Strain inevitably exists in practical GaN-based devices due to the mismatch of lattice structure and thermal expansion brought by heteroepitaxial growth and band engineering, and it significantly influences the thermal properties of GaN. In this work, thermal transport properties of GaN considering the effects from biaxial strain and electron-phonon coupling (EPC) are investigated using the first principles calculation and phonon Boltzmann transport equation. The thermal conductivity of free GaN is 263 and 257 W/mK for in-plane and cross-plane directions, respectively, which are consistent better with the experimental values in the literature than previous theoretical reports and show a nearly negligible anisotropy. Under the strain state, thermal conductivity changes remarkably. In detail, under +5% tensile strain state, average thermal conductivity at room temperature decreases by 63%, while it increases by 53% under the −5% compressive strain, which is mostly attributed to the changes in phonon relaxation time. Besides, the anisotropy of thermal conductivity changes under different strain values, which may result from the weakening effect from strain induced piezoelectric polarization. EPC is also calculated from the first principles method, and it is found to decrease the lattice thermal conductivity significantly. Specifically, the decrease shows significant dependence on the strain state, which is due to the relative changes between phonon-phonon and electron-phonon scattering rates. Under a compressive strain state, the decreases of lattice thermal conductivity are 19% and 23% for in-plane and cross-plane conditions, respectively, comparable with those under a free state. However, the decreases are small under the tensile strain state, because of the decreased electron-phonon scattering rates and increased phonon anharmonicity.

Tuning thermal transport in semiconductor nanostructures is of great significance for thermal management in power electronics. With excellent transport properties such as ballistic transport immune to point defects and backscattering forbidden, topological phonon surface states show remarkable potential in addressing this issue. Herein, topological phonon analyses are performed on hexagonal wurtzite GaN, as well as other nitrides of the same family, i.e. AlN, and AlGaN alloy in perspective of topological phonon phase transition. With the aid of first principle calculations and topological phonon theory, Weyl phonon states, which host surfaces states without backscattering are investigated for all these materials. The results show that there is no nontrivial topological phonon state in GaN. However, by introducing Al atoms, i.e. in wurtzite type AlN and AlGaN, more than one Weyl phonon points are found, confirmed by obvious topological characteristics, including non-zero topological charges, source/sink in Berry curvature distributions, surface local density of states and open surface arcs. As AlN and AlGaN are typical materials in AlGaN/GaN heterostructure based power electronics, existence of topological phonon states in them will benefit thermal management by designing one-way interfacial phonon transport without backscattering.

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