AlGaN Superlattice Buffer for GaN Crystallinity and Stress
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Solution Overview
Problem
Existing LED epitaxial growth technologies face challenges with poor GaN crystallinity, large warping stress, and reduced photoelectric conversion efficiency due to lattice and thermal mismatch between GaN and substrate materials, especially in large-size chip applications, leading to high work voltage and short service life.
Innovation Solution
The implementation of an AlGaN superlattice bottom buffer layer with a parabolic linear gradient in Al compositions and n-type impurity concentration, alternately stacked AlGaN/n-GaN layers to reduce film resistance and lattice stress, enabling improved crystallinity and photoelectric performance.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Manufacturing precision
If two-step growth method is used to grow GaN layers, then GaN crystallization quality is improved, but film resistance remains high and photoelectric conversion efficiency is reduced
Solution Approach 1:
The buffer layer is segmented into multiple thin AlGaN/n-GaN superlattice layers with alternating composition, creating a graded structure that progressively reduces dislocation density while maintaining low resistance pathways for current flow
Solution Approach 2:
The Al composition ratio is varied progressively through the superlattice structure, creating a gradient that transitions from high Al content (effective at blocking dislocations) to low Al content (effective at reducing resistance), thereby simultaneously improving crystallization quality and photoelectric efficiency
2Productivity
If large-size substrates are used to increase production output, then productivity is improved, but warping stress and lattice mismatch increase导致poor crystallinity
Solution Approach 1:
The buffer layer is divided into multiple thin superlattice layers that can better accommodate thermal and lattice mismatch on large substrates, preventing warping and maintaining crystallinity across the entire wafer surface
Solution Approach 2:
The superlattice structure combines AlGaN and n-GaN layers with different properties in a composite arrangement, creating a buffer that is more resilient to stress and mismatch effects, enabling high-quality growth on large-size substrates
3Manufacturing precision
If AlGaN superlattice buffer layer with parabolic linear gradient is implemented, then dislocation is reduced and crystallinity is improved, but device complexity increases
Solution Approach 1:
The complex gradient profile is achieved by segmenting the buffer into repeating superlattice units, where the complexity is managed at the unit level rather than requiring a continuously varying complex structure
Solution Approach 2:
The superlattice structure employs periodic alternation of AlGaN and n-GaN layers with parabolic gradient, creating a repeating pattern that achieves the desired crystallinity improvement while maintaining manufacturing simplicity through regular cycling of deposition parameters
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 effectively reduces dislocation and improves photoelectric performance and wavelength yield, particularly in large-size epitaxial wafers, enhancing the reliability and efficiency of high-power LED devices.
Implementation Method 1
a GaN nucleating layer epitaxially grown on the substrate; a superlattice buffer layer comprising a plurality of AlGaN/n-GaN superlattice layer pairs alternately stacked
Implementation Method 2
In consideration of large lattice mismatch and thermal expansivity difference between GaN and substrate
Implementation Method 3
large lattice mismatch and thermal expansivity difference between GaN and substrate
Implementation Method 4
first, grow one GaN nucleating bottom layer under low-temperature environment (400° C.-700° C.). Under low-temperature environment, a two-dimensional layered growth is impossible, but a GaN nucleating seed can be formed
Implementation Method 5
Most existing commercial LED epitaxial wafers are epitaxied through MOCVD
Data Source
AI summary
An LED epitaxial structure includes a substrate; a GaN nucleating layer; a superlattice buffer layer comprising a plurality pairs of alternately stacked AlGaN/n-GaN structures; an n-GaN layer; a MQW light-emitting layer, a p-GaN layer and a p-type contact layer. Al(n) represents Al composition value of the nth AlGaN/n-GaN superlattice buffer layer pair; N(n) represents n-type impurity concentration value of the nth AlGaN/n-GaN superlattice buffer layer pair; variation trend of Al(n) is from gradual increase to gradual decrease, and for N(n) is from gradual increase to gradual decrease. The structure can effectively and sufficiently release the lattice stress due to lattice mismatch between the sapphire substrate and GaN at the bottom layer growth section, thereby greatly reducing wrapping of the epitaxial wafer throughout high-temperature growth process, improving wavelength concentration and yield of the epitaxial wafer, improving GaN lattice quality, reducing lattice dislocation density and improving stability of photoelectric characteristics.