Electrical and thermal properties of polycrystalline Si thin films with phononic crystal nanopatterning for thermoelectric applications

Electrical and thermal properties of polycrystalline Si thin films with two-dimensional phononic patterning were investigated at room temperature. Electrical and thermal conductivities for the phononic crystal nanostructures with a variety of radii of the circular holes were measured to systematically investigate the impact of the nanopatterning. The concept of phonon-glass and electron-crystal is valid in the investigated electron and phonon transport systems with the neck size of 80 nm. The thermal conductivity is more sensitive than the electrical conductivity to the nanopatterning due to the longer mean free path of the thermal phonons than that of the charge carriers. The values of the figure of merit ZT were 0.065 and 0.035, and the enhancement factors were 2 and 4 for the p-doped and n-doped phononic crystals compared to the unpatterned thin films, respectively, when the characteristic size of the phononic crystal nanostructure is below 100 nm. The greater enhancement factor of ZT for the n-doped sample seems to result from the strong phonon scattering by heavy phosphorus atoms at the grain boundaries.