Crystal structure dependent thermal conductivity in two-dimensional phononic crystal nanostructures

TL;DR

This study investigates how the crystal lattice structure of silicon phononic crystal nanostructures affects thermal conductivity, revealing that triangular lattices have lower conductivity due to local heat flux disorder, with implications for thermoelectric design.

Contribution

It demonstrates the dependence of thermal conductivity on crystal structure in 2D phononic crystals through experimental measurements and numerical simulations.

Findings

Triangular lattice structures exhibit approximately 20% lower thermal conductivity than square lattices at 30% porosity.

Thermal conductivity varies significantly with lattice type when the period is within the phonon mean free path.

Numerical Monte Carlo simulations support experimental results, confirming structure-dependent heat flux disorder.

Abstract

Thermal phonon transport in square- and triangular-lattice Si phononic crystal (PnC) nanostructures with a period of 300 nm was investigated by measuring the thermal conductivity using micrometer-scale time-domain thermoreflectance. The placement of circular nanoholes has a strong influence on thermal conductivity when the periodicity is within the range of the thermal phonon mean free path. A staggered hole structure, i.e., a triangular lattice, has lower thermal conductivity, where the difference in thermal conductivity depends on the porosity of the structure. The largest difference in conductivity of approximately 20% was observed at a porosity of around 30%. This crystal structure dependent thermal conductivity can be understood by considering the local heat flux disorder created by a staggered hole structure. Numerical simulation using the Monte Carlo technique was also employed and also showed the lower thermal conductivity for a triangular lattice structure. Besides gaining a deeper understanding of nanoscale thermal phonon transport, this information would be useful in the design of highly efficient thermoelectric materials created by nanopatterning.