Nonequivalent Atomic Vibrations at Interfaces in a Polar Superlattice

• Published in: Advanced materials (Weinheim) 2024-08, Vol.36 (33), p.e2402925-n/a
• Main Authors: Hoglund, Eric R., Walker, Harrison A., Hussain, Kamal, Bao, De‐Liang, Ni, Haoyang, Mamun, Abdullah, Baxter, Jefferey, Caldwell, Joshua D., Khan, Asif, Pantelides, Sokrates T., Hopkins, Patrick E., Hachtel, Jordan A.
• Format: Article
• Language: English
• Subjects: Carrier gases; Electron energy; Electronic properties; Heterostructures; interface vibrations; Interfaces; Material properties; monochromated electron energy‐loss spectroscopy; Optical activity; Optical properties; phonon; Phonons; scanning transmission electron microscopy; Spectral resolution; Superlattices; Thermal conductivity; Vibration mode; wide bandgap
• ISSN: 0935-9648; 1521-4095; 1521-4095
• DOI: 10.1002/adma.202402925

In heterostructures made from polar materials, e.g., AlN–GaN–AlN, the nonequivalence of the two interfaces is long recognized as a critical aspect of their electronic properties; in that, they host different 2D 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 nonequivalence of the corresponding interface atomic vibrations, however, is not investigated so far due to a lack of experimental techniques with both high spatial and high spectral resolution. Herein, the nonequivalence of AlN–(Al0.65Ga0.35)N and (Al0.65Ga0.35)N–AlN interface vibrations is experimentally demonstrated using monochromated electron energy‐loss spectroscopy in the scanning transmission electron microscope (STEM‐EELS) and density‐functional‐theory (DFT) calculations are employed to gain insights in the physical origins of observations. It is demonstrated 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 nonequivalent 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. The influence of the structural nonequivalence of AlN–AlGaN interfaces on the interface vibrational modes is probed through off‐axis monochromated electron energy‐loss spectroscopy. By carefully aligning off‐axis deflection with the polar bonds, phonon softening at one interface (but not the other) are isolated and the nonequivalence is measured. This opens the way for polarization‐selective vibrational spectroscopy with ultrahigh spatial resolution.