ICT2014 Abstracts

Although bulk silicon is a poor thermoelectric due to its high thermal conductivity (~ 140 Wm⁻¹ K⁻¹ for non-doped Si at room temperature), there has been a continued interest in Si-based thermoelectric materials since silicon is one of the most abundant and non-toxic material. Until now, several studies have demonstrated that the thermal conductivity is drastically reduced for nanocrystalline Si due to enhanced scattering of phonons at the grain boundaries. In order to improve the figure of merit ZT, it is necessary to reduce thermal conductivity without much impact on the electron mobility, which means that the grain-boundary potential barriers should be minimized.

Quite recently, we synthesized highly efficient p-type and n-type composite films consisting of Si nanocrystals and Ni silicide nanocrystals (Ni-Si nanocomposite films) via phase separation from amorphous Si/Ni alloys. The resultant composite films have high electrical conductivities and low thermal conductivities, leading to higher ZT values than those of nanostructured bulk Si at room temperature. With a view to enhancing the ZT, it is important to understand the effect of the grain boundaries on the transport properties of carriers and phonons in the composite films.

The present work is an attempt to evaluate the effect of the grain boundaries on the transport properties using an energy filtering model. The experimental data were obtained for Ni-Si and M-Si nanocomposite films (M = Mo etc.). The results were compared with the theoretical calculations based on the Boltzmann transport equation to estimate the grain-boundary potential barriers of each nanocomposites. The difference of the grain boundary potential barriers between Ni-Si and M-Si nanocomposite films will be discussed.