Decoupling Composition and Band Gap in $\kappa$-Ga$_2$O$_3$ Heterostructures via STEM-EELS

TL;DR

This study uses advanced STEM-EELS techniques to precisely map and analyze band gap variations at oxide heterointerfaces, revealing strain effects dominate over composition in influencing local electronic properties.

Contribution

The paper introduces a novel automated EELS analysis framework combined with optimized STEM-EELS methods to accurately measure nanoscale band gap variations in $ppa$-Ga$_2$O$_3$ heterostructures.

Findings

Strain at defect-free interfaces causes a gradual band gap transition from 5.08 eV to 4.28 eV over 10 nm.

Structural defects relieve strain, resulting in band gaps consistent with composition.

STEM-EELS effectively resolves band gap variations at nanometer scale despite probe delocalization.

Abstract

High-resolution mapping of electronic properties at oxide heterointerfaces remains challenging due to probe delocalization and overlapping signals. In this work, we employ monochromated, probe-corrected scanning transmission electron microscopy combined with electron energy-loss spectroscopy (STEM-EELS) to resolve band gap variations across $\kappa$-Ga$_2$O$_3$-based multilayers with nanometer-scale precision. A custom automated quantitative-based EELS analysis framework enabled automated band gap fitting and visualization, ensuring reproducibility and high spatial resolution. By optimizing acquisition parameters and quantifying inelastic delocalization, we demonstrate reliable extraction of band gap excitations from layers only a few nanometers thick. For heterostructures grown on ITO templates, strain at defect-free interfaces induces a gradual band gap transition from $5.08~\mathrm{eV}$ to $4.28~\mathrm{eV}$ over $\sim 10~\mathrm{nm}$, despite an abrupt compositional change. In contrast, ZnO-based templates introduce structural defects that relieve strain, yielding band gaps consistent with composition. These results establish STEM-EELS as a powerful tool for nanoscale electronic characterization and highlight the dominant role of interfacial strain over composition in governing local band structure.