GaN LED Light Extraction via In-Situ Surface Roughening
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Solution Overview
Problem
Conventional LEDs fabricated using Group III-nitride semiconductors, such as GaN, face limited light extraction efficiency due to the critical angle of refraction, resulting in only about 4% of internal light being emitted, with the majority being trapped or reflected back into the substrate.
Innovation Solution
Surface roughening of the bottom n-GaN and top p-GaN layers is implemented to scatter internal light outward, creating etching pits or air voids that alter the refractive index, enhancing light extraction efficiency by scattering light rather than reflecting it back into the active layers.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Ease of manufacture
If conventional smooth surface LED structure is used, then manufacturing is simple, but light extraction efficiency is poor with only 4% of internal light being emitted
Solution Approach 1:
The patent applies surface curvature by creating a roughened surface topology on the GaN layer through chemical etching. The etching process forms curved, non-planar surface features that scatter light at multiple angles, enabling light rays that would normally be trapped by total internal reflection to escape. This curved surface structure transforms the optical path of trapped light without complicating the fundamental LED manufacturing process.
Solution Approach 2:
The patent changes the surface parameter from smooth to roughened by controlling the etching depth and morphology. By adjusting etching parameters such as chemical solution composition, temperature, and exposure time, the surface roughness is optimized to maximize light scattering while maintaining manufacturability. This parameter transformation increases the critical angle for total internal reflection, allowing more light to escape the high-index GaN material.
2Productivity
If surface roughening is applied to enhance light extraction, then light extraction efficiency improves, but manufacturing complexity increases
Solution Approach 1:
The patent employs a self-organized chemical etching process where the GaN surface automatically forms a uniform roughened morphology through controlled chemical reaction. The etching solution selectively removes material based on crystallographic orientation and local surface conditions, creating a self-regulating roughening pattern without requiring complex mask alignment or multiple processing steps. This self-service mechanism simplifies the overall manufacturing complexity despite the enhanced surface structure.
3Productivity
If surface roughening is applied to enhance light extraction, then light extraction efficiency improves, but the manufacturing process becomes more difficult
Solution Approach 1:
The patent merges the surface roughening step with the existing LED fabrication sequence by integrating chemical etching into the wet processing stage. The etching process is combined with other wet chemistry steps such as buffer layer removal or contact layer formation, allowing multiple functions to be achieved in a single process flow. This merging approach maintains ease of manufacture by utilizing existing wet processing infrastructure and chemical handling capabilities.
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 surface roughening process increases external quantum efficiency by scattering internal light outward, potentially doubling the light output compared to conventional LEDs, as shown in photoluminescence intensity measurements.
Implementation Method 1
Surface roughening of the bottom n-GaN and top p-GaN layers is implemented to scatter internal light outward, creating etching pits or air voids that alter the refractive index
Implementation Method 2
according to Snell's law (also known as the law of refraction), light traveling from a region having a higher index of refraction to a region with a low index of refraction that is within a certain critical angle
Data Source
AI summary
The embodiments of the present invention generally relates to methods for enhancing the light extraction by surface roughening of the bottom n-GaN layer and/or top p-GaN layer so that the internal light from the active region is scattered outwardly to result in a higher external quantum efficiency. In one embodiment, a surface roughening process is performed on the n-GaN layer to form etching pits in a top surface of the n-GaN layer. Once the etching pits are formed, growth of the n-GaN material may be resumed on the roughened n-GaN layer to partially fill the etching pits, thereby forming air voids at the interface of the n-GaN layer and the subsequent, re-growth n-GaN layer. These air voids provide one or more localized regions with indices of reflection different from that of the n-GaN layer, such that the internal light generated by the active layers (e.g., the InGaN MQW layer), when passing through the n-GaN layer, is scattered by voids or bubbles. The surface roughening process may be further performed on a top surface of a p-GaN layer to scatter the light emitted from the active layers outwardly rather than being reflected back to the active layers.