Universal Lateral Overgrowth for Low Dislocation GaN
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Solution Overview
Problem
The development of high-quality GaN substrates with low dislocation density is hindered by lattice and thermal mismatch issues with existing substrates, leading to high threading dislocation densities and cracking, which negatively impact the performance and longevity of nitride semiconductor devices.
Innovation Solution
The Universal Lateral Overgrowth (ULO) process controls growth conditions such as pressure, temperature, and surfactant addition to induce 90° bending of threading dislocations, creating dislocation loops and reducing defect density without the need for masks or external processing, applicable in various epitaxial growth technologies like MOVPE and HVPE.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Manufacturing precision
If heteroepitaxial deposition is used to grow GaN on sapphire, silicon, or SiC substrates, then GaN layers can be fabricated with reasonable thickness, but high threading dislocation densities (～10^9 cm^-2) are generated due to lattice and thermal mismatch
Solution Approach 1:
The invention segments the GaN layer growth into two distinct stages: first growing a buffer layer with relaxed strain to reduce dislocation density, then growing the device layer on top of this buffer. This segmentation allows each layer to serve a specific function - the buffer absorbs mismatch stress while the device layer achieves low defect density
Solution Approach 2:
The invention changes the compositional parameter of the buffer layer by incorporating aluminum (AlN or AlGaN) to create a gradient structure that gradually transitions from the substrate lattice constant to the GaN lattice constant. This parameter change enables strain relaxation and reduces threading dislocation propagation
2Reliability
If thick GaN layers are grown to compensate for dislocation filtering, then dislocation density may be reduced, but cracking and warping occur due to accumulated strain
Solution Approach 1:
The invention applies local quality by creating a buffer layer with specific AlN or AlGaN composition at the interface region where strain accumulation occurs, while the upper device layer maintains pure GaN composition. This localized compositional adjustment allows thick layers to be grown without cracking, as the buffer layer locally absorbs the strain
3Manufacturing precision
If buffer layers are used to relieve lattice strain, then single-crystalline epitaxial growth is enabled, but numerous defects remain due to uncompensated lattice mismatch
Solution Approach 1:
The invention uses composite materials by combining AlN or AlGaN buffer layer with GaN device layer. The AlN/AlGaN-GaN composite structure leverages the different lattice constants of the materials to create a gradual transition zone that reduces dislocation density while maintaining crystal quality
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 achieves GaN substrates with dislocation densities below 10^6 cm^-2, enabling the production of thick, crack-free, high-quality layers suitable for pseudo-substrates and homoepitaxy, improving the performance and reliability of nitride semiconductor devices.
Implementation Method 1
process for epitaxial growth where the lateral and vertical growth rates of the material are controlled only by growth conditions
Data Source
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AI summary
High quality free standing GaN is obtained using a new modification of the Epitaxial Lateral Overgrowth technology in which 3D islands or features are created only by tuning the growth parameters. Smoothing these islands (2D growth) is achieved thereafter by setting growth conditions producing enhanced lateral growth. The repetition of 3D-2D growth results in multiple bending of the threading dislocations thus producing thick layers or free standing GaN with threading dislocation density below 106cm-2.