Strained Buffer Layers for GaN-on-Silicon LED Cracking

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Solution Overview

Problem

The formation of gallium nitride (GaN) thin films on silicon substrates faces challenges due to lattice mismatch and coefficient of thermal expansion mismatch, leading to structural stresses, cracking, and poor device performance, which limits the lifetime and efficiency of light emitting diodes (LEDs).

Innovation Solution

The use of a buffer layer with strained aluminum nitride (AlN), aluminum gallium nitride (AlGaN), and gallium nitride (GaN) layers, where processing conditions are selected to generate defects and strain, balancing tensile and compressive stresses, thereby minimizing bowing and cracking issues. This approach includes forming layers with a predetermined defect density to maintain strain at both growth and room temperatures, ensuring minimal net strain in the light emitting device.

Engineering Contradictions & Design Principles

VSEngineering Contradiction Analysis

1Ease of manufacture

If GaN thin films are formed on silicon substrates, then LED device complexity is reduced and manufacturing cost decreases, but lattice mismatch and thermal expansion mismatch cause structural stresses, cracking, and poor device performance

Engineering Contradiction:

Improvemanufacturing costVSAvoiddevice performance

Core Design Contradiction:

Ease of manufactureVSReliability

Solution Approach 1:

A buffer layer comprising AlN, AlGaN, and GaN layers is introduced as an intermediary between the silicon substrate and the light emitting stack. This buffer layer mediates the lattice mismatch and thermal expansion mismatch between silicon and GaN, reducing structural stresses and cracking while enabling cost-effective silicon substrate usage.

Inventive Principle:

Principle #24Intermediary (Mediator)

Solution Approach 2:

The buffer layer composition is gradually changed from high aluminum content (AlN) at the silicon interface to gallium-rich layers (GaN) adjacent to the light emitting stack. This compositional gradient progressively adapts the lattice structure, reducing mismatch stresses while maintaining device performance.

Inventive Principle:

Principle #35Parameter changes

2Reliability

If buffer layers are added to reduce strain and cracking, then device reliability improves, but device complexity and manufacturing process complexity increase

Engineering Contradiction:

Improvedevice lifetimeVSAvoidbuffer layer structure

Core Design Contradiction:

ReliabilityVSDevice complexity

Solution Approach 1:

The buffer layer is segmented into multiple functional sub-layers: AlN layer for initial strain compensation at the silicon interface, AlGaN layer for gradual lattice transition, and GaN layer for matching the light emitting stack. Each segment addresses specific aspects of the mismatch problem.

Inventive Principle:

Principle #1Segmentation

Solution Approach 2:

Different regions of the buffer layer have different compositions and properties optimized for their specific functions: the AlN region handles high stress at the silicon interface, the AlGaN region provides gradual transition, and the GaN region interfaces with the light emitting stack. This local optimization reduces overall complexity.

Inventive Principle:

Principle #3Local quality

3Strength

If processing conditions are selected to generate defects and strain in buffer layers, then tensile and compressive stresses are balanced reducing cracking, but manufacturing precision requirements increase

Engineering Contradiction:

Improvestrain balanceVSAvoiddefect density control

Core Design Contradiction:

StrengthVSManufacturing precision

Solution Approach 1:

Processing conditions are intentionally selected to generate controlled defects and strain in the buffer layers. These normally harmful defects are converted into beneficial elements that balance tensile and compressive stresses, preventing catastrophic cracking and improving overall device reliability.

Inventive Principle:

Principle #22Blessing in disguise (Convert harm into benefit)

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 method effectively reduces strain and cracking in GaN thin films on silicon substrates, enhancing the quality and longevity of LEDs by maintaining a balanced strain state, which minimizes bowing and crack formation, resulting in improved device performance and efficiency.

Implementation Method 1

buffer layer with strained aluminum nitride (AlN), aluminum gallium nitride (AlGaN), and gallium nitride (GaN) layers, where processing conditions are selected to generate defects and strain, balancing tensile and compressive stresses

Methodology Applied

Scientific EffectStrain: Elasticity

Implementation Method 2

lattice mismatch and coefficient of thermal expansion mismatch, leading to structural stresses, cracking

Methodology Applied

Scientific EffectThermal expansion mismatch: Thermal Expansion

Implementation Method 3

an active layer configured to generate light upon the recombination of electrons and holes

Methodology Applied

Scientific EffectElectroluminescence: Electroluminescence

Data Source

PatentUS9130068B2Light emitting devices having dislocation density maintaining buffer layers

Publication Date: 2015.09.08 SAMSUNG ELECTRONICS CO LTD

AI summary

A method for forming a light emitting device comprises forming a buffer layer having a plurality of layers comprising a substrate, an aluminum gallium nitride layer adjacent to the substrate, and a gallium nitride layer adjacent to the aluminum gallium nitride layer. During the formation of each of the plurality of layers, one or more process parameters are selected such that an individual layer of the plurality of layers is strained.