Mie Scattering of Phonons by Point Defects in IV-VI Semiconductors PbTe and GeTe

TL;DR

This study reveals that phonon scattering by point defects in PbTe and GeTe exhibits Mie scattering rather than the traditionally assumed Rayleigh scattering, due to long-range strain fields, affecting thermal conductivity predictions.

Contribution

The paper demonstrates, using ab initio Green's function methods, that point defect scattering in IV-VI semiconductors follows Mie scattering, challenging the conventional Rayleigh scattering assumption.

Findings

Mie scattering characterized by weaker frequency dependence.

Asymmetric scattering phase functions observed.

Relaxation time approximation has ~20% error at 300K for vacancy fraction 1%.

Abstract

Point defects in solids such as vacancy and dopants often cause large thermal resistance. Because the lattice site occupied by a point defect has a much smaller size than phonon wavelengths, the scattering of thermal acoustic phonons by point defects in solids has been widely assumed to be the Rayleigh scattering type. In contrast to this conventional perception, using an ab initio Green's function approach, we show that the scattering by point defects in PbTe and GeTe exhibits Mie scattering characterized by a weaker frequency dependence of the scattering rates and highly asymmetric scattering phase functions. These unusual behaviors occur because the strain field induced by a point defect can extend for a long distance much larger than the lattice spacing. Because of the asymmetric scattering phase functions, the widely used relaxation time approximation fails with an error of ~20% at 300K in predicting lattice thermal conductivity when the vacancy fraction is 1%. Our results show that the phonon scattering by point defects in IV-VI semiconductors cannot be described by the simple kinetic theory combined with Rayleigh scattering.