Profiled Surface Roughness for LED Light Extraction
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Solution Overview
Problem
Light trapping and absorption issues in semiconductor emitting devices, such as LEDs, due to abrupt changes in optical properties and refractive indices between layers, lead to significant losses in light extraction efficiency.
Innovation Solution
A profiled surface with large and small roughness components is introduced, where the large roughness components have a characteristic scale an order of magnitude larger than the target wavelength and the small roughness components have a scale on the order of the target wavelength, superimposed on the large components, to facilitate improved light propagation and reduce Fresnel losses.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Loss of energy
If a smooth interface between layers is used, then Fresnel losses are minimized, but total internal reflection causes significant light trapping
Solution Approach 1:
The patent applies surface profiling (curvature) by introducing roughness elements with specific geometric shapes (spheres, hemispheres, cones, pyramids) on the interface surface. These curved or angled surfaces modify light propagation paths, reducing total internal reflection while maintaining low Fresnel losses through optimized geometric parameters.
Solution Approach 2:
The patent changes the surface topology parameters by introducing roughness elements with controlled characteristic scales (comparable to or larger than the emission wavelength), heights, and densities. This parameter modification transforms the flat interface into a profiled surface that optimizes light extraction efficiency by balancing TIR reduction and Fresnel loss minimization.
2Loss of energy
If roughness is introduced to reduce total internal reflection, then light extraction improves, but Fresnel losses increase
Solution Approach 1:
The patent optimizes the characteristic scale of roughness elements to be comparable to or larger than the emission wavelength, which reduces total internal reflection more effectively than conventional nanoscale roughness. The specific geometric parameters (height, width, spacing) are tuned to minimize Fresnel reflections while maximizing TIR reduction through controlled light path modifications.
Solution Approach 2:
The use of curved surface profiles (spheres, hemispheres, cones) creates gradual transitions in refractive index exposure, reducing abrupt Fresnel reflections. The curvature distributes the refractive index change over a longer optical path, minimizing reflection losses while the overall surface profiling maintains effectiveness against total internal reflection.
3Loss of energy
If conventional nanoscale roughness is used, then some light extraction improvement occurs, but the characteristic scale is too small to effectively reduce total internal reflection
Solution Approach 1:
The patent fundamentally changes the characteristic scale parameter from nanoscale (conventional) to microscale (comparable to or larger than emission wavelength). This parameter shift enables the roughness elements to effectively interact with and redirect light paths at angles that reduce total internal reflection, while the specific size optimization minimizes unwanted Fresnel losses.
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 profiled surface design enhances light extraction efficiency by reducing total internal reflection and Fresnel losses, allowing a higher fraction of radiation to escape and be transmitted through the interface, thereby improving the overall output of the emitting device.
Implementation Method 1
A larger change in the index of refraction between the layers, and between the substrate and its surroundings, results in a smaller total internal reflection (TIR) angle
Implementation Method 2
Fresnel losses are associated with light partially reflected at the interface for all the incident light angles
Data Source
AI summary
A profiled surface for improving the propagation of radiation through an interface is provided. The profiled surface includes a set of large roughness components providing a first variation of the profiled surface having a characteristic scale approximately an order of magnitude larger than a target wavelength of the radiation. The profiled surface also includes a set of small roughness components superimposed on the set of large roughness components and providing a second variation of the profiled surface having a characteristic scale on the order of the target wavelength of the radiation.