Mean Free Path–Thermal Conductivity Accumulation Calculations for Wurtzite Gallium Nitride: Two Approaches

ABSTRACT Understanding the mean free path distribution of the dominant heat carriers is very important in determining the ballistic to diffusive heat transport transition in nanoscale devices. This is true for the high electron mobility transistors made from GaN where both the thickness of the buffer layer and localized heating causing ballistic-diffusive heat transfer may complicate the transport properties needed to describe the device thermal response. In this work, we obtain the mean free path–thermal conductivity relation of phonons in bulk wurtzite GaN crystals using two different, ab-initio-based calculations. While the Vienna Ab-initio Simulation Package (VASP) is used in both approaches at the initial stage, the first method does not calculate the third-order force constants (FCs) and approximates the anharmonicity with a single fitting parameter in determination of discrete phonon properties thermal conductivity and relaxation time, while the second method uses third-order force constants directly. Results show that the third-order FCs are important in modeling the contribution of high-frequency optical phonons with relatively short MFPs, to the thermal conductivity of the material. Yet these effects are more significant at high temperatures and at samples without crystallographic disorders, and they can be omitted while modeling the real samples at low temperatures.