Enhancing Light Extraction in Deep‑UV Flip‑Chip LEDs Using Nanometer‑Scale Meshed Contacts and Inclined AlGaN Nanocones

Introduction

AlGaN‑based DUV LEDs are increasingly sought after for water purification, phototherapy, sensing, and photocatalysis. Yet achieving high external quantum efficiency (EQE) remains challenging, especially at shorter wavelengths. EQE equals internal quantum efficiency (IQE) multiplied by light‑extraction efficiency (LEE). Conventional flip‑chip DUV LEDs typically deliver EQE <10%, largely because LEE is limited to 7–9%. The current world‑record EQE of 20% at 275 nm was enabled by advanced LEE techniques such as patterned sapphire, transparent p‑electrodes, and refined packaging. Thus, raising LEE is pivotal for high‑efficiency DUV LEDs.

LEE suffers from total internal reflection (TIR) and Fresnel loss due to the high refractive index contrast between AlGaN (n≈2.6) and air. Increasing Al content in the quantum wells also favours TM‑polarized emission, which is difficult to escape the device. Strategies to enhance LEE include surface roughening, patterned sapphire, inclined sidewalls, and plasmonic resonances, all of which introduce scattering centres that redirect photons. Another key limitation is absorption in the p‑GaN contact layer, which cannot be easily replaced by Al‑rich p‑AlGaN due to poor hole conductivity. To mitigate this loss, meshed p‑type contacts, distributed Bragg reflectors, or photonic crystals have been proposed. Meshed contacts, in particular, are cost‑effective and have shown promise at the micron scale; however, nanometer‑scale meshes and their scattering impact have not been extensively studied.

Here we systematically explore nanometer‑scale meshed contact designs and an Al reflector to assess their influence on LEE for DUV LEDs. We compare p‑GaN nanorods, hybrid p‑GaN/p‑AlGaN nanorods, and hybrid p‑GaN/p‑AlGaN truncated nanocones. Our 3D‑FDTD analysis reveals that the hybrid truncated nanocone structure can deliver more than five‑fold enhancement for TE light and 24‑fold for TM light.

Model and Simulation Methods

Simulations were performed using Lumerical FDTD Solution, which solves Maxwell’s equations in the time domain. The baseline flip‑chip model (Fig. 1a) includes a 92 % reflective Al top layer to redirect photons toward the sapphire substrate. A 100 nm p‑GaN, 1.5 µm n‑AlGaN, and 1 µm sapphire stack houses 100 nm of multiple quantum wells (MQWs). A single dipole source positioned at the MQW centre, polarized either along X (TE) or Y (TM), excites emission at 280 nm. Absorption coefficients at 280 nm are 10 cm⁻¹ for AlGaN, 1000 cm⁻¹ for MQWs, and 170 000 cm⁻¹ for GaN. Refractive indices are 2.6 (AlGaN), 2.9 (GaN), and 1.8 (sapphire). The simulation domain is 8 µm × 8 µm laterally, with perfect‑matched layers (PML) on the top and bottom. A non‑uniform mesh with a minimum cell size of 5 nm ensures accurate field resolution. Power monitors placed 300 nm above the sapphire capture transmitted power, which is Fourier‑transformed to the far field. LEE is calculated as the ratio of transmitted power to total dipole emission.

a Side view of the 3D‑FDTD model for a conventional flip‑chip DUV LED. b TE and TM LEEs for LEDs with and without a 100‑nm p‑GaN layer versus p‑AlGaN thickness.

Results and Discussions

Effect of the Optical Cavity Thickness on LEE

The distance between the MQWs and the Al reflector, controlled by the p‑AlGaN thickness, forms an optical cavity that modulates LEE. Fig. 1b shows oscillatory LEE curves with a ~50 nm period, matching the theoretical λ/2n≈53 nm. TE and TM peaks are out of phase, as predicted by Fresnel and Mueller‑matrix analysis. The presence of a 100‑nm p‑GaN layer dampens the cavity effect, yet the minimum TE/TM LEE remains higher than for devices without p‑GaN. Consequently, eliminating or thinning the p‑GaN layer can more than double LEE.

