III-Nitride LED Templates for Strain Reduction
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Solution Overview
Problem
Semiconductor light emitting devices face challenges with strain in III-nitride layers, leading to reduced external quantum efficiency and difficulty in growing thicker or higher InN composition layers due to structural mismatch and strain, which affects emission wavelength and device performance.
Innovation Solution
A template structure is used with low temperature InGaN layers and graded composition regions to expand the lattice constant and reduce strain in the light emitting layer, incorporating multiple layer stacks and thermal annealing to control threading dislocation density and surface roughness, allowing for thicker and higher InN composition layers with reduced defects.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If conventional growth templates are used, then the device structure is simple, but strain in the light emitting layer increases leading to reduced external quantum efficiency
Solution Approach 1:
The template is segmented into multiple functional layers: low temperature InGaN layers for lattice constant expansion, high temperature GaN layers for surface smoothing and dislocation filtering, and graded composition regions for strain management. This segmentation allows each layer to perform its specific function optimally, resolving the contradiction between efficiency improvement and structural complexity.
Solution Approach 2:
The low temperature InGaN layers act as intermediaries between the substrate and the light emitting layer, expanding the lattice constant to reduce strain. The high temperature GaN layers serve as intermediary layers that smooth the surface and filter dislocations, mediating between the complex template structure and the light emitting layer to improve quantum efficiency.
2Length of moving object
If thicker light emitting layers are grown, then emission wavelength can be extended, but strain increases causing structural mismatch and reduced device performance
Solution Approach 1:
The lattice constant parameter is changed by incorporating low temperature InGaN layers with higher indium composition, which expand the lattice constant to better match the thicker light emitting layer. This parameter change allows thicker layers to be grown without excessive strain, enabling wavelength extension while maintaining device performance.
Solution Approach 2:
The problem of strain in the growth direction (vertical dimension) is addressed by introducing compositional grading in the template layers. The graded composition regions provide a gradual transition in lattice constant, distributing strain across multiple dimensions and enabling thicker light emitting layers to be grown without structural mismatch.
3Length of moving object
If higher InN composition layers are grown, then emission wavelength is extended, but strain and threading dislocation density increase
Solution Approach 1:
High temperature GaN layers are grown preliminarily before the light emitting layer to smooth the surface morphology and filter threading dislocations. This preliminary action creates a cleaner template that reduces the propagation of dislocations to higher InN composition layers, enabling better manufacturing precision despite increased strain.
Solution Approach 2:
The high temperature GaN layers serve as intermediary layers between the low temperature InGaN template and the high InN composition light emitting layer. These intermediary layers filter dislocations and smooth the surface, mediating the transition and reducing threading dislocation density in the final light emitting layer even when higher InN composition is used.
4Reliability
If low temperature InGaN layers are used to expand lattice constant, then strain is reduced, but surface roughness and threading dislocations increase
Solution Approach 1:
High temperature GaN layers are introduced as intermediary layers between the low temperature InGaN layers and the light emitting layer. These intermediary layers smooth the surface roughness created by the low temperature growth and filter threading dislocations, while the low temperature InGaN layers continue to provide lattice constant expansion and strain reduction.
Solution Approach 2:
The template structure is segmented into distinct low temperature InGaN layers for lattice expansion and high temperature GaN layers for surface smoothing. This segmentation allows each layer type to perform its optimal function without compromising the other, resolving the contradiction between strain reduction and surface 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 approach reduces strain in the light emitting layer, enabling the growth of thicker layers with lower defect densities and increased InN composition, thereby improving external quantum efficiency and extending emission wavelength without spinodal decomposition.
Implementation Method 1
The template may expand the lattice constant in the light emitting layer over the range of lattice constants available from conventional growth templates
Implementation Method 2
is formed by a process including a thermal anneal or thermal cycled growth step
Data Source
Figure 1~8
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Figure 15~16
AI summary
In a Ill-nitride light emitting device, the device layers 10 including the light emitting layer are grown over a template designed to reduce strain in the device, in particular in the light emitting layer. Reducing the strain in the light emitting device may improve the performance of the device. The template may expand the lattice constant in the light emitting layer over the range of lattice constants available from conventional growth templates. Strain is defined as follows: a given layer has a bulk lattice constant at,uik corresponding to a lattice constant of a free standing material of a same composition as that layer and an in-plane lattice constant ain-plane corresponding to a lattice constant of that layer as grown in the structure. The amount of strain in a layer is | (ain-plane - abulk) I / abulk. In some embodiments, the strain in the light emitting layer is less than 1%.