Ultrafast Laser-Induced Defects in β-Gallium Oxide Below Ablation Threshold

In this work, (2̅01)β-Ga₂O₃ was irradiated with 95 fs, 1030 nm laser pulses in order to investigate ultrafast laser-induced morphological and crystalline defects, which play an important role in the controlled transformation of materials at the nanomicro-scale. Kelvin probe force microscopy and depth-resolved cathodoluminescence spectroscopy (DRCLS) revealed laser-induced subsurface crystallographic defects below the ablation threshold that were undetectable by optical and atomic force microscopy. While DRCLS probed depths of 58-180 nm, scanning transmission electron microscopy provided complementary insights into regions beyond the reach of DRCLS, enabling direct imaging of the crystal structure and defects at or just below the surface. The analysis revealed a depth-dependent modification of the material, with an amorphous layer forming closest to the surface and damage site, transitioning to a defective region exhibiting a phase change to γ-Ga₂O₃, and further transitioning to a region rich in point defects, with defect concentrations decreasing with depth. A Keldysh ionization-based FDTD simulation of the single pulse interaction was carried out as well, revealing high density of carrier generation consistent with the depth scales observed by the measurements. These findings contribute to a deeper understanding of defect formation mechanisms in β-Ga₂O₃ and highlight the potential of ultrashort laser pulses for precision subsurface modification in wide bandgap semiconductors.