  Interface impedes heat flow in heterostructures and the interfacial thermal
resistance (ITR) has become a critical issue for thermal dissipation in
electronic devices. To explore the mechanism leading to the ITR, in this work,
the dynamic behaviors of phonons passing through the GaN/AlN interface with an
amorphous interlayer is investigated by using phonon wave packet simulation. It
is found the amorphous interlayer significantly impedes phonon transport across
the interface, and leads to remarkable phonon mode conversions, such as
LA$\rightarrow$TA, TA$\rightarrow$LA, and LA$\rightarrow$TO conversion.
However, due to mode conversion and inelastic scattering, we found a portion of
high-frequency TA phonons, which are higher than the cut-off frequency and
cannot transmit across the ideal sharp interface, can partially transmit across
the amorphous interlayer, which introduces additional thermal transport
channels through the interface and has positive effect on interfacial thermal
conductance. According to phonon transmission coefficient, it is found the ITR
increases with increasing of amorphous interlayer thickness L. The phonon
transmission coefficient exhibits an obvious oscillation behavior, which is
attributed to the multiple phonon scattering in the amorphous interlayer, and
the oscillation period is further revealed to be consistent with the
theoretical prediction by the two-beam interference equation. In addition,
obvious phonon frequency shifts and phonon energy localization phenomena were
observed in the amorphous interlayer. Finally, to improve phonon transmission,
the interface morphology was further optimized via the annealing reconstruction
technique, which results in re-crystallization of the amorphous interlayer and
the decrease of ITR by ~21% as L=2 nm.
