Ultimate confinement of phonon propagation in silicon nano-crystalline structure

TL;DR

This study demonstrates that silicon nano-crystalline structures with ultra-small grains and interfaces can achieve ultimate classical confinement of phonons, drastically reducing thermal conductivity across a wide temperature range.

Contribution

The paper introduces a phonon gas kinetics model validated by first-principles calculations that explains the anomalously low thermal conductivity in silicon nano-crystalline structures.

Findings

Thermal conductivity is significantly below bulk amorphous Si and SiO2.

Phonon mean free paths are below half the phonon wavelength.

Model accurately reproduces experimental data without fitting parameters.

Abstract

Temperature-dependent thermal conductivity of epitaxial silicon nano-crystalline (SiNC) structures composed of nanometer-sized grains separated by ultra-thin silicon-oxide (SiO2) films is measured by the time domain thermoreflectance technique in the range from 50 to 300 K. Thermal conductivity of SiNC structures with grain size of 3 nm and 5 nm is anomalously low at the entire temperature range, significantly below the values of bulk amorphous Si and SiO2. Phonon gas kinetics model, with intrinsic transport properties obtained by first-principles-based anharmonic lattice dynamics and phonon transmittance across ultra-thin SiO2 films obtained by atomistic Green's function, reproduces the measured thermal conductivity without any fitting parameters. The analysis reveals that mean free paths of acoustic phonons in the SiNC structures are equivalent or even below half the phonon wavelength, i.e. the minimum thermal conductivity scenario. The result demonstrates that the nanostructures with extremely small length scales and controlled interface can give rise to ultimate classical confinement of thermal phonon propagation.