Defect-Initiated Anisotropic Oxidation in Two-Dimensional GaSe

Oxidation governs the stability and functional conversion of two-dimensional semiconductors, yet its propagation through layered crystals remains poorly understood. Here, we investigate the thickness-dependent oxidation of GaSe using optical spectroscopy, cross-sectional microscopy, and first-principles calculations. Experiments reveal concurrent top-down and edge-initiated oxidation, accompanied by Se redistribution and transient Ga2Se3 formation. Calculations identify Se vacancies as favorable sites for O2 adsorption and activation, which weaken neighboring Ga–Se bonds and promote oxygen incorporation. Molecular O2 faces a 4.03 eV barrier when penetrating a monolayer Ga2O3 model, compared with 0.54 eV along a GaSe van der Waals gap. This contrast supports interlayer diffusion as a plausible pathway for lateral oxidation, consistent with the pronounced thickness dependence and spatial anisotropy observed experimentally. Oxidation also yields a progression from p-type GaSe through ambipolar Ga2O3/GaSe heterostructures to n-type Ga2O3, whose devices retain stable characteristics after 300 °C air annealing.