Ab-intio based electron-phonon scattering for 2D materials within the NEGF framework

For the last 15 years, two-dimensional (2D) materials are being actively studied as a possible replacement for silicon. Due to their layered nature, the materials can be scaled down to a single atomic layer, favouring extreme device scaling. In addition, surface effects such as roughness, dangling bonds, and interface states are also reduced/eliminated [1],[2]. Besides, these materials confine charge carriers to atomic-thin layers, which provides excellent gate-control and reduces short-channel effects [3]. Graphene (single layer graphite) was the first of the 2D material to be isolated, and very high electron mobility $(\gt 10^{5}$ cm$^{2} /$Vs) in its suspended state was reported [4]. However, it does not have a bandgap, which is a severe limitation for transistor applications (challenging to turn off the device). Still, it initiated the discovery of other 2D materials with a band gap, such as phosphorene (mono- or few-layer black phosphorous) [5] and transition metal dichalcogenides (TMDs) [6]. However, TMDs and phosphorene are found to have lower intrinsic mobility when compared to graphene [7],[8]. This can be attributed to the large electron-phonon scattering present in these materials. Theoretical calculations have predicted that, even in 2D material-based transistors of channel lengths as small as about 5 nm, electron-phonon scattering plays a significant role [9],[10].