  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.
