AlGaN Nucleation Layer Lattice Matching for SiC-Based GaN Devices
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Solution Overview
Problem
The lattice mismatch and energy level difference between gallium nitride (GaN) and silicon carbide (SiC) lead to increased defect density and forward voltage in semiconductor elements, compromising their conductivity.
Innovation Solution
An epitaxial structure is developed using a SiC substrate with an AlGaN nucleation layer doped with silicon, iron, magnesium, or carbon, where the aluminum content and doping concentration gradually reduce in the thickness direction, aligning the lattice constant and energy gap to improve lattice matching between GaN and SiC, thereby reducing defect density and forward voltage.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Power
If a heterojunction is formed between GaN and SiC, then high voltage withstanding capability and high frequency are achieved, but lattice mismatch and energy level difference lead to increased defect density and forward voltage
Solution Approach 1:
An AlGaN nucleation layer is introduced as an intermediary between the SiC substrate and the GaN layer. This intermediate layer has a lattice constant that gradually transitions from the SiC substrate value to the GaN layer value, serving as a buffer that reduces the lattice mismatch and minimizes defect density at the heterojunction interface.
Solution Approach 2:
The lattice constant of the AlGaN nucleation layer is systematically adjusted by varying the aluminum content, which changes from high to low in the thickness direction. This parameter change creates a gradient structure that bridges the lattice constant difference between SiC and GaN, reducing dislocation density and improving material quality.
2Strength
If a heterojunction is formed between GaN and SiC, then high voltage withstanding capability is achieved, but energy level difference leads to increased forward voltage reducing conductivity
Solution Approach 1:
The energy gap of the AlGaN nucleation layer is systematically adjusted by varying the aluminum content gradient. The energy gap changes from high to low in the thickness direction, creating a gradual energy level transition that reduces the energy barrier at the heterojunction interface, thereby lowering forward voltage and improving conductivity while maintaining voltage withstanding capability.
3Reliability
If AlGaN nucleation layer with gradient structure is used, then lattice matching is improved reducing defect density, but manufacturing complexity increases
Solution Approach 1:
The nucleation layer is formed with continuously changing parameters (aluminum content and doping concentration) in the thickness direction. This gradient structure can be achieved through modified molecular beam epitaxy or chemical vapor deposition processes, where the composition is dynamically adjusted during growth to create the desired lattice constant and energy gap gradient.
Solution Approach 2:
Different regions of the nucleation layer have different aluminum contents and doping concentrations, creating local variations in lattice constant and energy gap. The upper portion has lower aluminum content matching GaN, while the lower portion has higher aluminum content matching SiC, with a gradual transition zone in between that minimizes interface defects.
Data Source
AI summary
An epitaxial structure and a semiconductor device are provided in which the epitaxial structure includes at least a SiC substrate, a nucleation layer, and a GaN layer. The nucleation layer is formed on the SiC substrate. The material of the nucleation layer is aluminum gallium nitride doped with a dopant, the Al content in the nucleation layer changes from high to low in the thickness direction, the lattice constant of the nucleation layer is between 3.08 Å and 3.21 Å, and the doping concentration of the nucleation layer changes from high to low in the thickness direction. The GaN layer is formed on the nucleation layer.