  The breakdown of translational symmetry at heterointerfaces leads to the
emergence of new phonon modes localized near the interface. These interface
phonons play an essential role in thermal/electrical transport properties in
devices especially in miniature ones wherein the interface may dominate the
entire response of the device. Knowledge of phonon dispersion at interfaces is
therefore highly desirable for device design and optimization. Although
theoretical work has begun decades ago, experimental research is totally absent
due to challenges in achieving combined spatial, momentum and spectral
resolutions required to probe localized phonon modes. Here we use electron
energy loss spectroscopy in an electron microscope to directly measure both the
local phonon density of states and the interface phonon dispersion relation for
an epitaxial cBN-diamond heterointerface. In addition to bulk phonon modes, we
observe acoustic and optical phonon modes localized at the interface, and modes
isolated away from the interface. These features only appear within ~ 1 nm
around the interface. The experimental results can be nicely reproduced by ab
initio calculations. Our findings provide insights into lattice dynamics at
heterointerfaces and should be practically useful in thermal/electrical
engineering.
