Origin of ultralow thermal conductivity in amorphous Si thin films investigated using nanoindentation, 3ω method, and phonon transport analysis

The origin of the ultralow thermal conductivity in amorphous Si thin films was investigated by comparing their phonon transport properties with those of single-crystal Si. The group velocity and thermal conductivity were measured at 300 K using nanoindentation and the 3 omega method, respectively. The phonon mean free path (MFP) and phonon frequency were determined using the measured properties and models. The scattering in the disordered structure of amorphous Si thin films caused a significant decrease in the phonon MFP with an increase in the phonon frequency, leading to ultralow thermal conductivity. However, the group velocity was unaffected by the disordered structure.