Nanoscale heat transport in single-crystalline Si and amorphous SiGe phononic crystals

Thermal heat/phonon transport in single-crystalline Si and amorphous SiGe phononic crystal (PnC) nanos- tructures was investigated at room temperature. Thermal conductivities were compared between hexagonal-lattice PnCs, which have staggered alignment of circular holes, and square-lattice PnCs. In microscale structures, where thermal transport is diffusive, no difference can be expected. However, in our Si PnC nanostructures, a hexagonal- lattice PnC shows lower thermal conductivity than a square-lattice PnC by 20% at a neck size (separation between the adjacent two holes) of 75 nm. We also performed the same experiment using amorphous SiGe and found that the difference is much smaller (5%). In Si, thermal phonon transport is semi-ballistic and greater portion of thermal phonons have mean free path (MFP) around 300 nm, while phonon transport is mostly diffusive in SiGe. We conclude that thermal phonon MFP spectrum should be taken into account for efficient thermal conductivity nanoengineering.