Defect-Induced Phonon Redistribution at Semiconductor Heterointerfaces

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Wide-bandgap gallium nitride (GaN) is a foundational material platform for high-power electronics and optoelectronics [1-3], where interfacial phonon behaviors play critical roles in thermal transport and device reliability [4,5]. Although thermal boundary resistance at heterogeneous interfaces has been extensively studied at the macroscopic scale [6,7], direct experimental insight into the spatial distribution and modification of phonon populations at the atomic scale, particularly in the presence of interfacial defects, remains largely unexplored. Here, by performing vibrational electron energy-loss spectroscopy (EELS) across GaN/Al2O3 interfaces, we reveal interfacial vibrational modes and identify defect-mediated redistribution among phonon branches at the interface. The cross-sectional GaN/Al2O3 specimen was prepared from a bulk GaN film grown on an Al2O3 (0001) substrate using conventional focused ion beam lift-out procedures followed by ion milling for final thinning. Vibrational EEL spectra were acquired using a monochromated Nion UltraSTEM 200 with an energy resolution of 6 meV operated at 60 kV and a convergence semi-angle of 32 mrad. HAADF-STEM imaging, multi-slice ptychography reconstruction, and core-loss EELS measurements were performed at an accelerating voltage of 200 keV. Figure 1a presents a HAADF-STEM image of the GaN/Al2O3 heterointerface together with the corresponding EELS elemental maps, revealing an atomically abrupt interface with clear chemical and structural contrast between the GaN epilayer and the Al2O3 substrate. As shown in Figure 1b, a single-slice image from a multislice ptychographic reconstruction further resolves the atomic structure at the interface, showing that Al-terminated Al2O3 is coherently aligned with the N columns in GaN with no observable structural defects (Fig. 1b). To probe the local vibrational behavior across the interface, vibrational EELS hyperspectral imaging was performed in a momentum-averaged configuration. The resulting vibrational EEL spectra represent the total phonon density of states, incorporating vibrational modes with momentum components perpendicular to the electron beam direction. Figure 1c shows the vibrational EEL spectra extracted from bulk GaN, the interfacial region, and bulk Al2O3, each exhibiting distinct vibrational signatures characteristic of the respective materials. Quantitative fitting of the vibrational signal near 35 meV reveals a reduction in spectral intensity at the interface relative to both adjacent bulk regions, indicating suppressed interfacial phonon excitations over a broad energy range of ∼1.5 nm rather than within a single phonon branch (Fig. 1d). Figure 2 investigates the impact of interfacial defects by comparing defect-free regions with areas containing misfit dislocations. Figure 2a shows a representative ptychographic slice along the beam direction, with misfit dislocation cores highlighted by red arrows. The corresponding vibrational spectra from defect-free and dislocation-containing regions are shown in Fig. 2b. Regions containing misfit dislocations exhibit a clear suppression of acoustic phonon spectral weight accompanied by a relative enhancement of optical phonon contributions, indicating a systematic phonon redistribution of vibrational spectral weight from acoustic to optical phonon branches in the presence of misfit dislocations. These results demonstrate that interfacial defects modify local phonon mode populations at the interface rather than causing a uniform attenuation of vibrational signals. In summary, monochromated vibrational EELS enables direct, spatially resolved mapping of phonon distributions across the GaN/Al2O3 interface and reveals defect-mediated phonon branch redistributions associated with misfit dislocations. These results demonstrate that interfacial defects can fundamentally reshape local vibrational mode populations, and establish vibrational EELS as a powerful platform for probing phonon–defect interactions in technologically relevant semiconductor heterostructures [8]. Structural imaging and phonon response across the GaN/Al2O3 interface. a, HAADF-STEM image of the GaN/Al2O3 interface with corresponding Al and Ga elemental maps. Scale bar, 50 nm. b, Representative single-slice image from a multislice ptychographic reconstruction of an atomically coherent region of the GaN/Al2O3 interface, with atomic positions overlaid at the corresponding lattice sites. Scale bar, 5Å. c, Background-subtracted vibrational EELS spectra acquired from bulk GaN, the defect-free interfacial region, and bulk Al2O3. d, Integrated signal intensity near 35 meV. Error bars denote the standard deviation obtained from multiple independent measurements. Phonon response comparison between defect-free and misfit dislocation regions. a, Representative single-slice image from a multislice ptychographic reconstruction of a GaN/Al2O3 interfacial region containing a misfit dislocation. The defect region is highlighted by red arrows. Scale bar, 5Å. b, Background-subtracted vibrational EELS spectra acquired from defect-free and misfit dislocation regions. Black and red curves correspond to spectra from the defect-free and misfit dislocation regions, respectively. c, Integrated signal intensity using a 10 meV energy window from 20 to 100 meV. Gray and orange bars represent the integrated intensities from the dislocation and defect-free regions, respectively, while the red bars indicate the differential intensity (dislocation minus defect-free).

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