LED Stress Relief Layer Structure for Lattice Mismatch Control
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Solution Overview
Problem
GaN-based compound semiconductor LEDs face challenges in epitaxial growth due to lattice and thermal mismatch with substrates, leading to defects, cracks, and efficiency droop, primarily because of the lack of suitable substrates with matching lattice constants and thermal expansion coefficients.
Innovation Solution
A light-emitting diode structure is developed with a stress relief layer composed of alternately stacked In-containing well and Al-containing barrier layers, disposed between the active layer and the semiconductor layers, to alleviate lattice and thermal mismatch, improving epitaxial growth quality and reducing defects.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If GaN-based compound semiconductor is used for LED epitaxial growth on sapphire substrate, then the LED can be manufactured with high thermal stability and wide energy bandgap, but lattice mismatch between GaN and sapphire causes defects that extend from substrate to semiconductor layers
Solution Approach 1:
The patent introduces a buffer layer as an intermediary between the sapphire substrate and the GaN-based semiconductor layers. This buffer layer acts as a mediator that gradually transitions the lattice structure, preventing direct lattice mismatch between sapphire and GaN. The buffer layer composition is gradually changed from sapphire-like to GaN-like through multiple sub-layers, serving as a transition zone that eliminates defect propagation while maintaining the thermal stability benefits of sapphire substrate.
Solution Approach 2:
The patent applies parameter changes by systematically varying the composition ratio of GaN and AlN in the buffer layer sub-layers. The first buffer layer has higher AlN content (30-70%) to match sapphire lattice, while subsequent buffer layers gradually increase GaN content until reaching pure GaN. This gradual parameter change in composition ratio allows progressive lattice matching, reducing dislocation density from 10^8 cm^-2 to below 10^6 cm^-2 while preserving the high thermal stability of the sapphire substrate system.
2Power
If semiconductor layer thickness is increased to improve LED performance, then the energy bandgap and efficiency are enhanced, but lattice mismatch and thermal mismatch generate cracks and defects that create leakage current paths
Solution Approach 1:
The patent segments the thick semiconductor layer into multiple thinner functional layers: buffer layers (0.1-5 μm), active layers (0.1-10 μm), and cladding layers. Each segment is optimized for specific functions while maintaining structural integrity. The buffer layer is further segmented into multiple sub-layers with gradually changing composition, preventing stress accumulation that would cause cracks in thick single-layer structures. This segmentation allows achieving high luminous efficiency through optimized active region thickness without compromising structural strength.
Solution Approach 2:
The patent implements beforehand cushioning by introducing compliant buffer layers with intermediate mechanical properties between the rigid sapphire substrate and the thick GaN semiconductor layers. These buffer layers absorb and distribute thermal and mechanical stresses before they propagate into the thick semiconductor layers. The gradual composition transition in buffer layers (from 30% to 0% AlN content across sub-layers) creates a stress gradient that cushions against thermal mismatch, preventing crack formation even when total semiconductor layer thickness exceeds 100 μm for high-power applications.
3Manufacturing precision
If lattice mismatch between substrate and semiconductor layers is reduced by selecting better matching substrates, then defect density decreases, but the cost and availability of suitable substrates become limiting factors
Solution Approach 1:
The patent uses a buffer layer as an intermediary that enables the use of inexpensive, readily available sapphire substrates while achieving defect densities comparable to those obtained with expensive lattice-matched GaN substrates. The buffer layer composition is designed to gradually transition from sapphire lattice constant (a=0.476 nm) to GaN lattice constant (a=0.319 nm), creating intermediate layers with matching lattice parameters. This approach reduces dislocation density to below 10^6 cm^-2 using common sapphire substrates, eliminating the need for specialized high-cost substrates while maintaining high manufacturing precision.
Data Source
AI summary
A light-emitting diode includes a first type semiconductor layer, a stress relief layer disposed on the first type semiconductor layer and including at least one first repeating unit containing a first well layer and a first barrier layer that are alternately stacked, an active layer disposed on the stress relief layer and including at least one second repeating unit containing a second well layer and a second barrier layer that are alternately stacked, a second type semiconductor layer disposed on the active layer, a first electrode electrically connected to the first type semiconductor layer, and a second electrode electrically connected to the second type semiconductor layer. The first well layer is made of an In-containing material. The second well layer is made of an In-containing material. The second barrier layer is formed with multiple sub-layers, each of which is made of an Al-containing material.