Multi-scale anisotropic mechanisms in UV ultrafast laser machining of GaN

Gallium nitride (GaN) is a key wide-bandgap semiconductor for high-power electronics, yet its high hardness severely constrains high-precision machining. While ultrafast laser machining (ULM) offers a promising route, current strategies largely treat GaN as an isotropic material, overlooking the critical role of crystallographic orientation in laser-induced phase transitions and subsurface damage. To bridge this gap, this work elucidates the anisotropic mechanisms of UV picosecond ULM on single-crystal GaN through a multi-scale approach, combining molecular dynamics (MD) simulations with systematic experiments. We demonstrate that subsurface integrity is highly orientation-dependent. Scribing along the [10–10] direction triggers a pronounced hexagonal-to-cubic (h-GaN to c-GaN) phase transition, extensive dislocations, and deep subsurface cracking. In contrast, scribing along the [1–210] direction fundamentally suppresses this destructive phase transformation and defect proliferation. We further reveal that unlike subsurface damage, the formation of surface periodic structures (LIPSS) is almost completely isotropic, governed primarily by transient thermo-hydrodynamic effects. These multi-scale insights not only unlock the path to defect-minimized manufacturing of next-generation GaN devices but also provide a universal theoretical framework for controlling damage in advanced hard-brittle anisotropic crystals.