Thermal Conductance across Harmonic-Matched Epitaxial Al-Sapphire Heterointerfaces

A unified fundamental understanding of interfacial thermal transport is missing due to the complicated nature of interfaces which involves complex factors such as interfacial bonding and mixing, surface chemistry, crystal orientation, roughness, and interfacial disorder. This is especially true for metal-nonmetal interfaces which incorporate multiple fundamental heat transport mechanisms such as elastic and inelastic phonon scattering as well as electron-phonon coupling in the metal and across the interface. All these factors jointly affect thermal boundary conductance (TBC). Therefore, the experimentally measured interfaces may not be the same as the ideally modelled, thus obfuscating conclusions drawn from previous experimental and modeling comparisons. This work provides a systematic study of interfacial thermal conductance across well-controlled and ultraclean epitaxial (111) Al || (0001) sapphire interfaces, known as harmonic-matched interface. The measured high TBC is compared with theoretical models of atomistic Green’s function (AGF) and the non-equilibrium Landauer approach, showing that elastic phonon transport dominates the interfacial thermal transport of the Al-sapphire interfaces. By scaling the TBC with the Al heat capacity and sapphire heat capacity with phonon frequency lower than the max Al phonon frequency, a nearly constant transmission coefficient is observed, indicating that the phonons on the Al side limits the Al-sapphire TBC. This confirms that elastic phonon transport dominates interfacial thermal transport of the Al-sapphire interfaces and other mechanisms play negligible roles. Our work enables a quantitative study of TBC to validate theoretical models of thermal transport mechanisms across metal-nonmetal interfaces, and acts as a benchmark when studying how other factors impact TBC. KEYWORDS: Thermal boundary conductan.