Atomistic simulations of low-field mobility in Si nanowires: Influence of confinement and orientation

TL;DR

This study uses atomistic simulations to analyze how confinement and orientation affect low-field mobility in silicon nanowires, revealing size-dependent variations and potential mobility enhancements in ultra-scaled NWs.

Contribution

Developed a coupled atomistic electronic and transport simulation framework to systematically study size and orientation effects on silicon nanowire mobility.

Findings

Mobility decreases with diameter due to phonon and surface roughness scattering.

Large mobility enhancements (~4x) observed in certain p-type NWs at small diameters.

Orientation and doping type significantly influence mobility trends in scaled silicon NWs.

Abstract

A simulation framework that couples atomistic electronic structures to Boltzmann transport formalism is developed and applied to calculate the transport characteristics of thin silicon nanowires (NWs) up to 12nm in diameter. The sp3d5s*-spin-orbit-coupled atomistic tight-binding (TB) model is used for the electronic structure calculation. Linearized Boltzmann transport theory is applied, including carrier scattering by phonons, surface roughness (SRS), and impurities. We present a comprehensive investigation of the low-field mobility in silicon NWs considering: i) n- and p-type NWs, ii) [100], [110], and [111] transport orientations, and iii) diameters from D=12nm (electronically almost bulk-like) down to D=3nm (ultra-scaled). The simulation results display strong variations in the characteristics of the different NW types. For n-type NWs, phonon scattering and SRS become stronger as the diameter is reduced and drastically degrade the mobility by up to an order of magnitude depending on the orientation. For the [111] and [110] p-type NWs, on the other hand, large mobility enhancements (of the order of ~4X) can be achieved as the diameter scales down to D=3nm. This enhancement originates from the increase in the subband curvatures as the diameter is scaled. It overcompensates for the mobility reduction caused by SRS in narrow NWs and offers an advantage with diameter scaling. Our results may provide understanding of recent experimental measurements, as well as guidance in the design of NW channel devices with improved transport properties.