Thin-Film LED Light Extraction via Inverted Chip Structure
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Solution Overview
Problem
Conventional LEDs, particularly those with vertical chip structures, suffer from limited light extraction efficiency due to the absorption of reflected light by epitaxial layers, while horizontal chip structures face resistance issues with transparent conductive layers that reduce light extraction.
Innovation Solution
A thin-film LED design using a transparent substrate with metal p and n electrodes placed only in non-light-emitting areas, combined with a roughened n-type epitaxial layer surface and a reflector, enhances light extraction by minimizing absorption and increasing side light emission.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Illumination intensity
If a vertical chip structure with pyramid-shaped roughened surface is used, then light extraction efficiency is improved through surface roughening, but reflected light is absorbed by epitaxial layers reducing overall extraction efficiency
Solution Approach 1:
The patent inverts the conventional vertical chip structure to a horizontal chip structure, allowing light to exit through lateral surfaces rather than only through the top surface. This inversion enables reflected light to escape through the sides without being absorbed by the epitaxial layers, resolving the contradiction between surface roughening benefits and light absorption losses.
Solution Approach 2:
The patent transitions from a single-dimensional light extraction path (vertical through top surface) to multi-dimensional extraction paths (lateral surfaces and top surface). By utilizing the lateral dimensions for light emission, the design provides additional escape routes for reflected light that avoid absorption by the epitaxial layers.
2Illumination intensity
If a transparent conductive layer (ITO) is used as p-type conductive window, then light transmission is maintained, but resistance is higher than metal limiting current expansion under high current operation
Solution Approach 1:
The patent applies different material properties to different regions: metal electrodes are used in non-light-emitting areas for low resistance and high current carrying capacity, while transparent conductive materials are used only where light emission is required. This local differentiation resolves the contradiction between light transmission and current carrying capacity by optimizing each region for its primary function.
3Reliability
If a metal electrode is required in the light-emitting area to reduce resistance, then current expansion is improved, but light extraction efficiency is reduced by blocking the light
Solution Approach 1:
The patent segments the electrode placement into distinct regions: metal electrodes are confined to non-light-emitting areas where they provide low resistance paths without blocking light, while light-emitting areas remain free of metal electrodes to maintain high light extraction efficiency. This spatial segmentation resolves the contradiction between current expansion capability and light extraction efficiency.
Solution Approach 2:
The patent extracts the metal electrode function from the light-emitting area and relocates it to non-light-emitting areas. By separating the electrical conduction function (handled by metal electrodes in non-emitting regions) from the light emission function (handled by transparent regions), the design eliminates the conflict between low resistance and high light transmission.
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
This design significantly improves light extraction efficiency by reducing absorption and increasing lateral light emission, while maintaining low resistance for high current operations.
Implementation Method 1
the nitrogen-polar n-GaN top surface is roughened using a wet etch to create micro-pyramid structures in the surface, thus greatly improving the light extraction efficiency
Implementation Method 2
a reflector is provided between the light-emitting material layer and the permanent substrate to facilitate light extraction at the top side by reflecting the downward emissive light
Data Source
AI summary
An n-type layer, an active layer, and a p-type layer are grown on a growth substrate. Portions of the p-type layer and active layer are etched away to expose the n-type layer, and an n-electrode is formed over the exposed portions of the n-type layer. A first dielectric layer is formed over the n-electrodes. A transparent conductor layer is formed over the p-type layer and the first dielectric layer. A p-electrode is formed over the transparent conductor layer. A transparent bonding layer is deposited over the transparent conductor layer and the p-electrode. A transparent support substrate is bonded to the p-type layer via the bonding layer. The growth substrate is then removed to expose the n-type layer, and the layers are etched to expose the n and p electrodes for connection to a power source. A reflector layer is formed on the bottom surface of the substrate.