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

Abstract

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.

Phonon and heat transport control using pillar-based phononic crystals

“…However, the experimentally observed 20% reduction in thermal conductivity is interesting to compare with the reduction obtained in other silicon-based nanostructures. On one hand, a stronger reduction was measured in silicon with pores (>50%) [9,18,19], nanodots (>70%) [13,68], polycrystalline grains (>80%) [15,69,70], dopants (>50%) [71,72] or germanium atoms (>70%) [14,71,73]. On the other hand, the 20% reduction by the pillars [63] is comparable to the reduction by holes (20–25%) [21,74] or slits (20–30%) [75] covering the same relative area.…”

A phononic crystal strip based on silicon for support tether applications in silicon-based MEMS resonators and effects of temperature and dopant on its band gap characteristics

“…The one is the phonon Bragg scattering effect of dopant in silicon host. 27 The scattering mechanism based on Bragg scattering effect have been reported by M. Nomura et al for polycrystalline Si thin films with phononic crystal nanopatterning for thermoelectric applications. 27 The other is the change of elastic constant values due to dopant.…”

“…However, the Bragg scattering effect can be negligible. 27 Thus, as a part of investigation of influences of doping concentration, we only simulate to predict the effects of doped silicon using the Keys' prediction theory of variation of silicon elastic constants in this work. Similar to the discussions in section III C for temperature case, when a dopant is doped into silicon, its elastic constants are changed as predicted by Keys' theory.…”

“…However, the approach using the same material for the substrate and tethers is commonly utilized in silicon-based MEMS resonators 9 because of simple fabrication technology. Thirdly, the technique doping a dopant into silicon has been employed in many areas, e.g, in polycrystalline Si thin films phononic crystal nanopatterning for thermoelectric applications 27 or temperature-induced frequency stability of silicon-based MEMS resonators. 23,24,28 Obviously, the pattern of air hole in PnC-based MEMS resonators can reduce thermal conductance.…”

Control of Phonon Transport by Phononic Crystals and Application to Thermoelectric Materials

“…The details of the growth conditions are explained in Ref. 31. Samples used for measuring electrical properties have 2D PnC nanostructures grown on SiO 2 and Al electrical contacts evaporated onto the structure to allow for four terminal electrical measurements.…”