Non-equivalent Atomic Vibrations at Interfaces in a Polar Superlattice.

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 gases. Interfaces play an important 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-(Al 0.65 Ga 0.35 )N and (Al 0.65 Ga 0.35 )N-AlN interface vibrations using monochromated electron energy-loss spectroscopy in the scanning transmission electron microscope (STEM-EELS) and employ density-functional-theory (DFT) calculations to gain insights in the physical origins of observations. We demonstrate that STEM-EELS possesses sensitivity to the displacement vector of the vibrational modes as well as the frequency, which is as critical to understanding vibrations as polarization in optical spectroscopies. The combination enables direct mapping of the non-equivalent interface phonons between materials with different phonon polarizations. The results demonstrate the capacity to carefully assess the vibrational properties of complex heterostructures where interface states dominate the functional properties. This article is protected by copyright. All rights reserved.