Effect of the Meshed p‑GaN Contacts on LEE

To reduce p‑GaN absorption, we replaced the continuous layer with a square array of 250‑nm diameter, 100‑nm high nanorods embedded in the Al reflector (Fig. 2a). The optimal p‑AlGaN thickness is 125 nm. The inter‑nanorod spacing is critical: smaller spacing improves current spreading but increases the p‑GaN filling factor, enhancing absorption. Fig. 2c shows that TE LEE rises with spacing up to 125 nm, achieving a >3‑fold boost, then declines as scattering dominates. This trend is corroborated by a model with zero GaN absorption and a perfect‑electric‑conductor (PEC) reflector, where LEE increases and then decreases with spacing, confirming the role of scattering.

a Schematic of a flip‑chip DUV LED with meshed p‑GaN contacts. b Top view of the nanorod array. c TE and TM LEEs versus nanorod spacing (p‑AlGaN = 125 nm).

We further examined the influence of nanorod height. Reducing height from 100 nm to 25 nm increases LEE due to lower absorption, but the benefit saturates for heights below 25 nm because the Al reflector dominates the scattering behaviour (Fig. 4).

(a) LEEs versus spacing for nanorod heights of 10, 25, 50, and 100 nm. Inset: normal‑incidence reflectivity versus p‑GaN thickness. b Reflectivity versus spacing for each height.

Effect of the Hybrid p‑GaN/p‑AlGaN Meshed Contacts on LEE

Introducing p‑AlGaN nanorods alongside p‑GaN nanorods (Fig. 5a) further enhances scattering without compromising reflectivity. LEEs for various p‑AlGaN heights (0, 25, 75, 100 nm) all exceed those of the pure p‑GaN structure, with the 75‑nm case delivering the highest TE LEE (Fig. 5b). Far‑field patterns confirm stronger electric‑field intensity for devices with p‑AlGaN nanorods (Fig. 5c,d).

a Side view of a hybrid p‑GaN/p‑AlGaN nanorod meshed contact. b TE LEE versus spacing for p‑AlGaN heights 0, 25, 75, 100 nm (inset: reflectivity). c Far‑field at 125 nm spacing, 75‑nm p‑AlGaN. d Far‑field at 125 nm spacing, 0‑nm p‑AlGaN. e TM LEE versus spacing for the same heights.

Effect of the Hybrid p‑GaN/p‑AlGaN Truncated Nanocone on LEE

To tackle the persistently low TM LEE, we replaced the p‑AlGaN nanorods with truncated nanocones that feature an inclination angle α (Fig. 6a). Decreasing α increases both TE and TM LEEs. At α = 30°, the TE LEE peaks at 26% (spacing = 375 nm) and the TM LEE rises to 12% (spacing = 260 nm), representing 5‑fold and 24‑fold gains relative to the baseline. This demonstrates that inclined nanocones dramatically improve TM scattering.

a Side view of a hybrid p‑GaN nanorod/p‑AlGaN truncated‑nanocone meshed contact. b TE LEE versus spacing for α = 30°, 50°, 75°, 90°. c TM LEE versus spacing for the same angles.

Conclusions

Our comprehensive simulation study confirms that nanometer‑scale meshed contacts—p‑GaN nanorods, hybrid p‑GaN/p‑AlGaN nanorods, and hybrid p‑GaN/p‑AlGaN truncated nanocones—substantially raise LEE in deep‑UV LEDs. The key trade‑offs are absorption in the p‑GaN layer and Al plasmonic losses versus scattering benefits. The hybrid truncated nanocone, with a 30° inclination, achieves the most dramatic improvement, delivering a 24‑fold increase in TM LEE while maintaining TE LEE gains.

Abbreviations

3D FDTD:

Three‑dimensional finite‑difference time‑domain method

DBR:

Distributed Bragg reflector

DUV LEDs:

Deep ultraviolet light‑emitting diodes

EQE:

External quantum efficiency

IQE:

Internal quantum efficiency

LEE:

Light extraction efficiency

MQWs:

Multiple quantum wells

ODR:

Omni‑directional reflector

PEC:

Perfect electrical conductor

PML:

Perfectly matched layer

TE:

Transverse electric

TIR:

Total internal reflection

TM:

Transverse magnetic