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

## TL;DR

This paper shows how surface sulfidation improves the efficiency of red micro-LEDs by reducing defects and enhancing performance.

## Contribution

The study demonstrates that surface sulfidation reduces S-droop in AlGaInP LEDs by stabilizing surface defects and unpinning the Fermi level.

## Key findings

- Surface sulfidation with ammonium sulfide increases maximum external quantum efficiency by 120.6% at 5 A/cm².
- Sulfidation replaces surface defects with stable sulfur bridge bonds, reducing nonradiative recombination.
- Electrical characterization shows sulfidation improves both top and sidewall interface properties.

## Abstract

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.