Single Crystal ZnO Current Spreading Layer for LED Light Extraction
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Solution Overview
Problem
Current light emitting diode (LED) devices based on Group III-Nitride semiconductors face challenges with low electrical conductivity in p-type layers, leading to inefficient light generation and high current injection, and existing current-spreading layers like ITO have defects that reduce efficiency due to high optical absorption.
Innovation Solution
The use of a zinc oxide (ZnO) based LED device with a single crystal ZnO structure, including a bottom and top single crystal ZnO portion formed through thermal hydro synthesis, which can include voids to enhance light extraction and electrical performance, and a ZnO seed layer and bulk layer configuration to improve conductivity and optical properties.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If ITO is used as current-spreading layer, then electrical conductivity is improved, but optical absorption increases due to defects in ITO films
Solution Approach 1:
The patent changes the material parameter from ITO to ZnO, which has fundamentally different optical and electrical properties. ZnO exhibits lower optical absorption in the visible range while maintaining high electrical conductivity, thus resolving the contradiction between electrical performance and optical transparency.
Solution Approach 2:
The patent employs a low-cost aqueous deposition method to produce ZnO films, replacing expensive vacuum deposition techniques. This approach achieves comparable or superior electrical conductivity without the defect-related optical absorption problems of ITO, providing a cost-effective solution.
2Manufacturing precision
If vapor phase methods are used to produce ZnO films, then film quality is improved, but manufacturing cost and complexity increase
Solution Approach 1:
The patent replaces complex vacuum-based vapor phase deposition equipment with simple aqueous solution deposition equipment. The process uses liquid-phase chemistry at atmospheric pressure and low temperature, eliminating the need for expensive vacuum chambers and high-temperature furnaces while producing high-quality epitaxial ZnO films.
Solution Approach 2:
The patent changes the deposition parameters from high-temperature vapor phase to low-temperature liquid phase. This parameter change enables the use of simple equipment and atmospheric pressure processing while maintaining film quality through controlled chemical reactions in aqueous solutions.
3Illumination intensity
If p-type GaN layers are used in LED devices, then light generation is improved, but electrical conductivity decreases leading to current crowding
Solution Approach 1:
The patent introduces ZnO as an intermediary layer between the p-type GaN active region and the electrical contact. This ZnO current-spreading layer has high electrical conductivity that compensates for the low conductivity of p-GaN, distributing current uniformly across the active region while maintaining efficient light generation.
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 ZnO based LED device exhibits improved light extraction efficiency, reduced optical absorption, and enhanced electrical performance compared to traditional ITO-based devices, while using a cost-effective low-temperature aqueous deposition method.
Implementation Method 1
a bottom single crystal ZnO portion and a top single crystal ZnO portion, both of which can be grown by known low temperature aqueous solution methods such as thermal hydro-synthesis (THS)
Implementation Method 2
the large amount of defects in ITO films lends to high optical absorption in the visible range, which is generally reported to be in the 650-2000 cm-1 range
Data Source
AI summary
LED devices having high-quality single crystal ZnO structures for spreading currents and extracting light out of the LEDs are disclosed. In one aspect, a LED device is provided to include a substrate; a first semiconductor layer exhibiting a first conductivity type and formed over the substrate; an active light-emitting structure formed over the first semiconductor layer, the active light-emitting structure operable to emit light under electrical excitation; a second semiconductor layer exhibiting a second conductivity type and formed over the active light-emitting structure; and a single crystal ZnO structure formed over the second semiconductor layer and including a bottom single crystal ZnO portion over the second semiconductor layer and a top single crystal ZnO portion extending from the bottom single crystal ZnO portion, wherein the bottom single crystal ZnO portion is a contiguous single crystal ZnO portion without having voids.