SiC Epitaxial Wafer Buffer Layer Lattice Misfit
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Solution Overview
Problem
Conventional silicon carbide epitaxial wafers and semiconductor elements experience degraded crystal quality and reduced carrier mobility due to lattice constant differences between the substrate and epitaxially-grown layers, leading to increased element resistance, especially with thicker epitaxial layers.
Innovation Solution
A multi-layer buffer layer with decreasing doping concentration is introduced between the substrate and epitaxially-grown layer, with each layer having approximately the same thickness and a doping concentration that gradually decreases from the epitaxial layer side, effectively mitigating lattice misfit and preventing crystal defects.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Length of moving object
If the epitaxially-grown layer has a larger thickness to achieve higher voltage operation, then the voltage sustaining capability is improved, but the crystal quality degrades due to lattice constant difference between substrate and epitaxial layer
Solution Approach 1:
The buffer layer is divided into multiple sub-layers (first buffer layer and second buffer layer) with different doping concentrations. The first buffer layer has a doping concentration of 1×10^18 to 1×10^19 cm^-3, while the second buffer layer has a doping concentration of 1×10^16 to 1×10^17 cm^-3. This segmentation allows each sub-layer to address specific aspects of lattice mismatch separately, enabling the epitaxial layer to achieve both large thickness and high crystal quality.
Solution Approach 2:
Different regions of the buffer layer are assigned different doping concentrations tailored to their specific functions. The first buffer layer with higher doping concentration is positioned adjacent to the substrate to handle the most severe lattice mismatch, while the second buffer layer with lower doping concentration is positioned adjacent to the epitaxial layer to provide a smoother transition. This local optimization of doping concentration resolves the contradiction between thickness and crystal quality.
2Reliability
If the doping concentration of the epitaxially-grown layer is reduced to maintain low resistance, then the element resistance decreases, but the carrier mobility decreases due to crystal defects from lattice misfit
Solution Approach 1:
The buffer layers are formed in advance before growing the epitaxial layer, creating a pre-conditioned interface that gradually transitions from the substrate's high doping concentration to the epitaxial layer's low doping concentration. This preliminary action prevents lattice misfit defects from forming during epitaxial growth, thereby maintaining high carrier mobility even when the epitaxial layer has low doping concentration for low resistance.
Solution Approach 2:
The buffer layers act as intermediary structures between the heavily-doped substrate and the lightly-doped epitaxial layer. These intermediary layers with intermediate doping concentrations (1×10^18 to 1×10^19 cm^-3 and 1×10^16 to 1×10^17 cm^-3) mediate the lattice constant transition, preventing direct lattice misfit between substrate and epitaxial layer, thus preserving carrier mobility while allowing low resistance in the epitaxial layer.
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 maintains high crystal quality and carrier mobility even with large epitaxial layer thicknesses, reducing element resistance and preventing the introduction of crystal defects, thereby enhancing the performance of silicon carbide epitaxial wafers and semiconductor elements.
Implementation Method 1
the doping concentration differs considerably between the epitaxially-grown layer (voltage sustaining layer) and the substrate, which leads to a lattice constant difference therebetween. In a case where the epitaxially-grown layer has a larger thickness, the crystal quality of the epitaxially-grown layer degrades due to a lattice constant difference, that is, the introduction of a crystal defect accompanying lattice misfit.
Implementation Method 2
the crystal quality of the epitaxially-grown layer degrades due to a lattice constant difference, that is, the introduction of a crystal defect accompanying lattice misfit. As a result, the carrier mobility decreases, leading to a problem of an increase in element resistance.
Data Source
AI summary
A silicon carbide semiconductor element, including: i) an n-type silicon carbide substrate doped with a dopant, such as nitrogen, at a concentration C, wherein the substrate has a lattice constant that decreases with doping; ii) an n-type silicon carbide epitaxially-grown layer doped with the dopant, but at a smaller concentration than the substrate; and iii) an n-type buffer layer doped with the dopant, and arranged between the substrate and the epitaxially-grown layer, wherein the buffer layer has a multilayer structure in which two or more layers having the same thickness are laminated, and is configured such that, based on a number of layers (N) in the multilayer structure, a doping concentration of a K-th layer from a silicon carbide epitaxially-grown layer side is C·K/(N+1).