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 -oriented nanowire has the highest ballistic thermal conductance, followed by the and finally the 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 -type nanowires only a small orientation-dependence of the electric power factor was observed. In the case of -type silicon nanowires, however, they showed that nanowires have significantly higher power factors than differently orientated nanowires. The silicon nanowire channel is, therefore, the most advantageous for -type thermoelectric applications, since it simultaneously provides the highest power factor and lowest thermal conductance compared with other transport orientations.