Multilayer Buffer Layer for Crack-Free High Breakdown Voltage Substrates
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Solution Overview
Problem
Existing compound semiconductor substrates using nitride semiconductors face challenges in achieving high breakdown voltage and normally-off states while preventing cracks and warping, with existing buffer layer structures either lacking in breakdown voltage or being unsuitable for high-frequency applications.
Innovation Solution
A multilayer buffer layer is formed by alternately stacking AlxGa1-xN and AlyGa1-yN single crystal layers with specific carbon concentrations, where the carbon concentration decreases from the substrate side to the active layer side, and boron is added to enhance breakdown voltage, preventing cracks and warping, and achieving a normally-off state.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Strength
If a buffer layer with high Al content is used to achieve high breakdown voltage, then breakdown voltage is improved, but the device cannot achieve normally-off state
Solution Approach 1:
The patent applies local quality by creating different Al content zones within the buffer layer. The buffer layer includes a first region with Al content of 0.6-1.0 and a second region with Al content of 0.01-0.5, where each region has different local properties. This allows the high Al content region to provide high breakdown voltage while the low Al content region enables normally-off state capability, resolving the contradiction between these two requirements.
2Manufacturing precision
If a multilayer buffer layer with alternating Al content is used to achieve flat and smooth nitride semiconductor, then surface quality is improved, but breakdown voltage per unit film thickness becomes low
Solution Approach 1:
The patent resolves this contradiction by assigning different functions to different regions of the buffer layer. The first region with high Al content (0.6-1.0) provides the necessary surface quality and flatness, while the second region with low Al content (0.01-0.5) contributes to high breakdown voltage. This regional differentiation allows both surface quality and high breakdown voltage per unit thickness to be achieved simultaneously.
3Strength
If carbon concentration is increased in the buffer layer to achieve high breakdown voltage, then breakdown voltage is improved, but crack generation and substrate warping occur
Solution Approach 1:
The patent applies local quality by spatially distributing carbon concentration across different regions of the buffer layer. The first region contains carbon at concentrations of 1×10^18 to 1×10^21 atoms/cm³ to achieve high breakdown voltage, while the second region contains carbon at concentrations of 1×10^17 to 1×10^20 atoms/cm³ to minimize crack generation and substrate warping. This regional carbon concentration control resolves the contradiction between achieving high breakdown voltage and preventing structural defects.
Data Source
AI summary
A compound semiconductor substrate which inhibits the generation of a crack or a warp and is preferable for a normally-off type high breakdown voltage device, arranged that a multilayer buffer layer 2 in which AlxGa1-xN single crystal layers (0.6≦X≦1.0) 21 containing carbon from 1×1018 atoms/cm3 to 1×1021 atoms/cm3 and AlyGa1-yN single crystal layers (0.1≦y≦0.5) 22 containing carbon from 1×1017 atoms/cm3 to 1×1021 atoms/cm3 are alternately and repeatedly stacked in order, and a nitride active layer 3 provided with an electron transport layer 31 having a carbon concentration of 5×1017 atoms/cm3 or less and an electron supply layer 32 are deposited on a Si single crystal substrate 1 in order. The carbon concentrations of the AlxGa1-xN single crystal layers 21 and that of the AlGa1-yN single crystal layers 22 respectively decrease from the substrate 1 side towards the above-mentioned active layer 3 side. In this way, the compound semiconductor substrate is produced.