Anomalously High Thermal Conductivity of Amorphous Silicon Films Prepared by Hot-wire Chemical Vapor Deposition

We report anomalously high thermal conductivities of amorphous Si (a-Si) films prepared by hot-wire chemical-vapor deposition (HWCVD) at the National Renewable Energy laboratory (NREL), that is a factor of 4 similar to 6 higher than predicted by the model of minimum thermal conductivity. The temperature dependent thermal conductivities are measured with the time-domain thermoreflectance method on two thin films and with the 3 omega method on a thick film. For all these films, the thermal conductivity shows a strong phonon mean free path dependence that has So far only been found in crystalline semiconductor alloys. Similar HWCVD a-Si films prepared at the U. Illinois do not show an enhanced thermal conductivity even though the Raman spectra of the NREL and the U. Illinois samples are essentially identical. We also applied a Kubo based theory using a tight-binding method on three 1000 atom continuous random network models. The theory gives higher thermal conductivity for more ordered models, but not high enough to explain our results, even after extrapolating to lower frequencies with a Boltzmann approach. Our results show that the thermal conductivity of a-Si depends strongly on the details of their microstructure that are not revealed by vibrational spectroscopy.