Thermal conduction in Si and SiGe phononic crystals explained by phonon mean free path spectrum

Abstract

Thermal phonon transport in single-crystalline Si, amorphous SiGe, and poly-SiGe nanostructures was investigated experimentally at room temperature. The characteristic length dependence of thermal conductivity was compared across these three materials by changing the shortest distance between the circular holes of phononic crystals formed in the membranes. The dependences clearly differ for these materials, and these differences can be explained by the thermal phonon mean free path spectra of the three materials. Nanostructuring has a larger impact on thermal conductivity reduction when the characteristic length is comparable to that in the region where the thermal phonon mean free path spectrum is dense. The results suggest that thermal phonon mean free path spectra can be estimated qualitatively by thermal conductivity measurements with characteristic length sweeps.

Constructed Ge Quantum Dots and Sn Precipitate SiGeSn Hybrid Film with High Thermoelectric Performance at Low Temperature Region

“…The thermal phonons of SiGe based polycrystalline film are distributed in very short (<20 nm) and long (>200 nm) mean free path (MFP) ranges. [ 52 ] The short MFP phonons are strongly scattered by QDs, polycrystalline boundaries, and a fraction of small size Sn nanoprecipitation particles, while the long MFP phonons can be significantly scattered by amounts of Sn nanoprecipitation particles. Hence, the hybrid nanostructure provides a broader range of tunable MFP that can reduce thermal conductivity.…”

Ultra-low thermal conductivity of two-dimensional phononic crystals in the incoherent regime

“…[1][2][3][4][5][6][7][8][9][10][11] Since different length scales are associated with phonons and electrons, thermal and electrical conductivity can be independently optimized in a periodic nanomesh of holey structure. Reproducible low-thermal conductivity, sufficient electrical properties and good mechanical strength make 2D Si PnCs as a promising candidate for thermoelectric applications, such as electrical power generation and onchip thermal management for solid state devices.…”

Thermal Studies of Nanoporous Si Films with Pitches on the Order of 100 nm —Comparison between Different Pore-Drilling Techniques

“…Pores with >100 nm diameters were also drilled with a focused ion beam (FIB) 21 . Besides some studies 5 , 13 – 20 , the in-plane k L at room temperature was much lower than theoretical predictions that assumed bulk-like phonon transport and diffusive phonon scattering by pore edges 22 , 23 . Further considering the diffusive phonon scattering by the top and bottom surfaces of a thin film did not largely reduce the divergence for ~100 nm or shorter pitches.…”

“…Furthermore, some distortion and damage of a fragile nanoporous film during the film transfer and the following fabrication processes may also strongly affect k L . Such issues were addressed in more recent studies using an integrated device fabricated from the same Si film or using micro time-domain thermoreflectance measurements on a suspended sample, where the measured k values were mostly comparable to or higher than the theoretical predictions at 300 K 5 , 13 – 16 , 18 – 20 .…”