Thermal boundary conductance and thermal conductivity strongly depend on nearby environment

KZ Adnan and TL Feng, PHYSICAL REVIEW B, 109, 245302 (2024).

DOI: 10.1103/PhysRevB.109.245302

At the nanoscale, thermal boundary conductance (TBC) and thermal conductivity are not intrinsic properties of interfaces or materials but depend on the nearby environment. However, most studies focus 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 m - 2 K - 1 . 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, the thermal conductivity of standalone 30 nm thick Si can be increased from 50 to 280 W m - 1 K - 1 , a more than fourfold 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; these phonons carry more heat than optical phonons. This work opens up an area of successive interface thermal transport and is expected to be important for nanoscale thermal characterization and thermal management of semiconductor devices.

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