Sulfur Passivation Engineering of Carbon Defects in N-Surface GaN: Suppressing Nonadiabatic Carrier Recombination Via Self-Compensated S_N-C_N Complexes

In gallium nitride (GaN), the carbon-on-nitrogen substitutional defect (C_N) has been extensively investigated as a prototypical deep acceptor center, particularly for its well-characterized yellow luminescence associated with the (-/0) transition. However, the carrier dynamics involving its secondary (0/+) transition remains poorly understood. Combining first-principles calculations and nonadiabatic molecular dynamics simulations, we systematically investigate the nonradiative carrier capture processes mediated by the C_N defects on nitrogen-terminated GaN surfaces, along with their sulfur-based passivation mechanisms. Our results demonstrate that the neutral C_N defect serves as a critical nonradiative recombination center, exhibiting an ultrafast hole capture rate (τ ≈ 10^-12 s). Notably, sulfur atoms can migrate with a low energy barrier (0.64 eV) to occupy adjacent nitrogen vacancies on the N surface, forming S_N-C_N complex defects through a self-compensation mechanism. This structural modification induces a significant charge redistribution, shifting the defect level from deep within the bandgap to near the valence band maximum. Such electronic structure modulation effectively suppresses nonadiabatic transitions between defect states and the valence band. This work provides the first atomistic visualization of sulfur passivation mechanisms for C_N defects in GaN, establishing a quantitative relationship between defect configuration engineering and carrier recombination dynamics.