Thermal Conductivity of amorphous and nanocrystalline silicon films prepared by hot-wire chemical-vapor deposition*

We measured thermal conductivity of amorphous and nanocrystalline silicon (a-Si and nc-Si) thin films. The films were prepared by hot-wire chemical-vapor deposition. The crystallinity of the films is controlled by the hydrogen dilution during growth. For a-Si, the result is in good agreement with those of previous reports. The thermal conductivity of the as-grown nc-Si is 70% higher and increases 35% more after annealing at 600^οC. The results of all a-Si and nc-Si films have similarly weak temperature dependence from 85 to 300 K. Structural analysis shows that the as-grown nc-Si is approximately 60% crystalline. The nanograins, averaging 9.1 nm, are embedded in an amorphous matrix. The grain size increases to 9.7 nm upon annealing, accompanied by the disappearance of the amorphous phase. We extend the models of grain boundary scattering of phonons with two different non-Debye dispersion relations to explain our result of nc-Si, confirming that the strong grain size dependent heat transport mechanism still applies to grain sizes in the 10-nm range. However, the similarity in thermal conductivity between a-Si and nc-Si suggests the heat transport mechanisms in both structures may not be as dissimilar as we currently understand.