Observation of Conductive Interstitial Ga Line Defects in <i>β</i> ‐Ga ₂ O ₃

Beta-phase gallium sesquioxide (β-Ga₂O₃), possessing an ultrawide bandgap and high breakdown voltage, exhibits strong potential for deep-ultraviolet photodetection and high-power electronics. However, nanometer-scale line defects, prevalent in β-Ga₂O₃ growth, degrade device performance by increasing leakage currents and reducing breakdown voltages, thus termed "killer defects". Critically, the impact of these defects at the atomic scale remains unclear due to limited characterization and a lack of detailed understanding. Here, the observation of novel conductive atomic line defects is reported within β-Ga₂O₃ nanoflakes using near-field infrared imaging. Combining atomic-resolution imaging with density functional theory calculations, these defects are identified as interstitial Ga atoms migrating along the c-axis. These atomic line defects exhibit a broadband infrared response and quenched cathodoluminescence, indicative of significantly enhanced local conductivity. This elevated conductivity enables subsurface near-field detection of the defects and remote excitation of phonon polaritons in a hexagonal boron nitride (hBN) capping layer. These findings underscore the distinct conductivity of atomic-scale line defects, emphasizing the need for their controlled management during material synthesis and device fabrication, while simultaneously suggesting opportunities for their exploitation in nanophotonic applications.