Disparity in electron and phonon mean-free-paths (Vol. 46 No. 3)

Disparity in electron and phonon mean-free-paths
Electrical and lattice thermal conductivity accumulation with respect to the mean free paths (MFPs).

There has been intensified interest in high performance thermoelectric materials in the last two decades, largely owing to the nanostructuring approaches that significantly reduce heat leakage by phonons. A condition for the nanostructuring approaches to be effective is the phonon mean free path (MFP) much longer than that of electrons so that phonons are more frequently scattered at the interfaces. While there has been a recent significant progress in the first-principles understanding of phonon MFP spectral distributions, the spectral distribution of electron MFPs remains unknown.

In this work the authors compute from first-principles the energy dependent electron scattering and MFPs in silicon. They show that electrons and phonons have very different MFP distributions with phonon MFPs significantly longer than those of electrons. The authors show with a model that silicon 20 nm grains can yield a more than five times enhancement in the thermoelectric figure of merit, since the grain boundaries scatter phonons more significantly than electrons due to their different MFP distributions. This work provides new fundamental insights by quantitatively revealing the disparity in electron and phonon MFP distributions from first principles.

B. Qiu, Z. Tian, A. Vallabhaneni, B. Liao, J. M. Mendoza, O. D. Restrepo, X. Ruan and G. Chen, First-principles simulation of electron mean-free-path spectra and thermoelectric properties in silicon, EPL, 109, 57006 (2015)