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

TL;DR

This paper introduces a mode space method for efficient atomistic Green's function simulations of graphene nanoribbon FETs with phonon scattering, significantly reducing computation time while maintaining accuracy.

Contribution

The authors develop a decoupled mode space approach for atomistic simulations that accurately includes electron-phonon interactions and improves computational efficiency.

Findings

Speed-up factor of 40 for a 1.5-nm-wide device

Decoupled modes retain accuracy in ideal electrostatic conditions

Coupled mode space approach needed for disordered potentials

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 self-energies: 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.