  At the nanoscale, the thermal boundary conductance (TBC) and thermal
conductivity are not intrinsic properties of interfaces or materials but depend
on the nearby environment. However, most studies focused on single interfaces
or superlattices, and the thermal transport across heterostructures formed by
multiple different materials is still mysterious. In this study, we demonstrate
how much the TBC of an interface is affected by the existence of a second
interface, as well as how much the thermal conductivity of a material is
affected by the nearby materials. Using Si and Ge modeled by classical
molecular dynamics simulations, the following phenomena are demonstrated. (1)
The existence of a nearby interface can significantly change the TBC of the
original interface. For example, by adding an interface after Si/Ge, the TBC
can be increased from 400 to 700 MW/m2K. This is because the nearby interface
serves as a filter of phonon modes, which selectively allows particular modes
to pass through and affect the TBC of the original interfaces. This impact will
disappear at the diffusive limit when the distance between interfaces is much
longer than the phonon mean free path so that phonon modes recover equilibrium
statistics before arriving at the second interface. (2) The thermal
conductivity of a material can be significantly changed by the existence of
neighboring materials. For example, a standalone 30-nm-thick Si's thermal
conductivity can be increased from 50 to 280 W/mK, a more than 4-fold increase,
beating the bulk thermal conductivity of Si, after being sandwiched between two
Ge slabs. This is because the Ge slabs on the two sides serve as filters that
only allow low-frequency phonons to transport heat in Si, which carry more heat
than optical phonons. This work opens a new area of successive interface
thermal transport and is expected to be important for nanoscale thermal
characterization and thermal management of semiconductor devices.
