Direct Measurement of Room Temperature Non-diffusive Thermal Transport Over Micron Distances in a Silicon Membrane

TL;DR

This study demonstrates that at micron scales, room temperature thermal transport in silicon deviates from classical diffusion, revealing non-diffusive, ballistic phonon behavior for low-frequency phonons over micron distances.

Contribution

The paper introduces a simple experimental method to directly measure non-diffusive thermal transport in silicon at micron scales, highlighting deviations from diffusion at room temperature.

Findings

Thermal transport deviates from diffusion at micron distances.

Ballistic phonon transport becomes significant for low-frequency phonons.

Effective thermal conductivity decreases at small distances.

Abstract

The "textbook" phonon mean free path (MFP) of heat carrying phonons in silicon at room temperature is ~40 nm. However, a large contribution to the thermal conductivity comes from low-frequency phonons with much longer MFPs. We present a simple experiment demonstrating that room temperature thermal transport in Si significantly deviates from the diffusion model already at micron distances. Absorption of crossed laser pulses in a freestanding silicon membrane sets up a sinusoidal temperature profile that is monitored via diffraction of a probe laser beam. By changing the period of the thermal grating we vary the heat transport distance within the range ~1-10 {\mu}m. At small distances, we observe a reduction in the effective thermal conductivity indicating a transition from the diffusive to the ballistic transport regime for the low-frequency part of the phonon spectrum.