  Interface phonon modes that are generated by several atomic layers at the
heterointerface play a major role in the interface thermal conductance for
nanoscale high-power devices such as nitride-based high-electron-mobility
transistors and light emitting diodes. Here we measure the local phonon spectra
across AlN/Si and AlN/Al interfaces using atomically resolved vibrational
electron energy-loss spectroscopy in a scanning transmission electron
microscope. At the AlN/Si interface, we observe various localized phonon modes,
of which the extended and interfacial modes act as bridges to connect the bulk
AlN modes and bulk Si modes, and are expected to boost the inelastic phonon
transport thus substantially contribute to interface thermal conductance. In
comparison, no such phonon bridge is observed at the AlN/Al interface, for
which partially extended modes dominate the interface thermal conductivity.
This work provides valuable insights into understanding the interfacial thermal
transport in nitride semiconductors and useful guidance for thermal management
via interface engineering.
