Effect of surface and interfaces on longitudinal thermal transport in thin-film Si/Ge structures

One of the main approaches to increase the thermoelectric figure of merit of materials is to reduce their thermal conductivity and, in this respect, the surface and possible interfaces play an important role in low-dimensional structures. By means of nonequilibrium molecular dynamics simulations at 300 K we investigate the longitudinal phonon thermal conductivity in Si/Ge multilayered thin-film structures having sharp interfaces and (100), (110), (111) crystallographic orientations with respect to the number of Si/Ge periods (or film thickness) and in comparison with Ge films of equivalent thickness. It is shown that as the thickness of the Si/Ge layered film decreases from $\sim$ 50 to 1 nm and heat flux propagates along the [110] direction, significant phonon-surface scattering occurs for the (100) orientation, which leads to a decrease in the phonon thermal conductivity by almost a factor of 4 (from 19.1 to 5.12 W/(m $\cdot$ K)) and to insignificant change ($\sim$ 22 $\pm$ 1 W/(m $\cdot$ K)) for the (110) and (111) orientations. In comparison with the Si/Ge films, the Ge films of equivalent thickness display a qualitative and quantitative agreement indicating the scattering of phonons at the Si/Ge interface to be balanced by the higher thermal conductivity of the Si layers.