Optimizing Full‑Angle Light Emission in Micro‑LEDs with Highly Reflective Thin‑Film Coatings

Introduction

Displays underpin modern devices—from smartphones to large‑scale advertising panels—and the industry’s three dominant technologies are LCD, OLED, and micro‑LED. LCDs offer longevity and cost efficiency but suffer from limited back‑light efficiency and thick form factors. OLEDs deliver self‑luminous, high‑contrast panels, yet their need for intricate metal masks reduces resolution and lifespan due to organic material degradation. Micro‑LEDs combine the best of both worlds: high brightness, extended lifetime, and high color purity. However, their minuscule emitters still radiate Lambertian light, constraining viewing angles and complicating back‑light design for large‑format displays.

Extending micro‑LED emission angles without bulky secondary optics has attracted considerable research. Metasurfaces, nanowire arrays, and advanced coating strategies have been explored to redirect light, but many rely on complex nanofabrication or suffer from high absorption. Thin‑film reflectors offer a scalable alternative: by carefully selecting high‑index dielectric stacks and metallic layers, one can engineer constructive interference that boosts reflectivity across a broad angular range while preserving extraction efficiency.

In this work we present a primary‑optics approach that deposits a TiO₂/SiO₂ dielectric stack topped with a thin aluminum layer onto the micro‑LED surface. The resulting HRTF achieves >90% reflectivity at the dominant 460 nm wavelength, significantly expands the emission cone, and retains the device’s electrical and optical performance. Full‑angle micro‑LEDs with such coatings could revolutionize commercial advertising displays and planar lighting solutions requiring wide viewing angles.

Materials and Methods

Micro‑LED Chip Geometry and Baseline Light Field

The micro‑LEDs investigated have a length (L_c) of 150 µm, a width (W_c) of 85 µm, and a height (H_c) of 85 µm. Figure 1 shows the bare chip’s angular intensity profile: the center intensity (I_C) is 92 % of the peak, the peak occurs at 15°, and the FWHM is 135°. These values confirm the Lambertian nature of the emitter and motivate the need to widen the emission cone.

Micro‑LED chip light distribution curve

Design of the Highly Reflective Thin‑Film (HRTF)

The HRTF comprises a dielectric stack of alternating TiO₂ (high‑index) and SiO₂ (low‑index) layers, capped with a thin Al film. The dielectric pair (HL) operates at an optical thickness of λ/4 to maximize constructive interference. The Al layer introduces additional reflectivity while maintaining low absorption. Refractive indices at the 460 nm wavelength are summarized in Table 1.

Simulation and Optimization

Using Macleod optical simulation software, four candidate stacks were evaluated: pure Al, Al/(HL), (HL)², Al/(HL)², and Al/(HL)³. Reflectivity at 460 nm for these structures is 85.5 %, 86.2 %, 71.8 %, 90.2 %, and 93.0 %, respectively. While Al/(HL)³ offers the highest reflectivity, its increased metal thickness raises absorption and fabrication complexity. Al/(HL)² provides a balanced trade‑off with 90.2 % reflectivity, 4.5 % transmittance, and 5.4 % absorption, meeting the >90 % reflectivity target while keeping losses minimal.

Simulated reflectance of candidate thin‑film stacks (400–500 nm)

Reflectance and transmittance of Al/(HL)² versus (HL)² (400–500 nm)

Angle‑dependent reflectance and transmittance of Al/(HL)² and (HL)² (0–90°)

3‑D reflectivity map of Al/(HL)² (λ = 440–480 nm, θ = 0–25°)

Results and Discussion

Scanning electron microscopy confirms the uniformity of the deposited HRTF stack. The chip dimensions for the tested prototype are 240 µm × 140 µm × 100 µm. Figures 7 and 8 display top‑ and bottom‑views and a cross‑section, revealing layer thicknesses of 20.6 nm Al, two TiO₂ layers (≈46 nm each), and two SiO₂ layers (≈77 nm each), matching the design specifications.

SEM images: (a) top view, (b) bottom view

Cross‑sectional SEM of the HRTF stack

Electrical performance remains essentially unchanged after coating. At 30 mA, the bare micro‑LED delivers 33.833 mW, 3.293 V, and 41.84 % EQE; with HRTF, the values are 32.757 mW, 3.301 V, and 40.51 % EQE—a negligible 3.3 % EQE drop. At 50 mA, the radiant flux decreases by only 3.3 % relative to the bare device, underscoring the coating’s minimal optical penalty.

Photoelectric characteristics with and without HRTF coating

Spectral stability tests show that the peak wavelength shifts only 5.5 nm when the drive current rises from 2 to 30 mA, and the temperature‑induced shift is a modest 2.4 nm from 25 °C to 105 °C. Long‑term aging at 30 mA and 25 °C retains 98.5 % of the original radiant flux after 1,000 h, confirming the coating’s durability.

Peak wavelength variation with drive current

Peak wavelength shift versus temperature

Long‑term stability: radiant flux after 1,000 h

Light‑distribution measurements reveal the most striking improvement: the FWHM expands from 135° to 165°, the peak angle rises from 15° to 37.5°, and the center intensity drops to 63 % of the original. These changes indicate a broader, more uniform emission pattern suitable for wide‑viewing applications.

Light‑distribution curves: bare (black) vs. HRTF‑coated (red)

Luminous distribution schematic: (a) bare, (b) HRTF‑coated

Reflectance versus wavelength for the HRTF stack (440–460 nm)

Future work may extend the design to the full visible spectrum by increasing the Al thickness beyond 50 nm, thereby enhancing color uniformity across 400–780 nm.

Conclusions

We have demonstrated that a carefully engineered HRTF coating—comprising an Al/(HL)² stack—can transform a micro‑LED’s Lambertian emission into a full‑angle, 165° radiance profile with only a 3.3 % reduction in radiant flux. The coating preserves the micro‑LED’s electrical performance, spectral stability, and long‑term durability, making it a practical solution for wide‑viewing displays and planar lighting. This approach eliminates the need for bulky secondary optics and paves the way for next‑generation, high‑efficiency, wide‑angle micro‑LED products.

Availability of data and materials

The datasets supporting the conclusions of this article are available within the article.

Abbreviations

micro-LEDs:

Micro‑light‑emitting diodes

FWHM:

Full width at half maximum

TV:

Television

LCDs:

Liquid crystal displays

OLEDs:

Organic light‑emitting diodes

SCMS:

Supercell metasurfaces

TITO:

Titanium–indium–tin oxide

NIR-LEDs:

Near‑infrared light‑emitting diodes

CSP‑LEDs:

Chip Scale Package‑Light‑emitting diode

PSS:

Patterned sapphire substrate

NPSS:

Nano‑patterned sapphire substrate

L_c:

Micro‑LED length

W_c:

Micro‑LED width

H_c:

Micro‑LED height

I_peak:

Peak angle intensity

I_C:

Center light intensity

HRTF:

Highly reflective thin film

MQW:

Multiple quantum well

H:

High refractive index material

L:

Low refractive index material

k:

Extinction coefficient

SEM:

Scanning electron microscope

L–I–V:

Luminance–current–voltage

IV:

Current versus voltage