we show that, in the ideal case of a smooth electrostatic potential, the modes can be decoupled in different groups without any loss of accuracy. Thus, inter-subband scattering due to electron-phonon interactions is properly accounted for, while the overall simulation time considerably improves with respect to real-space, with a speed-up factor of 40 for a 1.5-nm-wide device. Such factor increases with the square of the device width. We also discuss the accuracy of two commonly used approximations of the scattering selfenergies: the neglect of the off-diagonal entries in the mode-space expressions and the neglect of the Hermitian part of the retarded self-energy. While the latter is an acceptable approximation in most bias conditions, the former is somewhat inaccurate when the device is in the off-state and optical phonon scattering is essential in determining the current via band-to-band tunneling. Finally, we show that, in the presence of a disordered potential, a coupled mode space approach is necessary, but the results are still accurate compared to the real-space solution.

Parity conservation in electron-phonon scattering in zigzag graphene nanoribbon

“…In contrast with carbon nanotubes (CNTs), where the electron/phonon subband numbers can be interpreted as momentum in the circumferrencial direction (which is conserved during scattering), such intepretation does not exist in GNRs. The electron-phonon scattering in armchair graphene nanoribbons (AGNRs) has been studied extensively [7][8][9][10]. This is, however, not the case for zigzag graphene nanoribbons (ZGNRs).…”

The scaling laws of edge vs. bulk interlayer conduction in mesoscale twisted graphitic interfaces

“…23,32 with no edge passivation. Such tight-binding calculations are known to be very successful in capturing electronic structure and optical characteristics of single and few-layered graphene 3,37-42 as well as their electronic transport behavior [43][44][45] . Furthermore, they capture the main physical characteristics of zigzag edge states in these systems 11,[46][47][48][49] .…”

Büttiker probes and the recursive Green's function: An efficient approach to include dissipation in general configurations

“…To provide a simple description, dangling bonds are passivated by hydrogen, whose states are too far away from the energy scales of interest and can be safely ignored [25]. No special treatment is given for the edge carbon atoms though it has been suggested that their hopping and on-site terms be modified to adequately represent edge bond distortion [25,26]. In our model we have included only the first and third hopping terms as both the on-site energy and second-nearest neighbour interactions serve to only rigidly shift the position of the conduction and valence bands and therefore without loss of generality are set to zero.…”

“…We now provide examples of dissipation in graphene nanoribbon based transistors. Though graphene is well studied, the particular choice of parameters governing the strength of the optical and acoustic contributions is contentious [26][27][28][29]. We choose the numerical coefficients FIG.…”