Wafer-Bonded AlGaInP Red LEDs with Suppressed S-Droop through Surface Sulfidation

Micro-light-emitting diode (micro-LED) technology enables high pixels-per-inch (PPI) displays using conventional semiconductor processes and relies on extremely miniaturized mesa structures derived from traditional LEDs. Scaling to smaller sizes leads to a significant decrease in emission efficiency because of the stronger influence of sidewall damage. This efficiency degradation, known as the size-effect or S-droop, primarily arises from surface defects introduced by dry etching. These defects promote nonradiative Shockley-Read-Hall (SRH) recombination and create pathways for surface leakage current. In aluminum gallium indium phosphide (AlGaInP) LEDs, chemical passivation with ammonium sulfide is widely used to mitigate sidewall damage. However, the underlying reaction mechanism remains unclear, and most studies address only the sidewall regions. In this work, we fabricated a wafer-bonded vertical AlGaInP LED structure compatible with light-emitting diode on silicon (LEDoS) integration and applied ammonium sulfide surface sulfidation to investigate its broader effects. Chemical changes from sulfidation were examined using energy dispersive spectroscopy (EDS) and X-ray photoelectron spectroscopy (XPS). These analyses revealed that surface defects were replaced by stable sulfur bridge bonds, leading to Fermi level unpinning. Electrical characterization further separated parallel and series resistances, which revealed the influence of surface sulfidation across both the top and sidewall interfaces. As a result, the maximum external quantum efficiency (EQE) increased by 120.6% at 5 A/cm² in a 10 μm pixel, accompanied by a clear reduction in S-droop.