Profiled Surface for LED Light Extraction
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Solution Overview
Problem
Semiconductor emitting devices, such as LEDs and laser diodes, face issues with light trapping due to abrupt changes in optical properties and refractive indices, leading to significant Fresnel losses and internal reflection, which hinder efficient light extraction.
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 light extraction by reducing total internal reflection and Fresnel losses.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Ease of manufacture
If a smooth interface is used between layers, then manufacturing is simpler, but light extraction efficiency deteriorates due to total internal reflection and Fresnel losses
Solution Approach 1:
The patent applies surface curvature by introducing roughness components with characteristic scales larger than the wavelength of light at the interface. This curvature modifies the optical path of light rays, enabling them to escape the high-index semiconductor layer by reducing total internal reflection. The rough surface creates varying normal vectors across the interface, allowing light to find escape paths that would be blocked by a smooth planar interface.
Solution Approach 2:
The patent changes the surface geometry parameter by introducing roughness with specific characteristic scales (larger than wavelength) at the interface. This parameter change transforms the optical interaction at the boundary, converting a high-reflection smooth interface into a high-transmission rough interface by modifying how light rays interact with the boundary conditions.
2Loss of energy
If roughness is introduced at the interface to reduce total internal reflection, then light extraction improves, but manufacturing complexity increases
Solution Approach 1:
The patent specifies particular parameter ranges for the roughness (characteristic scale larger than wavelength) to achieve optimal light extraction. By defining these parameters, the patent balances the complexity of creating roughness with the substantial improvement in light extraction efficiency, making the added manufacturing complexity worthwhile for the performance gain.
3Adaptability or versatility
If abrupt changes in molar fractions are used to achieve band gap control, then device functionality is achieved, but light trapping increases due to abrupt optical property changes
Solution Approach 1:
The patent applies surface curvature at the heterojunction interface to counteract the abrupt optical property changes caused by discrete molar fraction transitions. The rough surface geometry creates gradual effective optical transitions by scattering light and providing multiple escape paths, compensating for the abrupt material property changes needed for band gap control.
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 significantly enhances light extraction efficiency by increasing the amount of radiation that exits the device, compared to devices with smooth boundaries, while minimizing losses associated with total internal reflection and Fresnel reflections.
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 set of large roughness components can include a series of truncated shapes. 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.