Unusually High and Anisotropic Thermal Conductivity in Amorphous Silicon Nanostructures

Amorphous Si (a-Si) nanostructures are ubiquitous in numerous electronic and optoelectronic devices. Amorphous materials are considered to possess the lower limit to the thermal conductivity (κ), which is ∼1 W·m^-1 K^-1 for a-Si. However, recent work suggested that κ of micrometer-thick a-Si films can be greater than 3 W·m^-1 K^-1, which is contributed to by propagating vibrational modes, referred to as "propagons". However, precise determination of κ in a-Si has been elusive. Here, we used structures of a-Si nanotubes and suspended a-Si films that enabled precise in-plane thermal conductivity (κ_∥) measurement within a wide thickness range of 5 nm to 1.7 μm. We showed unexpectedly high κ_∥ in a-Si nanostructures, reaching ∼3.0 and 5.3 W·m^-1 K^-1 at ∼100 nm and 1.7 μm, respectively. Furthermore, the measured κ_∥ is significantly higher than the cross-plane κ on the same films. This unusually high and anisotropic thermal conductivity in the amorphous Si nanostructure manifests the surprisingly broad propagon mean free path distribution, which is found to range from 10 nm to 10 μm, in the disordered and atomically isotropic structure. This result provides an unambiguous answer to the century-old problem regarding mean free path distribution of propagons and also sheds light on the design and performance of numerous a-Si based electronic and optoelectronic devices.