Graded III-N Buffer Layers for Crack-Free Silicon Growth
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Solution Overview
Problem
The growth of thick III-Nitride (III-N) layers on silicon substrates is hindered by lattice and thermal expansion coefficient mismatches, leading to tensile stress and cracking, making it difficult to achieve crack-free and structurally adequate layers.
Innovation Solution
A method involving the formation of multiple III-N buffer layers with varying aluminum compositions and thicknesses is employed, where the aluminum composition either remains constant or decreases from the substrate side to the surface, with specific thicknesses and doping, to manage stress and prevent cracking.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Length of stationary object
If thick III-N layers are grown on silicon substrates, then the desired thick layer is achieved, but tensile stress causes cracking
Solution Approach 1:
The buffer layer is divided into multiple discrete layers with different aluminum compositions (e.g., AlN, AlGaN layers with varying Al content). Each layer segment manages stress differently, allowing the total structure to support much greater thicknesses (microns to millimeters) without cracking, resolving the contradiction between achieving thick layers and maintaining crack resistance.
Solution Approach 2:
The aluminum composition parameter is varied systematically through the buffer layer structure. By changing the aluminum content from high near the substrate to lower toward the III-N layer, the lattice constant and thermal expansion properties are adjusted at each interface, managing stress accumulation and enabling thick crack-free layer growth.
2Ease of manufacture
If III-N layers are grown directly on silicon substrates, then ease of manufacture is improved, but lattice mismatch and thermal expansion coefficient mismatch cause tensile stress
Solution Approach 1:
Multiple buffer layers with graded aluminum compositions serve as intermediary structures between the silicon substrate and the III-N device layer. These intermediary layers gradually transition the lattice constant and thermal properties, mediating the mismatch between silicon and III-N materials while maintaining ease of silicon-based manufacturing.
Solution Approach 2:
Different regions of the buffer structure have different aluminum compositions tailored to local requirements: high aluminum content near the silicon substrate to match thermal expansion, transitioning to lower aluminum content near the III-N layer to match lattice constants. This local optimization reduces overall tensile stress while maintaining manufacturability.
3Reliability
If the maximum thickness of III-N layers is limited to avoid cracking, then crack resistance is maintained, but the ability to grow sufficiently thick layers for many applications is reduced
Solution Approach 1:
The buffer is segmented into multiple layers with progressively varying aluminum compositions, allowing stress to be managed in discrete steps throughout the thickness. This segmentation enables the structure to support much greater total thicknesses (microns to millimeters) while maintaining crack-free reliability.
Solution Approach 2:
The buffer structure uses composite material design with multiple AlN/AlGaN layers of different compositions. This composite approach combines the advantages of high-aluminum layers (stress management near substrate) with low-aluminum layers (lattice matching near device), enabling both crack-free reliability and sufficient thickness for applications.
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 allows for the growth of thick III-N layers with reduced stress and defects, enabling the formation of high-quality semiconductor devices by maintaining compressive stress at growth temperature and minimizing tensile stress at room temperature.
Implementation Method 1
due to the large lattice mismatch and thermal expansion coefficient mismatch between silicon and III-N materials, there is typically a net tensile stress in III-N epitaxial layers deposited directly on silicon substrates
Data Source
AI summary
Embodiments of the present disclosure include a buffer structure suited for III-N device having a foreign substrate. The buffer structure can include a first buffer layer having a first aluminum composition and a second buffer layer formed on the first buffer layer, the second buffer layer having a second aluminum composition. The buffer structure further includes a third buffer layer formed on the second buffer layer at a second interface, the third buffer layer having a third aluminum composition. The first aluminum composition decreases in the first buffer layer towards the interface and the second aluminum composition throughout the second buffer layer is greater than the first aluminum composition at the interface.