Epitaxial Substrate with Superlattice Strain Gradient
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Solution Overview
Problem
Forming high-quality nitride films on silicon substrates is challenging due to lattice constant mismatch, thermal expansion coefficient differences, and the formation of liquid-phase compounds, leading to dislocation issues, warping, and low crystal quality, which affects the performance of HEMT devices.
Innovation Solution
A crack-free epitaxial substrate is created using a (111)-oriented single crystal silicon base substrate with a superlattice layer group composed of alternately laminated group-III nitride unit layers, where the second unit layer has a greater in-plane lattice constant than the first unit layer, and an intermediate layer enhances compressive strain to cancel tensile stress, resulting in reduced warping and improved crystal quality.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Ease of manufacture
If a nitride film is formed on a silicon substrate, then cost reduction and integration with silicon-based circuit devices are achieved, but lattice constant mismatch causes misfit dislocation and three-dimensional growth mode, leading to poor crystal quality
Solution Approach 1:
A buffer layer made of group-III nitride is introduced as an intermediary between the silicon substrate and the nitride film. This buffer layer has a lattice constant intermediate between silicon and the nitride material, reducing lattice mismatch and preventing misfit dislocation. The buffer layer enables two-dimensional growth mode and improves crystal quality while maintaining the cost benefits of using silicon substrates.
Solution Approach 2:
The lattice constant parameter is gradually changed through the buffer layer, which has a composition gradient or thickness gradient. This gradual parameter change reduces the abrupt lattice mismatch at interfaces, suppressing dislocation formation and enabling high-quality nitride film growth on silicon substrates.
2Ease of manufacture
If a nitride film is formed on a silicon substrate, then cost-effective substrate is used, but thermal expansion coefficient difference causes tensile stress and warping upon temperature drop
Solution Approach 1:
The buffer layer is designed with specific thickness and material composition to compensate for thermal expansion coefficient differences between silicon and nitride materials. During cooling from growth temperature to room temperature, the buffer layer's thermal expansion characteristics help balance the differential contraction, reducing tensile stress and warping of the nitride film.
3Productivity
If trimethylgallium is used as material gas for vapor-phase growth, then GaN film can be deposited, but liquid-phase compound formation with silicon hinders epitaxial growth
Solution Approach 1:
The buffer layer acts as a protective intermediary between the silicon substrate and the GaN film during vapor-phase growth. It prevents direct interaction between trimethylgallium and silicon, avoiding liquid-phase compound formation while allowing controlled epitaxial growth of high-quality GaN film.
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
The approach results in a crack-free epitaxial substrate with reduced warping and excellent crystal quality, enhancing the breakdown voltage and electron mobility of HEMT devices while using a cost-effective silicon substrate.
Implementation Method 1
The superlattice layer group has a compressive strain contained therein, and, in the superlattice layer group, the more distant the superlattice layer is from the base substrate, the greater the compressive strain becomes
Implementation Method 2
the nitride material has a higher thermal expansion coefficient value than that of silicon. Therefore, in the step of lowering the temperature to the vicinity of the room temperature after a nitride film is epitaxially grown on the silicon substrate at a high temperature a tensile stress acts in the nitride film
Implementation Method 3
an intermediate layer enhancing the compressive strain introduced in the epitaxial substrate by the superlattice layer group
Implementation Method 4
a group of group-III nitride layers are formed on a base substrate made of (111)-oriented single crystal silicon such that a (0001) crystal plane of the group of group-III nitride layers is substantially in parallel with a substrate surface of the base substrate
Data Source
AI summary
Provided is a crack-free epitaxial substrate with reduced warping, in which a silicon substrate is used as a base substrate. The epitaxial substrate includes a (111) single crystal Si substrate, a superlattice layer group in which a plurality of superlattice layers are laminated, and a crystal layer. The superlattice layer is formed of a first unit layer and a second unit layer made of group-III nitrides having different compositions being alternately and repeatedly laminated. The crystal layer is made of a group-III nitride and formed above the base substrate so as to be positioned at an upper side of the superlattice layer group relative to the base substrate. The superlattice layer group has a compressive strain contained therein. In the superlattice layer group, the more distant the superlattice layer is from the base substrate, the greater the compressive strain becomes.