Atomistic Study of Acoustic Phonon Limited Mobility in Extremely Scaled Si and Ge Films

We explore the impact of Silicon (Si) and Germanium (Ge) thickness scaling (from 1 to 5 nm) on their electronic and transport properties using density functional theory. We find that in Ge, the lowest conduction band valley shifts from L in bulk to X in ultrathin slabs, and at 1 nm thickness Ge reduces to a direct band gap semiconductor. On the other hand, Si changes to a direct band-gap semiconductor in the thickness range of 1 to 5 nm, as opposed to its indirect nature in bulk form. We show that the electron-phonon coupling, which is the dominant scattering mechanism in these materials, is found to be very weak in scaled Ge-slabs as compared to Si-slabs. This in combination with drastically reduced longitudinal effective mass in thin Geslabs, leads to very high electron mobility in scaled Ge films, which increases with decreasing thickness. Our reported mobility trend in Ge-slabs is in agreement with the recently reported experimental results. For Si-slabs, we show that 3 nm is the most suitable thickness for future ultrathin Si devices as it has the lowest conduction band deformation potential and highest electron mobility.