Revealing Vibrational Nonequivalence at Interfaces in Polar Heterostructures using STEM-EELS and DFT*

Distinct material properties and emergent phenomena stem from structural and chemical discontinuities at interfaces. In heterostructures made from polar materials, e.g., piezoelectric AlN-GaN-AlN, the two interfaces are nonequivalent, hosting distinct 2D carrier gases, resulting from piezoelectric and spontaneous polarization effects. Likewise, the modes of atomic vibrations at interfaces are also affected by the corresponding structural and chemical discontinuities. The nonequivalence of these interface vibrations has not yet been investigated due to a lack of experimental techniques with both high spatial and spectral resolution. We demonstrate the nonequivalence of AlN-(Al_0.65Ga_0.35)N and (Al_0.65Ga_0.35)N-AlN interface vibrations using monochromated electron-energy-loss spectroscopy in the scanning transmission electron microscope (STEM-EELS) and explicate the physical origin of the nonequivalent interface behavior using density-functional theory (DFT). Our work leverages the mixed real- and reciprocal space selectivity of the vibrational response in angle-resolved STEM-EELS to directly map nonequivalent interface phonons between materials with different stacking order. The findings elucidate the vibrational characteristics of interfaces, which can enhance thermal conductivity.