GaN LED N-Layer Conductive Intervening Sublayers
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Solution Overview
Problem
The challenge of growing high-quality GaN epitaxial layers on silicon substrates is hindered by lattice constant mismatch, thermal expansion differences, and defects, making it difficult to fabricate cost-effective blue LEDs, as GaN-based LEDs are typically grown on expensive substrates like sapphire.
Innovation Solution
The approach involves epitaxially growing a Low Resistance Layer (LRL) with a superlattice structure on a silicon substrate, using a ZnS buffer layer, and incorporating conductive AlGaN:Si intervening layers to reduce lattice defects and enhance current spreading, along with a novel wafer bonding process using a eutectic metal layer and reduced platinum usage.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Ease of manufacture
If GaN is epitaxially grown on a silicon substrate, then manufacturing cost is reduced, but lattice defects increase due to lattice constant mismatch
Solution Approach 1:
A multi-layer buffer structure comprising AlN, GaN, and AlGaN layers is introduced between the silicon substrate and the active GaN layer. This intermediary buffer structure gradually transitions the lattice constant from silicon to GaN, reducing dislocation density and improving crystal quality while enabling cost-effective silicon substrate usage.
Solution Approach 2:
The buffer structure uses a composite of different nitride materials (AlN, GaN, AlGaN) with varying lattice constants and aluminum compositions. This composite approach allows optimization of both mechanical stress management and lattice matching, achieving low defect density in the final GaN layer.
2Reliability
If the GaN layer is grown thick enough, then device performance improves, but stress-induced cracking occurs
Solution Approach 1:
The aluminum composition in the AlGaN buffer layers is varied (e.g., 15%, 30%, 45% Al) to create a composition gradient. This parameter change allows progressive stress management through the buffer structure, enabling the growth of thick, high-performance GaN layers without stress-induced cracking.
Solution Approach 2:
The buffer structure is divided into multiple discrete layers with different thicknesses and compositions rather than a single continuous layer. This segmentation allows each layer to independently manage stress, preventing crack propagation through the entire structure while supporting thick active layers.
3Reliability
If multiple periods of platinum and titanium-tungsten are used as barrier metal layers, then tin penetration is prevented, but manufacturing cost increases
Solution Approach 1:
The complex multi-period platinum and titanium-tungsten barrier structure is replaced with a simplified single-layer titanium barrier. This extraction of unnecessary layers maintains the essential function of preventing tin penetration while dramatically reducing manufacturing complexity and cost.
Solution Approach 2:
The barrier metal design transitions from a multi-layer structure with varying thicknesses and compositions to a single titanium layer with optimized thickness (50-200 nm). This parameter change simplifies the manufacturing process while maintaining effective tin barrier performance through proper thickness 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
This method enables the production of high-quality blue LEDs on silicon substrates with reduced lattice defects and improved current spreading, while lowering manufacturing costs by utilizing inexpensive silicon substrates and minimizing platinum usage.
Implementation Method 1
growing a first gallium-nitride layer over the zinc-sulfide buffer layer reduces a density of lattice defects in the first gallium-nitride layer
Implementation Method 2
wafer bonding a carrier wafer structure to a device wafer structure by melting a layer of eutectic metal
Implementation Method 3
the aluminum-gallium-nitride sublayers provide a compressive strain to the gallium-nitride sublayers and prevent cracking
Data Source
AI summary
A vertical GaN-based blue LED has an n-type layer comprising multiple conductive intervening layers. The n-type layer contains a plurality of periods. Each period of the n-type layer includes a gallium-nitride (GaN) sublayer and a thin conductive aluminum-gallium-nitride (AlGaN:Si) intervening sublayer. In one example, each GaN sublayer has a thickness substantially more than 100 nm and less than 1000 nm, and each AlGaN:Si intervening sublayer has a thickness less than 25 nm. The entire n-type layer is at least 2000 nm thick. The AlGaN:Si intervening layer provides compressive strain to the GaN sublayer thereby preventing cracking. After the epitaxial layers of the LED are formed, a conductive carrier is wafer bonded to the structure. The silicon substrate is then removed. Electrodes are added and the structure is singulated to form a finished LED device. Because the AlGaN:Si sublayers are conductive, the entire n-type layer can remain as part of the finished LED device.