Surface Faceting Dependence of Thermal Transport in Silicon Nanowires

Surface faceting on sidewalls is ubiquitously observed during crystal growth of semiconductor nanowires. However, predicting the thermal transport characteristics of faceted nanowires relevant to thermoelectric applications remains challenging. Here, direct molecular dynamics simulations show that thermal conductivity is considerably reduced in crystalline < 111 > Si nanowires with periodic sawtooth faceting compared to nanowires of same size with smooth sidewalls. It is discovered that surface phonon scattering is particularly high with {100} facets, but less pronounced with {113} facets and remarkably low with {111} facets, which suggests a new means to optimize phonon dynamics for nanoscale thermoelectric devices. This anomaly is reconciled by showing that the contribution of each facet to surface phonons is due to diffuse scattering rather than to backward scattering. It is further shown that this property is not changed by addition of an amorphous shell to the crystalline core, similar to the structure of experimental nanowires.