AlGaN Quantum Wells for UV Light Extraction
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Solution Overview
Problem
Current ultraviolet light-emitting diodes (UVLEDs) face challenges in achieving high extraction efficiency due to the optical polarization anisotropy of AlGaN quantum wells, which leads to trapped light within the device, as the polarization of light emission is not optimally aligned with the light escape cones, resulting in reduced efficiency.
Innovation Solution
The design involves AlGaN quantum wells with a high Al composition (>0.3), compressive strain (>70% to AlN), and thin thickness (<2.5 nm) to preferentially polarize light parallel to the quantum well plane, along with a multiplicity of quantum wells to reduce carrier density and enhance light extraction efficiency.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Illumination intensity
If high Al composition (>0.6) is used in AlGaN quantum wells to achieve desired emission wavelength, then the emission wavelength is improved, but the optical polarization shifts to perpendicular orientation causing light trapping and reduced extraction efficiency
Solution Approach 1:
The patent applies local quality by creating quantum wells with non-uniform Al composition profiles, including gradient compositions and localized high-Al regions, to achieve wavelength tuning while maintaining favorable parallel polarization in specific zones where light extraction occurs
Solution Approach 2:
The patent uses composite AlGaN quantum well structures combining multiple Al composition regions (gradient wells, dual-barrier wells with different Al contents) within a single active region to simultaneously achieve desired emission wavelength and optimized optical polarization for light extraction
2Illumination intensity
If Al composition is increased to achieve shorter wavelength emission, then the emission wavelength is improved, but the degree of perpendicular polarization increases causing light to be trapped within the LED layers
Solution Approach 1:
The patent changes the Al composition parameter spatially within the quantum well structure, using gradients and localized variations to decouple the relationship between wavelength determination and polarization orientation, allowing independent optimization of both parameters
3Ease of operation
If compressive strain is increased to modify valence band structure, then the optical polarization can be controlled, but the Al composition required for polarization switching increases beyond 0.80
Solution Approach 1:
The patent applies local quality by introducing localized strain through dislocation structures, piezoelectric layers, or compositional gradients in specific regions of the quantum well to achieve polarization control without requiring extreme Al compositions throughout the entire structure
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 results in a significant increase in light extraction efficiency by aligning the polarization of emitted light with the efficient light escape cones, improving the overall performance of UVLEDs.
Implementation Method 1
an ultraviolet light-emitting diode, comprising a p-type structure comprised of AlGaN layers and an n-type structure comprised of AlGaN layers to form a p-n junction, and a multiplicity of quantum wells disposed between the p- and n-type structures
Implementation Method 2
the quantum well layers are sufficiently compressively strained, the quantum well layers are sufficiently thin, and the carrier density is sufficiently low such that the multiple quantum wells emit light with a degree of polarization that is sufficiently high so that it is preferentially polarized parallel to the plane of the quantum well layers
Data Source
AI summary
Ultraviolet light-emitting diodes with tailored AlGaN quantum wells can achieve high extraction efficiency. For efficient bottom light extraction, parallel polarized light is preferred, because it propagates predominately perpendicular to the QW plane and into the typical and more efficient light escape cones. This is favored over perpendicular polarized light that propagates along the QW plane which requires multiple, lossy bounces before extraction. The thickness and carrier density of AlGaN QW layers have a strong influence on the valence subband structure, and the resulting optical polarization and light extraction of ultraviolet light-emitting diodes. At Al>0.3, thinner QW layers (<2.5 nm are preferred) result in light preferentially polarized parallel to the QW plane. Also, active regions consisting of six or more QWs, to reduce carrier density, and with thin barriers, to efficiently inject carriers in all the QWs, are preferred.