Sub-Wavelength Antireflective Grating for Directed MicroLED Emission

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Solution Overview

Problem

Existing optoelectronic devices with surface-emitting structures, such as LEDs and VCSELs, face significant light reflection issues due to the high optical index difference between semiconductor materials and the medium they emit into, leading to reduced light delivery and undesirable emission patterns, particularly in microLEDs where characteristics of antireflective layers are hard to control and reproduce.

Innovation Solution

A sub-wavelength periodic grating with hollow and protruding parts is used as an antireflective structure at the emission surface, which modifies the emission cone's angular aperture and improves directivity without diffractive effects, allowing for better control and reproducibility of luminous efficiency and emission patterns.

Engineering Contradictions & Design Principles

VSEngineering Contradiction Analysis

1Loss of energy

If a homogeneous antireflective layer is deposited on the outlet surface, then reflection is reduced and light transmission is improved, but the emission cone angular aperture becomes too wide and directivity is lost

Engineering Contradiction:

Improvelight reflection lossVSAvoidemission directivity

Core Design Contradiction:

Loss of energyVSEase of operation

Solution Approach 1:

The homogeneous antireflective layer is segmented into a periodic grating structure with alternating high-index and low-index regions. This segmentation allows the structure to simultaneously reduce reflection through effective index matching and control emission directionality through geometric configuration, resolving the contradiction between improving light transmission and maintaining emission directivity.

Inventive Principle:

Principle #1Segmentation

Solution Approach 2:

Different regions of the outlet surface are given different optical properties through the periodic grating structure. The high-index regions provide reflection reduction while the low-index regions and overall geometric configuration control the emission cone angular aperture. This local differentiation allows simultaneous optimization of both reflection reduction and emission directivity.

Inventive Principle:

Principle #3Local quality

2Ease of operation

If no antireflective layer is used, then emission directivity is maintained, but significant light reflection occurs at the outlet surface

Engineering Contradiction:

Improveemission directivityVSAvoidlight reflection loss

Core Design Contradiction:

Ease of operationVSLoss of energy

Solution Approach 1:

The optical parameters at the outlet surface are changed by introducing a periodic grating structure with specific pitch, depth, and filling factor. These parameter changes create an effective medium with optimized refractive index that reduces reflection while the geometric parameters control the emission pattern, allowing both reflection reduction and directivity control to be achieved.

Inventive Principle:

Principle #35Parameter changes

Solution Approach 2:

The grating structure acts as a composite optical system combining regions of different refractive indices (high-index semiconductor material and low-index air or cladding material). This composite structure provides effective index matching to reduce reflection while the overall geometric configuration maintains emission directivity, resolving the contradiction between the two requirements.

Inventive Principle:

Principle #40Composite materials

3Loss of energy

If a porous ATO layer with tilted nanocolumns is deposited, then reflection is reduced by about 20%, but manufacturing complexity increases and control over layer characteristics is limited

Engineering Contradiction:

Improvelight reflection lossVSAvoidmanufacturing process complexity

Core Design Contradiction:

Loss of energyVSDevice complexity

Solution Approach 1:

The manufacturing approach is simplified by changing from a complex tilted vapor deposition process to a more controllable lithography and etching process. The grating parameters (pitch, depth, filling factor) can be precisely controlled through standard semiconductor fabrication techniques, reducing manufacturing complexity while maintaining or improving reflection reduction performance.

Inventive Principle:

Principle #35Parameter changes

Solution Approach 2:

The mechanical vapor deposition process with tilted flux is replaced by a lithography-based patterning process followed by etching. This substitution allows for better control over the grating structure characteristics and simplifies the manufacturing process by using more conventional and controllable fabrication techniques.

Inventive Principle:

Principle #28Mechanics substitution (Replace mechanical system)

4Ease of manufacture

If the grating pitch is increased to reduce manufacturing difficulty, then diffractive effects appear and emission pattern control is compromised

Engineering Contradiction:

Improvegrating fabrication easeVSAvoidemission pattern precision

Core Design Contradiction:

Ease of manufactureVSManufacturing precision

Solution Approach 1:

The grating pitch is optimized to a specific range that balances manufacturing feasibility with optical performance. The pitch is small enough to avoid diffractive effects and maintain precise emission pattern control, yet large enough to be manufacturable with current lithography techniques. This parameter optimization resolves the contradiction between manufacturing ease and emission pattern precision.

Inventive Principle:

Principle #35Parameter changes

Applied Scientific Principles

This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.

Function Achieved in This Case

The sub-wavelength grating effectively reduces reflection, concentrates emission into a narrower cone, and enhances luminous efficiency and reproducibility, particularly in microLEDs, by acting as an effective homogeneous medium that adjusts its optical index to optimize light transmission.

Implementation Method 1

by acting as an effective homogeneous medium that adjusts its optical index to optimize light transmission

Methodology Applied

Scientific EffectEffective medium theory:

Implementation Method 2

A sub-wavelength periodic grating with hollow and protruding parts is used as an antireflective structure at the emission surface

Methodology Applied

Scientific EffectAntireflection: Anti-Reflective Coating

Implementation Method 3

Methodology Applied

Scientific EffectSub-wavelength grating effect:

Data Source

PatentUS20230268461A1Optoelectronic device with sub-wavelength antireflective structure, associated screen and manufacturing method

Publication Date: 2023.08.24 COMMISSARIAT A LENERGIE ATOMIQUE ET AUX ENERGIES ALTERNATIVES

AI summary

An optoelectronic device includes an emissive structure, at least a part of which is formed of one or more semiconductor materials, configured to produce a luminous radiation when it has an electric current flowing therethrough, the luminous radiation being produced within the emissive structure and having an average wavelength λ, the emissive structure having an average optical index n and being delimited by an outlet surface, through which at least a part of the luminous radiation exits, the device further including an antireflective structure includes a sub-wavelength periodic grating which includes hollow parts and protruding parts forming a periodic structure with a pitch lower than λ/[2.n].