(Invited) Limits to Phonon Thermal Boundary Resistance across GaN Interfaces

Abstract

As material length scales decrease below typical vibrational heat carrier (e.g., phonon) mean free paths, the thermal boundary resistances (TBR) at heterogeneous material interfaces become the dominant thermal resistances that dictate temperature rises, thermal runaway, and power density at failure in a wide array of devices. In this work, I will discuss the phonon scattering processes that dictate the thermal boundary conductance (TBC, the inverse of TBR, so TBC = 1/TBR) across GaN-based interfaces, including the maximum limits to TBC at GaN interfaces and the roles of point defect scattering and interfacial layers on GaN TBC. I will first present experimental measurements of the 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. I will then turn my discussion to focus on the role that interfacial imperfections, such as point defects and continuous films at the interface, have on GaN TBC. These results demonstrate the large variability that can occur in TBC at GaN-based interfaces based on interfacial non-idealities. 1. J. T. Gaskins, et al. "Thermal boundary conductance across heteroepitaxial zno/gan interfaces: Assessment of the phonon gas model," Nano Letters, 18, 7469–7477 (2018). 2. B. F. Donovan, et al. "Thermal boundary conductance across metal-gallium nitride interfaces from 80 - 450 K," Applied Physics Letters, 105, 203502 (2014).