Sidewall optical leakage and scaling limits in sub-20  μ m AlGaInP micro-resonant cavity LEDs

Micro-resonant cavity light-emitting diodes (RCLEDs) are attractive for next-generation augmented reality displays and visible-light communication systems because of their superior spectral purity and color stability; however, their optical behavior in the sub-20 μm regime remains insufficiently understood. In this work, we demonstrate 620 nm AlGaInP micro-RCLEDs with 13 × 20 μm2 mesas and ± 2–3 nm cavity detuning, achieving pronounced spectral narrowing (full width at half maximum <10 nm) and excellent current-dependent wavelength stability. Far-field characterization reveals an unexpectedly large beam divergence, which is identified as being dominated by sidewall emission. By employing a hybrid optical model combining transfer-matrix calculations with geometric optics, we verify that this behavior originates from enhanced coupling into laterally guided cavity modes that are efficiently extracted at the sidewalls of sub-20 μm mesas. This work reveals, for the first time, a fundamental scaling trade-off between cavity confinement and beam directionality, highlighting the critical need for sidewall-emission mitigation strategies in the design of high-efficiency, high-resolution micro-RCLED pixels. Simulations further indicate that an Au sidewall reflector can suppress lateral leakage and increase the coupling efficiency into a ±20° acceptance cone from 8.4% to 20.0%.