Bandstructure Effects in Silicon Nanowire Hole Transport

TL;DR

This paper investigates how bandstructure effects influence hole transport in silicon nanowire FETs, revealing orientation-dependent dispersion shapes, charge distributions, and performance metrics using an atomistic tight-binding model.

Contribution

It introduces a detailed 20-band atomistic tight-binding model to analyze valence band dispersion and transport in silicon nanowire FETs, highlighting orientation and size effects.

Findings

Dispersion shapes vary significantly with orientation and size.

Charge prefers accumulation on (110) and (112) surfaces over (100).

[111] and [110] nanowires show higher carrier velocities and better ON-current.

Abstract

Bandstructure effects in PMOS transport of strongly quantized silicon nanowire field-effect-transistors (FET) in various transport orientations are examined. A 20-band sp3d5s* spin-orbit-coupled (SO) atomistic tight-binding model coupled to a self consistent Poisson solver is used for the valence band dispersion calculation. A ballistic FET model is used to evaluate the capacitance and current-voltage characteristics. The dispersion shapes and curvatures are strong functions of device size, lattice orientation, and bias, and cannot be described within the effective mass approximation. The anisotropy of the confinement mass in the different quantization directions can cause the charge to preferably accumulate in the (110) and secondly on the (112) rather than (100) surfaces, leading to significant charge distributions for different wire orientations. The total gate capacitance of the nanowire FET devices is, however, very similar for all wires in all the transport orientations investigated ([100], [110], [111]), and is degraded from the oxide capacitance by ~30%. The [111] and secondly the [110] oriented nanowires indicate highest carrier velocities and better ON-current performance compared to [100] wires. The dispersion features and quantization behavior, although a complicated function of physical and electrostatic confinement, can be explained at first order by looking at the anisotropic shape of the heavy-hole valence band.