Variable-Lattice Superlattice Epitaxy for High-Voltage GaN Buffers
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Solution Overview
Problem
The lattice mismatch between silicon substrates and wide bandgap semiconductor materials like GaN leads to defects and dislocations in epitaxially grown layers, degrading the performance of high-electron-mobility transistors and light-emitting diodes, due to the difference in lattice parameters across operational temperatures.
Innovation Solution
A superlattice epitaxial structure is grown on a substrate with alternating layers of varying lattice parameters, engineering the strain to create misfit dislocations further down in the structure, allowing the upper layers to act as insulators and enabling thinner structures and increased operating voltages by varying the composition percentage of semiconductor materials like AlN and AlGaN.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Stability of the object's composition
If a superlattice buffer layer is used to overcome lattice mismatch, then structural stability is improved, but device complexity increases
Solution Approach 1:
The buffer layer is segmented into multiple alternating layers of different semiconductor materials (e.g., AlN and GaN) with varying thicknesses, creating a superlattice structure. This segmentation allows each layer to contribute differently to strain management, effectively reducing lattice mismatch while maintaining structural stability without requiring a single complex material system
Solution Approach 2:
Different regions of the superlattice buffer layer have different material compositions and thicknesses tailored to specific locations. The lower layers may have higher aluminum content to handle initial strain, while upper layers gradually transition to GaN-rich compositions. This local variation in material quality optimizes strain distribution across the structure, improving overall stability while allowing systematic design
2Ease of manufacture
If the lattice parameter difference is kept constant through the superlattice structure, then manufacturing simplicity is maintained, but threading dislocation density increases
Solution Approach 1:
The lattice parameter difference is deliberately varied through the superlattice structure by changing the composition percentages of semiconductor materials in alternating layers. For example, the aluminum nitride content may decrease from bottom to top layers, creating a gradient in lattice parameters. This parameter variation engineers strain distribution to promote misfit dislocations in lower layers, preventing their propagation to upper layers and reducing threading dislocation density in the final device layer
Solution Approach 2:
The superlattice structure introduces dynamic strain management through alternating layers with different elastic properties. As each layer is deposited, it experiences strain from underlying layers, but the alternating composition creates a dynamic balance of tensile and compressive stresses. This dynamic strain distribution allows the structure to accommodate lattice mismatch progressively, reducing dislocation formation compared to static, uniform structures
3Device complexity
If the superlattice structure is made thinner to reduce device complexity, then manufacturing is simplified, but voltage handling capability decreases
Solution Approach 1:
The superlattice buffer layer uses composite materials consisting of alternating layers of different semiconductor compounds (e.g., AlN-GaN superlattice). This composite structure combines the high breakdown voltage capability of wide-bandgap materials like AlN with the lower strain of GaN layers. The composite nature allows the thin structure to maintain high voltage handling capability through the combined electronic properties of constituent materials while keeping the overall thickness reduced for simpler manufacturing
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 approach reduces electrical resistance and current leakage, allowing for the growth of thinner superlattice structures that can support higher voltages and improve the performance of high-power electronic devices.
Implementation Method 1
by varying the difference between the lattice parameters, the strain endured at that level may be engineered
Implementation Method 2
there is a significant lattice mismatch between the two materials, which can lead to defects and dislocations in the GaN layer
Implementation Method 3
a superlattice structure is conventionally used as a buffer layer between the silicon substrate and the GaN layer
Data Source
AI summary
A superlattice epitaxial structure epitaxially grown on a substrate. The superlattice epitaxial structure includes epitaxial layers composing multiple layer sequences. Each of the multiple layer sequences includes a corresponding lower layer and a corresponding upper layer epitaxially grown on the corresponding lower layer. The lattice parameter of the epitaxial layers alternate lower and higher (or higher and lower) moving up through the superlattice epitaxial structure. The difference in lattice parameters in the neighboring lower and higher epitaxial layers may also vary moving up through the epitaxial structure. Thus, by varying the difference between the lattice parameters, the strain endured at that level may be engineered with the effect of increasing the electrical resistance seen vertically through the superlattice epitaxial structure, thus allowing the structure to be thinner and/or operate with higher voltages.