Phonon thermal transport in silicon thin films with nanoscale constrictions and expansions

In patterned thin film devices, abrupt geometric changes can introduce thermal constriction and expansion resistances whose magnitude and relative importance depends on the device's size and geometry as well as the dominant heat carrier mean free path spectra of the comprising material. Existing analytical models of thermal constriction and expansion resistances at the nanoscale have focused primarily on semi-infinite geometries or other situations which are quite different from those encountered in modern nanopatterned thin film devices. In this work, Monte Carlo methods are used to simulate phonon transport in silicon thin films patterned with a commonly utilized source-channel-drain geometry. The length, width, and thickness of the channel region were varied, and the dependence of the thermal constriction-expansion resistance on these parameters was determined. Results show that thin film source-drain reservoirs with diffuse boundary scattering do not behave as semi-infinite reservoirs for feature sizes smaller than approximately 100 nm in silicon near 300 K, and that existing analytical models cannot be readily applied to such systems. In addition, our results support the case that ballistic phonon effects in silicon nanowires at room temperature, if present, are small and not easily observable. Finally, we provide guidance and perspective for Si nanowire measurements near room temperature as to what scenarios may lead to a non-negligible amount of error if constriction-expansion geometry effects are ignored.