Phonon modes and topological phonon properties in (GaN)x/(AlN)x and (AlGaN)x/(GaN)x superlattices

To effectively regulate thermal transport for the near-junction thermal management of GaN electronics, it is imperative to gain an understanding of the phonon characteristics of GaN nanostructures, particularly the topological phonon properties connected to low-dissipation surface phonon states. In this work, a comprehensive study on phonon modes and topological phonon properties is performed from first principles in (GaN)x/(AlN)x and (AlGaN)x/(GaN)x (x=1,2,3) superlattices. Phonon modes, including the dispersion relation, density of states, and participation ratio, were calculated for six GaN superlattices. The participation ratio results did not reveal the localized phonon mode. In topological phonon analyses, it is found that Weyl phonons with a Chern number of 1(-1) are present in all six GaN superlattices, consisting of trivial (GaN) and nontrivial (AlN and AlGaN) combinations. These phonons are located on either side of the kz = 0 plane symmetrically in the Brillouin zone. With the increase in the number of phonon branches in superlattices, the number of Weyl phonon points also increases from dozens to hundreds. One Weyl phonon with significant and clean surface states is selected and analyzed for each GaN superlattice. Among them, the Weyl phonon in (GaN)2/(AlN)2 superlattice mainly results from the lattice vibrations of Al and Ga atoms, while the Weyl phonons in other superlattices mainly result from the lattice vibrations of N atoms. The Weyl phonons at opposite kz planes form pairs in (GaN)2/(AlN)2, AlGaN/GaN, and (AlGaN)2/(GaN)2. Effects of strain including biaxial and uniaxial strain on Weyl phonons in GaN/AlN and AlGaN/GaN superlattices are investigated. Results indicate that Weyl phonons persist in large strain states, however, no monoclinic trend is observed due to the accidental degeneracy of these superlattices.