Compound Semiconductor Substrate for Low-Dislocation AlGaN Growth
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Solution Overview
Problem
Conventional ultraviolet LEDs using AlGaN suffer from low light emission efficiency (external quantum efficiency below 10%) due to high threading dislocation densities, which also affect the internal quantum efficiency, electron/hole injection efficiency, and light extraction efficiency.
Innovation Solution
A compound semiconductor substrate is designed with a base layer, a stress relaxation layer that relaxes strain from the base layer, and a functional layer with a dominant lattice relaxation region, achieving a threading dislocation density lower than 2.0×10^9 cm^-2.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Manufacturing precision
If epitaxial growth of GaN is performed on an AlN substrate, then the in-plane lattice constant mismatch causes compressive strain in the GaN epitaxial layer, but when the thickness is increased to achieve necessary semiconductor element performance, strain energy accumulates leading to lattice relaxation, misfit dislocation, and surface roughening
Solution Approach 1:
The patent segments the epitaxial growth process into multiple stages with different Al composition ratios. The AlGaN layer is grown with gradually decreasing Al composition (from 0.7 to 0.0) in multiple steps, allowing strain to be managed incrementally rather than accumulated in a single thick layer. This segmentation enables the formation of a thick functional layer while maintaining crystal quality and preventing surface roughening.
2Illumination intensity
If the Al composition ratio in AlGaN is increased to achieve deeper ultraviolet light emission, then the band gap increases and light emission wavelength decreases, but the lattice mismatch with AlN substrate increases causing higher threading dislocation density
Solution Approach 1:
The patent applies local quality by having different Al composition ratios in different regions of the AlGaN layer. The lower AlGaN layer has higher Al composition (0.5-0.7) for deep UV emission, while the upper AlGaN layer has lower Al composition (0.0-0.3) to reduce lattice mismatch. This spatial variation in composition allows the structure to simultaneously achieve deep UV light emission and low threading dislocation density.
3Productivity
If a thick AlGaN functional layer is formed to achieve necessary device thickness, then the device performance improves, but lattice relaxation occurs leading to increased threading dislocation density and reduced light emission efficiency
Solution Approach 1:
The patent changes the Al composition ratio parameter through the thickness of the AlGaN layer. By gradually decreasing the Al composition from 0.7 at the bottom to 0.0 at the top in multiple growth steps, the structure maintains a total thickness sufficient for device performance (over 3 μm) while preventing lattice relaxation. The parameter change in Al composition allows the thick layer to remain strain-free and maintain high light emission efficiency.
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
The proposed substrate configuration improves the device characteristics by reducing threading dislocation density, enhancing crystal quality, and securing in-plane uniformity of physical properties such as sheet resistance.
Implementation Method 1
a stress relaxation layer that relaxes strain received from the base layer
Implementation Method 2
crystal growth for laminating the same material as or a dissimilar material from a substrate on the substrate is necessary. The crystal growth of a dissimilar material is called epitaxial growth
Data Source
AI summary
This compound semiconductor substrate has a base layer having an in-plane lattice constant of a, a stress relaxation layer that relaxes strain that is received from the base layer and a functional layer having an in-plane lattice constant of b (a≠b), the base layer, the stress relaxation layer and the functional layer are disposed in order of the base layer, the stress relaxation layer and the functional layer, in the functional layer, a region where a lattice is relaxed from a crystal lattice of the base layer is dominant, and the threading dislocation density of the functional layer is lower than 2.0×109 cm−2.