Graded AlN/GaN Substrate Structure for Lower Threading Dislocations
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Solution Overview
Problem
Conventional nitride semiconductor layers containing Al often have high internal threading dislocation densities, which are carried over to the GaN layer, increasing threading dislocations within the GaN layer.
Innovation Solution
A compound semiconductor substrate is designed with a graded Al concentration in the first nitride semiconductor layer, followed by a second nitride semiconductor layer with a lower Al concentration, and a third nitride semiconductor layer with a higher Al concentration, resulting in a lower threading dislocation density in the third layer compared to the first layer.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Stability of the object's composition
If a nitride semiconductor layer containing Al is used as a buffer layer, then the lattice constant difference between GaN and Si substrate is reduced, but threading dislocation density increases
Solution Approach 1:
The nitride semiconductor layer is divided into three distinct layers with different Al concentrations: a first layer with higher Al concentration for lattice matching, a second layer with lower Al concentration to reduce dislocations, and a third layer with intermediate Al concentration. This segmentation allows each layer to perform its specific function independently, resolving the contradiction between lattice matching and dislocation reduction.
Solution Approach 2:
Each nitride semiconductor layer is designed with a specific local Al concentration tailored to its position and function. The first layer has high Al concentration for lattice matching with Si substrate, the second layer has low Al concentration for dislocation reduction, and the third layer has intermediate concentration for transition. This local quality optimization allows simultaneous achievement of lattice matching and low dislocation density.
2Strength
If a SiC layer is introduced between Si substrate and GaN layer, then warpage and cracks are suppressed, but device complexity increases
Solution Approach 1:
The nitride semiconductor layers with graded Al concentrations serve multiple functions simultaneously: they provide lattice constant matching with the Si substrate, reduce threading dislocation density through the concentration gradient, and suppress warpage and cracks by managing thermal expansion differences. This multi-functionality eliminates the need for a separate SiC layer, reducing overall device complexity while maintaining substrate stability.
Data Source
AI summary
A compound semiconductor substrate has a Si (silicon) substrate, a first Al nitride semiconductor layer which is a graded layer formed on the Si substrate and whose Al concentration decreases as the distance from the Si substrate increases along the thickness direction, a GaN (gallium nitride) layer formed on the first Al nitride semiconductor layer and having a lower average Al concentration than the average Al concentration of the first Al nitride semiconductor layer, and a second Al nitride semiconductor layer formed on the GaN layer and having a higher average Al concentration than the average Al concentration of the GaN layer. The threading dislocation density at any position in the thickness direction within the second Al nitride semiconductor layer is lower than the threading dislocation density at any position in the thickness direction within the first Al nitride semiconductor layer.