Transparent Ohmic Contacts for LED Light Extraction
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Solution Overview
Problem
Light emitting diodes (LEDs) face inefficiencies in external quantum efficiency due to internal reflection and absorption of photons within the diode structure, particularly when using carrier substrates after removing the growth substrate, which affects the output and brightness of Group III nitride LEDs.
Innovation Solution
The implementation of a light emitting diode structure with a conductive carrier substrate, transparent ohmic contacts, and a lenticular surface to minimize Fresnel loss and enhance light extraction, along with a reflective layer and passivation layer to reduce absorption, is used to increase the external quantum efficiency.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If a carrier substrate is added after removing the growth substrate, then the mechanical support and electrical contact are improved, but internal reflection and absorption of photons increase, reducing external quantum efficiency
Solution Approach 1:
A transparent ohmic contact layer is introduced as an intermediary between the carrier substrate and the active region. This transparent contact allows electrical current to flow while simultaneously permitting photons to pass through with minimal absorption and reflection, thus maintaining both electrical functionality and optical efficiency
Solution Approach 2:
The patent employs transparent contacts with optical properties matched to the LED's emission wavelength. By selecting materials whose transmission characteristics align with the operating wavelength, the contact becomes effectively 'transparent' to the emitted light, minimizing optical losses while maintaining electrical conductivity
2Power
If traditional metal contacts are used, then electrical conductivity is improved, but light absorption and Fresnel loss increase, reducing photon output
Solution Approach 1:
The patent uses transparent conductive materials whose optical transmission is optimized for the LED's emission wavelength. These materials appear transparent at the operating wavelength while maintaining sufficient electrical conductivity, thereby allowing maximum photon output without sacrificing electrical performance
Solution Approach 2:
The transparent ohmic contact is formed as a composite structure combining transparent conductive oxide layers with metallic layers. This composite provides both the electrical conductivity of metals and the optical transparency of dielectric materials, resolving the contradiction between electrical and optical requirements
3Adaptability or versatility
If the growth substrate is removed and carrier substrate is added, then device flexibility and application versatility are improved, but internal reflection at interfaces increases, reducing light extraction efficiency
Solution Approach 1:
The transparent ohmic contact serves as an optical intermediary layer between the carrier substrate and the active region. This intermediate layer has refractive index properties that reduce Fresnel reflection at the interface, allowing more photons to escape while the carrier substrate provides the necessary mechanical support
Solution Approach 2:
The patent optimizes the refractive index, thickness, and optical constants of the transparent contact layer to minimize reflection losses. By carefully controlling these parameters, the interface between the carrier substrate and active region becomes more transparent to emitted light, improving light extraction efficiency
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 configuration enhances the external quantum efficiency and brightness of LEDs by reducing internal losses and improving light extraction, leading to increased photon output and improved diode performance.
Implementation Method 1
minimize Fresnel loss and enhance light extraction
Implementation Method 2
lenticular surface to minimize Fresnel loss and enhance light extraction
Implementation Method 3
reflective layer and passivation layer to reduce absorption
Data Source
Figure 1~2
Figure 3~4
AI summary
A light emitting diode is disclosed that includes an active structure formed of at least p-type and n-type epitaxial layers of Group III nitride on a conductive carrier substrate. A conductive bonding system joins the active structure to the conductive carrier substrate. A first transparent ohmic contact is on the active structure adjacent the conductive carrier substrate, a second transparent ohmic contact is on the active structure opposite the conductive carrier substrate, and a third ohmic contact is on the conductive carrier substrate opposite from the active structure.