Decoupling composition and band gap in κ-Ga2O3 heterostructures via STEM-EELS.

High-resolution band-gap mapping at oxide heterointerfaces is challenged by probe delocalization, overlapping spectral contributions, and the interplay between strain and composition. Here, we combine monochromated, probe-corrected scanning transmission electron microscopy with low-loss electron energy-loss spectroscopy (STEM-EELS) and a transparent Python workflow to quantify local band-gap variations in ultrathin κ-Ga2O3 multilayers. The band gap fitting procedure enables reproducible extraction of band-gap onsets from layers only a few nanometers thick. In heterostructures grown on ITO, we identify a strain-induced reduction of the band gap, resolving a ∼10 nm transition from 5.08 to 4.28 eV that far exceeds the calculated inelastic delocalization length and cannot be explained by alloy composition. GPA strain maps and plasmon-based composition tracking confirm that a continuous compressive strain field across the (Al0.27Ga0.73)2O3/(In0.18Ga0.82)2O3 interface drives the reduced band gap. In contrast, ZnO-templated multilayers exhibit relaxed interfaces, structural defects, and band gaps consistent with composition. These results demonstrate that physically informed VEELS analysis can reliably decouple strain, composition, and measurement artefacts, establishing STEM-EELS as a robust tool for nanoscale electronic-structure characterization in complex oxide heterostructures.