Thermal Boundary Conductance Across Heteroepitaxial ZnO/GaN Interfaces: Assessment of the Phonon Gas Model

We present experimental
measurements of the thermal boundary conductance
(TBC) from 78–500 K across isolated heteroepitaxially grown
ZnO films on GaN substrates. This data provides an assessment of the
underlying assumptions driving phonon gas-based models, such as the
diffuse mismatch model (DMM), and atomistic Green’s function
(AGF) formalisms used to predict TBC. Our measurements, when compared
to previous experimental data, suggest that TBC can be influenced
by long wavelength, zone center modes in a material on one side of
the interface as opposed to the ‘“vibrational mismatch”’
concept assumed in the DMM; this disagreement is pronounced at high
temperatures. At room temperature, we measure the ZnO/GaN TBC as 490­[+150,–110]
MW m–2 K–1. The disagreement among
the DMM and AGF, and the experimental data at elevated temperatures,
suggests a non-negligible contribution from other types of modes that
are not accounted for in the fundamental assumptions of these harmonic
based formalisms, which may rely on anharmonicity. Given the high
quality of these ZnO/GaN interfaces, these results provide an invaluable,
critical, and quantitative assessment of the accuracy of assumptions
in the current state of the art computational approaches used to predict
phonon TBC across interfaces.