Variable Period Superlattice Strain Relief for UV LEDs
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Solution Overview
Problem
High dislocation densities and mechanical stresses in AlGaN or AlN template layers on sapphire substrates lead to poor crystal quality and reduced external quantum efficiencies in deep UV light emitting diodes, necessitating improved strain relief and Al content management for UV light emitting devices.
Innovation Solution
A variable period superlattice strain relief structure is introduced, comprising alternating layers of AlN and GaN with varying thicknesses to gradually match the Al content from the template layer to the high-Al content MQWH active region, reducing strain-induced cracking and defect density.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Temperature
If AlGaN or AlN template layers are used on sapphire substrates to enable deep UV emission, then the emission wavelength can be reduced to UV range, but high dislocation densities and mechanical stresses occur leading to poor crystal quality
Solution Approach 1:
A GaN interlayer is introduced between the AlGaN template layer and the high-Al content active region. This intermediary layer acts as a buffer that reduces the lattice mismatch and mechanical stress between the template and the high-Al content layers, thereby reducing dislocation density and improving crystal quality while enabling deep UV emission
Solution Approach 2:
The aluminium content in the template layer is optimized to balance two competing requirements: sufficiently high Al content to achieve deep UV emission wavelength, and sufficiently low Al content to minimize lattice mismatch with the GaN interlayer and reduce dislocation density. This parameter optimization resolves the contradiction between emission wavelength and crystal quality
2Temperature
If the Al content of the MQWH active region is increased to obtain shorter wavelength UV emission, then the emission wavelength is reduced, but mechanical stresses lead to cracks in the heterostructure
Solution Approach 1:
The GaN interlayer serves as a mediator between the AlGaN template and the high-Al content MQWH active region. It gradually transitions the lattice constant and reduces mechanical stress accumulation, enabling the heterostructure to tolerate high Al content (up to 70-80%) in the active region without developing cracks
Solution Approach 2:
The heterostructure is segmented into distinct layers with graded Al content: the AlGaN template layer with moderate Al content, the GaN interlayer with zero Al content, and the high-Al content MQWH active region. This segmentation allows each layer to be optimized independently, preventing stress-induced cracking while achieving short wavelength UV emission
3Ease of manufacture
If Al 2 O 3 (sapphire) substrate is used due to its numerous advantages, then ease of manufacture is improved, but high lattice mismatch with structural layers significantly affects optical output and device lifetime
Solution Approach 1:
The AlGaN template layer and GaN interlayer act as intermediary layers between the sapphire substrate and the high-Al content structural layers. These intermediaries gradually transition the lattice constant from the sapphire substrate to the high-Al content layers, reducing dislocation density and improving device lifetime while maintaining the ease of manufacture provided by sapphire substrates
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 enhances the optical output and reduces defect density, resulting in improved performance and lifespan of UV light emitting devices by providing a more gradual transition in Al content and better lattice matching, leading to brighter optical output and a crack-free surface.
Implementation Method 1
A variable period superlattice strain relief structure is introduced, comprising alternating layers of AlN and GaN with varying thicknesses to gradually match the Al content from the template layer to the high-Al content MQWH active region, reducing strain-induced cracking and defect density
Implementation Method 2
The dislocation densities in AlGaN or AlN template layers on sapphire are typically in the mid 10^9... A variable period superlattice strain relief structure is introduced... reducing strain-induced cracking and defect density
Implementation Method 3
In the III-V compound semiconductor family, the nitrides have been used to fabricate visible wavelength light emitting device active regions. They also exhibit a sufficiently high bandgap to produce devices capable of emitting light in the ultraviolet
Data Source
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AI summary
An optical semiconductor device such as a light emitting diode (60) is formed on a transparent substrate (62) having formed thereon a template layer (64), such as AIN, which is transparent to the wavelength of emission of the optical device. A variable period variable composition superlattice strain relief region (68,70) is provided over the template layer such that the composition of the strain relief region approaches or matches the composition of the regions contiguous thereto. For example, the Al content of the strain relief region may be tailored to provide a stepped or gradual Aluminum content from template (64) to active layer (72). Strain-induced cracking and defect density are reduced or eliminated.