Effect of phonon dispersion on thermal conduction across Si/Ge interfaces

TL;DR

This study uses advanced simulations of phonon transport to analyze how phonon dispersion affects thermal conductivity in Si/Ge superlattices, revealing significant reductions in thermal conductivity due to phonon spectrum mismatch.

Contribution

First detailed simulation incorporating phonon dispersion and polarization in Si/Ge interfaces, revealing new insights into phonon energy exchange and thermal resistance mechanisms.

Findings

Thermal conductivity in Si/Ge superlattices is much lower than bulk materials.

Phonon spectrum mismatch causes significant non-equilibrium energy exchange.

Conditions identified where phonon mismatch greatly increases thermal resistance.

Abstract

We report finite-volume simulations of the phonon Boltzmann transport equation (BTE) for heat conduction across the heterogeneous interfaces in SiGe superlattices. The diffuse mismatch model incorporating phonon dispersion and polarization is implemented over a wide range of Knudsen numbers. The results indicate that the thermal conductivity of a Si/Ge superlattice is much lower than that of the constitutive bulk materials for superlattice periods in the submicron regime. We report results for effective thermal conductivity of various material volume fractions and superlattice periods. Details of the non-equilibrium energy exchange between optical and acoustic phonons that originate from the mismatch of phonon spectra in silicon and germanium are delineated for the first time. Conditions are identified for which this effect can produce significantly more thermal resistance than that due to boundary scattering of phonons.