Multiscale phonon blocking in Si phononic crystal nanostructures

TL;DR

This study demonstrates that combining phononic patterning with grain boundary scattering in silicon nanostructures effectively reduces thermal conductivity by blocking a broad spectrum of phonon mean free paths, especially in polycrystalline silicon.

Contribution

It reveals the multiscale phonon blocking mechanism in silicon phononic crystal nanostructures, highlighting the enhanced impact in polycrystalline silicon due to grain boundary scattering.

Findings

Polycrystalline Si PnCs show larger thermal conductivity reduction than single-crystalline PnCs.

Grain boundaries and phononic patterning scatter phonons at different MFP scales.

Multiscale phonon blocking covers a broad MFP distribution, reducing thermal conduction effectively.

Abstract

In-plane thermal conduction and phonon transport in both single-crystalline and polycrystalline Si two-dimensional phononic crystal (PnC) nanostructures were investigated at room temperature. The impact of phononic patterning on thermal conductivity was larger in polycrystalline Si PnCs than in single-crystalline Si PnCs. The difference in the impact is attributed to the difference in the thermal phonon mean free path (MFP) distribution induced by grain boundary scattering in the two materials. Grain size analysis and numerical simulation using the Monte Carlo technique indicate that grain boundaries and phononic patterning are efficient phonon scattering mechanisms for different MFP length scales. This multiscale phonon blocking structure covers a large part of the broad distribution of thermal phonon MFPs and thus efficiently reduces thermal conduction.