4.1.3 Discussion

In this section we have studied the thermal properties of ultra-narrow silicon nanowires using the atomistic MVFF method for the computation of the phonon bandstructure. We have extracted the thermal properties using the ballistic Landauer formalism. We have addressed the effects of structural confinement on the phonon dispersion, the phononic density of states, the phononic transmission function, the sound velocity, and the effective group velocity. Our results show that differently oriented nanowires can have up to a factor of two difference in their effective group velocity, transmission function, and ballistic thermal conductance. The $\textless 110\textgreater$ -oriented nanowire has the highest ballistic thermal conductance, followed by the $\textless 100\textgreater$ and finally the $\textless 111\textgreater$ nanowire.

Neophytou et al. have recently studied the role of transport orientations and diameter on the thermoelectric power factor of nanowires using atomistic calculations of the electronic bandstructure [97]. In the case of $n$ -type nanowires only a small orientation-dependence of the electric power factor was observed. In the case of $p$ -type silicon nanowires, however, they showed that $\textless 111\textgreater$ nanowires have significantly higher power factors than differently orientated nanowires. The $\textless 111\textgreater$ silicon nanowire channel is, therefore, the most advantageous for $p$ -type thermoelectric applications, since it simultaneously provides the highest power factor and lowest thermal conductance compared with other transport orientations.