Effects of atomic relaxation on phonon dispersion relations and thermal properties of ultrathin (Si)n(Ge)n[001] superlattices

Abstract

Using ab initio density-functional perturbation theory we have examined the effects of atomic relaxation on the phonon dispersion relations and thermal properties of ultrathin (Si)${}_{n}$(Ge)${}_{n}$[001] ($1\ensuremath{\le}n\ensuremath{\le}5$) superlattices. It is found that atomic relaxation effects governed by the minimum energy requirement lead to significant changes in the location of phonon frequencies above 200 cm${}^{\ensuremath{-}1}$ as well as in the location and width of phononic gaps. These changes result in a decrease of around $7%$ in the zone-average phonon relaxation time and up to a $5%$ decrease in the thermal conductivity tensor components ${\ensuremath{\kappa}}_{zz}$ and ${\ensuremath{\kappa}}_{xx}$ of the (Si)${}_{1}$(Ge)${}_{1}$[001] superlattice.