Lifetimes of Confined Acoustic Phonons in Ultra-Thin Silicon Membranes

TL;DR

This study investigates how the lifetime of high-frequency acoustic phonons in ultra-thin silicon membranes decreases with reduced thickness, providing insights into nanomechanical and thermal properties at the nanoscale.

Contribution

It presents experimental measurements of phonon decay times in silicon membranes and compares them with theoretical models including surface roughness effects.

Findings

Decay time decreases from ~4.7 ns to 5 ps as thickness reduces from 194 nm to 8 nm.

Experimental results align with models considering phonon-phonon interactions and surface roughness scattering.

Provides insights into limits of nanomechanical resonators and thermal transport in nanostructures.

Abstract

We study the relaxation of coherent acoustic phonon modes with frequencies up to 500 GHz in ultra-thin free-standing silicon membranes. Using an ultrafast pump-probe technique of asynchronous optical sampling, we observe that the decay time of the first-order dilatational mode decreases significantly from \sim 4.7 ns to 5 ps with decreasing membrane thickness from \sim 194 to 8 nm. The experimental results are compared with theories considering both intrinsic phonon-phonon interactions and extrinsic surface roughness scattering including a wavelength-dependent specularity. Our results provide insight to understand some of the limits of nanomechanical resonators and thermal transport in nanostructures.