Effect of surface morphology and interfaces on longitudinal phonon thermal conductivity in Ge(001) and Si/Ge(001) thin-film structures

In this work the method of non-equilibrium molecular dynamics was used to study the longitudinal phonon thermal transport at 300 K in nanoscale homogenous Ge(001) and layered Si/Ge(001) thin films with $p$ (2$\times$1) surface reconstruction along different directions. The appearance of thermal transport anysotropy in the films under consideration, which is due to both the surface morphology and sharp Si/Ge interfaces, has been established. For the direction when the dimers and Si–Ge bonds at the interface lie in a plane parallel to the direction of the heat flow, the lowest thermal conductivity is observed ($\sim$5–18 W/(m$\cdot$K) in the range from $\sim$1 to 27 nm). It is shown that for films with thicknesses of more than 13 nm for all directions, layered films have a lower thermal conductivity compared to homogenous ones. In this case, the role of the surface morphology and interfaces is reduced to different degrees of phonon localization and compensation for more heat-conducting Si layers, respectively.