Superlattice Composite Buffer Structure for GaN Stress Relief
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Solution Overview
Problem
GaN epitaxy grown on heterogeneous substrates experiences high-density dislocations and mismatch stress due to lattice constant and thermal expansion coefficient mismatches, leading to crystal defects and wafer warping.
Innovation Solution
A superlattice composite structure is developed, comprising a first and second superlattice stack layer with alternating AlN, AlGaN, and GaN layers, where the thickness of each unit varies gradually, optimizing stress release and reducing crack line generation.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If GaN epitaxy is grown on heterogeneous substrates, then high-frequency transmission capability and high-voltage transistor element performance are achieved, but high-density dislocations and mismatch stress are generated due to lattice constant and thermal expansion coefficient mismatches
Solution Approach 1:
The patent divides the buffer layer into multiple segments: a first buffer layer directly on the substrate, a second buffer layer with different composition ratios, and a superlattice buffer layer with alternating high-Al and low-Al AlGaN layers. This segmentation allows each layer to address specific stress and dislocation issues, reducing overall mismatch stress while maintaining device performance.
Solution Approach 2:
The patent changes the composition ratio parameter of AlGaN layers across different buffer layers. The first buffer layer has AlGaN with a first composition ratio, the second buffer layer has AlGaN with a second composition ratio, and the superlattice contains layers with varying Al content. This parameter variation enables gradual lattice matching and stress management, reducing dislocation density while preserving the high-frequency and high-voltage performance of GaN devices.
2Reliability
If GaN epitaxy is grown on heterogeneous substrates, then wide energy gap and high critical electric field characteristics are obtained, but crystal defects such as dislocations and cracks are generated
Solution Approach 1:
The patent implements preliminary stress management by designing a multi-layer buffer structure before growing the active GaN device layers. The first buffer layer provides initial lattice matching, the second buffer layer adjusts composition to reduce stress, and the superlattice buffer layer further refines stress distribution. This preliminary action prevents crystal defects from forming during subsequent epitaxial growth, ensuring high crystal quality while maintaining the desired electrical characteristics.
Solution Approach 2:
The patent uses composite material structures combining multiple AlGaN layers with different composition ratios and a superlattice of alternating high-Al and low-Al layers. This composite buffer structure effectively manages lattice mismatch and thermal expansion differences between the heterogeneous substrate and GaN epitaxy, reducing dislocation and crack formation while preserving the wide energy gap and high critical electric field properties necessary for high-performance semiconductor devices.
3Reliability
If GaN epitaxy is grown on heterogeneous substrates, then high thermal conductivity and high saturation electron velocity are achieved, but wafer warping is generated due to large mismatch stress
Solution Approach 1:
The patent applies local quality by creating regions with different AlGaN composition ratios at specific locations within the buffer structure. The first buffer layer has a composition optimized for substrate bonding, the second buffer layer has a different composition to manage stress, and the superlattice contains localized high-Al and low-Al layers. This local variation in material properties allows differential stress management across the wafer, maintaining flatness while preserving the high thermal conductivity and electron velocity properties of GaN.
Data Source
AI summary
A superlattice composite structure includes a first superlattice stack layer and a second superlattice stack layer. The first superlattice stack layer includes a plurality of first units stacked along a vertical direction. Each of the first units includes an aluminium nitride (AlN) layer, an aluminium gallium nitride (AlGaN) layer and a gallium nitride (GaN) layer stacked in sequence along the vertical direction. The second superlattice stack layer is stacked with the first superlattice stack layer along the vertical direction. The second superlattice stack layer includes a plurality of second units stacked along the vertical direction. Each of the second units includes another AlN layer, another AlGaN layer and another GaN layer stacked in sequence along the vertical direction.