Semi-Polar InGaN Substrates for Strain Relaxation
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Solution Overview
Problem
Current visible-spectrum light-emitting diodes (LEDs) and laser diodes in the ultraviolet to green regime face challenges with high built-in stress in InGaN active layers, leading to reduced radiative efficiency and material breakdown due to phase separation, especially when grown on c-plane substrates, and semi-polar orientations do not fully address strain-related issues.
Innovation Solution
The method involves activating (0001)/1/3 slip planes in GaN substrates using semi-polar oriented material and controlled stress at heterointerfaces to form a relaxed InGaN layer, which serves as a seed for further growth, reducing dislocation density through misfit dislocation generation and annihilation, allowing for the formation of a bi-axially relaxed epitaxial layer suitable for LED and LD fabrication.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Adaptability or versatility
If InGaN active layers are grown with increased InN mole fraction to achieve longer wavelength devices, then the wavelength range is extended, but built-in stress increases leading to phase separation and material breakdown
Solution Approach 1:
The patent changes the crystal orientation parameter from c-plane to semi-polar planes, which fundamentally alters the stress state and polarization fields in the InGaN layers. This parameter change enables higher InN mole fractions to be achieved without phase separation, thus extending wavelength range while maintaining material stability
Solution Approach 2:
The patent employs composite heterostructure design with multiple layers including InGaN active layers, GaN barrier layers, and AlGaN electron blocking layers. This composite structure manages stress distribution and prevents phase separation, allowing reliable operation at extended wavelength ranges
2Manufacturing precision
If InGaN layers are grown thicker to reduce dislocation density, then material quality improves, but built-in stress increases leading to phase separation
Solution Approach 1:
Changing to semi-polar orientation modifies the critical thickness for phase separation, allowing much thicker InGaN layers to be grown before reaching the phase separation limit. This enables dislocation density reduction through thickness while maintaining material stability
Solution Approach 2:
The patent utilizes the third dimension (layer thickness) more effectively by growing thicker InGaN layers on semi-polar planes. The altered stress state in semi-polar orientation allows thickness to be increased for dislocation reduction without triggering phase separation as occurs in c-plane structures
3Manufacturing precision
If c-plane substrates are used for InGaN growth, then pseudomorphic growth is achieved, but built-in electric fields increase reducing electron-hole overlap and radiative efficiency
Solution Approach 1:
The patent changes the substrate orientation parameter from c-plane to semi-polar planes, which eliminates or reduces spontaneous and piezoelectric polarization fields. This parameter change directly improves radiative efficiency by enhancing electron-hole overlap while maintaining good epitaxial growth control
Solution Approach 2:
The patent converts the previously harmful polarization fields in c-plane structures into a beneficial neutral state by using semi-polar orientation. The same crystal growth mechanisms that caused polarization issues in c-plane are transformed into advantages in semi-polar structures, eliminating efficiency loss
4Loss of energy
If semi-polar orientations are used to reduce polarization fields, then radiative efficiency improves, but misfit dislocations form at heterointerfaces acting as non-radiative recombination centers
Solution Approach 1:
The patent optimizes composition parameters of barrier and electron blocking layers to manage misfit dislocation formation. By carefully controlling Al and In mole fractions, the stress at heterointerfaces is managed to reduce misfit dislocation density, thereby improving device longevity while maintaining the radiative efficiency benefits of semi-polar orientation
Solution Approach 2:
The patent introduces GaN and AlGaN barrier layers as intermediary structures between the InGaN active layer and the semi-polar substrate. These intermediary layers act as dislocation filters and stress management layers, reducing the impact of misfit dislocations on device longevity while preserving the radiative efficiency improvements
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 significantly reduced dislocation density, enhancing the material quality and radiative efficiency of InGaN layers, enabling the production of high-performance LEDs and LDs across a broader wavelength range with improved long-term operation and efficiency.
Implementation Method 1
forming at least one epitaxial AlxInyGa(1-x-y)N layer having a thickness of at least 100 nanometers formed overlying at least a portion of the semipolar surface orientation such that a plurality of misfit dislocations are included in one or more portions of the thickness to reduce a bi-axial strain in the thickness to a relaxed state
Data Source
AI summary
A relaxed epitaxial AlxInyGa(1-x-y)N layer on a substrate having a semipolar surface orientation includes a plurality of misfit dislocations in portions of the thickness of the epitaxial layer to reduce bi-axial strain to a relaxed state.