Nitride Semiconductor Dual Superlattice Buffer for Crack Resistance
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Solution Overview
Problem
Nitride semiconductor devices face challenges with increased leak current and cracking due to the thickness limitations of buffer layers, which restrict the improvement of breakdown voltage when using different substrates like Si or SiC, as the buffer layer's lattice relaxation leads to stress on the GaN electron transport layer.
Innovation Solution
A nitride semiconductor device with a dual superlattice buffer layer structure, where the second buffer layer has an intermediate lattice constant between the first buffer layer and the device operation layer, allowing for increased thickness without lattice relaxation, and an AlN layer between the substrate and the first buffer layer to prevent crystal defects.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Strength
If the thickness of the buffer layer is increased to improve breakdown voltage, then the breakdown voltage is improved, but the buffer layer experiences lattice relaxation and misfit transition which causes cracking of the GaN electron transport layer
Solution Approach 1:
The buffer layer is divided into multiple superlattice periods with alternating high-Al composition and low-Al composition layers. This segmentation allows the buffer layer to accommodate misfit dislocations within the superlattice structure itself, preventing crack propagation to the GaN electron transport layer while maintaining the necessary thickness for high breakdown voltage.
Solution Approach 2:
Different regions of the buffer layer have different Al compositions - high-Al composition layers provide stress relief and dislocation accommodation, while low-Al composition layers maintain lattice matching closer to GaN. This local variation in composition allows the buffer layer to simultaneously achieve sufficient thickness for high breakdown voltage while preventing crack formation.
2Strength
If the number of superlattice periods is increased to increase buffer layer thickness, then the breakdown voltage is improved, but the leak current increases
Solution Approach 1:
The Al composition ratio is varied within the superlattice structure - high-Al composition layers (first buffer layer) provide stress relief and dislocation accommodation, while low-Al composition layers (second buffer layer) maintain better lattice matching and lower defect density. This parameter variation allows increasing the number of superlattice periods for higher breakdown voltage while controlling leak current through optimized composition distribution.
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 configuration effectively suppresses lattice relaxation, improves breakdown voltage, and reduces leak current while maintaining the buffering effect, even with increased superlattice periods, and prevents crystal defects between the Si substrate and the GaN layer.
Implementation Method 1
The buffer layer is made of a composition having a lower average lattice constant than the epitaxial layer, thereby relieving the warpage of the epitaxial wafer which may otherwise occur due to an internal stress caused by a difference in linear expansion coefficient between the different type substrate and the epitaxial layer during cooling after the epitaxial growth.
Implementation Method 2
If having a thickness greater than the critical thickness, on the other hand, the buffer layer is liable to experience misfit transition to reduce a stress energy. Thus, the buffer layer experiences lattice relaxation
Implementation Method 3
an AlN layer provided between the substrate and the first buffer layer
Data Source
AI summary
An inventive nitride semiconductor device includes: a substrate; a first buffer layer provided on the substrate, and having a superlattice structure which includes two types of Group III nitride semiconductor sublayers having different compositions and alternately stacked in pairs; a second buffer layer provided on the first buffer layer in contact with the first buffer layer, and having a superlattice structure which includes two types of Group III nitride semiconductor sublayers having different compositions and alternately stacked in pairs; and a device operation layer of a Group III nitride semiconductor provided on the second buffer layer; wherein an average lattice constant LC1 of the first buffer layer, an average lattice constant LC2 of the second buffer layer and an average lattice constant LC3 of the device operation layer satisfy the following expression (1):LC1<LC2<LC3 (1).