Thermal conductance across harmonic-matched epitaxial Al-sapphire heterointerfaces

Abstract

A unified fundamental understanding of interfacial thermal transport is missing due to the complicated nature of interfaces. Because of the difficulty to grow high-quality interfaces and lack of materials characterization, the experimentally measured thermal boundary conductance (TBC) in the literature are usually not the same as the ideally modelled interfaces. This work provides a systematic study of TBC across the highest-quality (atomically sharp, harmonic-matched, and ultraclean) epitaxial (111) Al||(0001) sapphire interfaces to date. The comparison of measured high TBC with theoretical models shows that elastic phonon transport dominates the interfacial thermal transport and other mechanisms play negligible roles. This is confirmed by a nearly constant transmission coefficient by scaling the TBC with the Al heat capacity and sapphire heat capacity with phonon frequency lower than 10 THz. Finally, the findings in this work will impact applications such as electronics thermal management, thermoelectric energy conversion, and battery safety.

Inelastic phonon transport across atomically sharp metal/semiconductor interfaces

“…A recent work by Cheng et al claimed that no inelastic phonon transport is observed in atomically sharp Al/Al2O3 interface grown by MBE 6 . We would like to point out that at least 5 monolayers of Al2O3 near the interface are distorted, as shown in their TEM image, which is similar to our Al/Si Sample 2 and not as sharp as our Al/Si Sample 1.…”

“…The scattering of heat carriers, predominately phonons, leads to interface thermal resistance. Over the past several decades, there were extensive studies on thermal conductance across metal/semiconductor interfaces, both experimentally and theoretically [6][7][8][9][10][11][12] . However, the rare agreement between experiments and calculations as well as the scattered experimental results even for the same interface warrant study for much better understanding of thermal transport across interfaces 6,[13][14][15] .…”

“…The elastic transport assumption predicts that interface thermal conductance will reach a plateau at temperatures higher than the lower Debye temperature (TD) of the two materials that formed the interface, when all phonons in this side have been excited. In recent years, advanced calculation methods such as molecular dynamics (MD) and atomistic Green function (AGF) have been used to study phonon transport process across interfaces [6][7][8]10,11,18 . Inelastic phonon transport process has been predicted to exist in interfaces between very dissimilar materials, where the transmitted phonons do not have the same frequency as the incident phonons 7,11,19 , and anharmonicity was found to be of fundamental importance for the inelastic phonon transport across interfaces.…”

“…Most experimental results show that phonon transport across metal/semiconductor interface is an elastic process, as interface thermal conductance saturates at high temperatures for most interfaces under study 6,15,20 . There are only a few exceptions to this trend 12,13,21 , and all of these suggest a large Debye temperature difference across interface leads to the observation of non-saturated thermal conductance at high temperatures.…”

“…Extrinsic phonon scattering centers, such as atomic intermixing, interface roughness and contamination, would easily scatter phonons and bury the phonon's intrinsic elastic and inelastic transport processes across interfaces 22 . Epitaxial metal/semiconductor interfaces are usually used to study the intrinsic interface phonon transport due to their importance and high quality 6,[22][23][24] . However, previous studies on epitaxial interfaces used typically lacked atomic-level structural details 12,21 with interface roughness and atomic intermixing often ignored, thus only lead to qualitative analysis and limit our understanding of intrinsic phonon transport across interfaces.…”