Non-equivalent Atomic Vibrations at Interfaces in a Polar Superlattice

Abstract In heterostructures made from polar materials, e.g., AlN-GaN-AlN, the non-equivalence of the two interfaces has long been recognized as a critical aspect of their electronic properties, in that they host different two-dimensional carrier gasses. Interfaces play as important a role in the vibrational properties of materials, where interface states enhance thermal conductivity and can generate unique infrared-optical activity. The non-equivalence of the corresponding interface atomic vibrations, however, has not been investigated so far due to a lack of experimental techniques with both high spatial and high spectral resolution. Herein we experimentally demonstrate the non-equivalence of AlN-(Al0.65Ga0.35)N and (Al0.65Ga0.35)N-AlN interface vibrations using monochromated electron energy-loss spectroscopy in the scanning transmission electron micro-scope (STEM-EELS) and employ density-functional-theory (DFT) calculations to gain insights in the origins of observations. We demonstrate that STEM-EELS possesses sensitivity to the dis-placement vector of the vibrational modes as well as the frequency, which enables direct mapping of the non-equivalent interface phonons between materials with different phonon polarizations in the interface direction. The physical origin of the non-equivalent interface behavior is then unraveled by understanding the localization and anisotropy of phonon displacements using DFT calculations. The results demonstrate the capacity to carefully assess the vibrational properties of com-plex heterostructures where interface states dominate the functional properties.