GaN Epitaxial Layer on Silicon Substrate
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Solution Overview
Problem
Current methods for growing GaN layers on silicon substrates face challenges due to significant lattice mismatch and thermal expansion differences, leading to crystalline defects, leakage currents, and cracking, which limits the thickness of crack-free GaN layers to around 2.5 µm, insufficient for most applications.
Innovation Solution
A method involving epitaxial growth of GaN using a silicon substrate with a buffer layer of AIN, followed by intermediate layers of BAlGaInN with varying aluminum content and dopant concentrations, allowing for the growth of a crack-free GaN layer exceeding 5 µm thickness with a dislocation density of ≤5×10^8 cm^-2, utilizing metalorganic vapour phase epitaxy (MOVPE) and incorporating transition layers to manage strain and dislocation density.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Ease of manufacture
If a silicon substrate is used for growing GaN layers, then cost and substrate availability are improved, but lattice mismatch and thermal expansion differences cause crystalline defects and cracking
Solution Approach 1:
The patent introduces a buffer layer of AlN and intermediate layers of BAlGaInN between the silicon substrate and the GaN layer. These intermediary layers serve as a transition zone that gradually adapts the lattice structure and thermal expansion properties, reducing the shock of the mismatch between silicon and GaN, thereby preventing cracking while maintaining the use of inexpensive silicon substrates.
Solution Approach 2:
The patent varies the aluminum content and dopant concentrations in the intermediate BAlGaInN layers. By gradually changing the composition parameters (aluminum content from 0.8 to 0.2, dopant concentrations), the lattice constant and thermal expansion coefficient are continuously adjusted between the silicon substrate and the GaN layer, reducing mismatch-induced stresses and preventing crack formation.
2Length of moving object
If the thickness of GaN layer is increased to meet application requirements, then device performance is improved, but thermal mismatch causes cracking in thicker layers
Solution Approach 1:
The buffer and intermediate layers act as strain-absorbing intermediaries that prevent the propagation of thermal mismatch stresses through the entire structure. This allows the GaN layer to be grown much thicker (exceeding 5 µm) without the cracks that would normally form in conventional structures where the full thermal stress is transmitted directly to the GaN layer.
Solution Approach 2:
By gradually changing the aluminum content in the intermediate layers (from 0.8 near the substrate to 0.2 near the GaN layer), the patent creates a gradient structure that continuously adapts to the thermal expansion differences. This parameter gradient allows thick GaN layers to be grown without accumulating sufficient stress to cause cracking.
3Manufacturing precision
If lattice mismatch is addressed by using GaN buffer layer, then crystalline defects are reduced, but the compressive strain from GaN on AIN is insufficient to compensate for tensile strain during cooling
Solution Approach 1:
The patent creates a composite structure with multiple layers of different compositions (AlN buffer, BAlGaInN intermediate layers with varying Al content, and GaN layer). This composite structure provides both the crystalline quality improvement from the AlN buffer and the strain compensation through the graded BAlGaInN layers, combining the benefits of both approaches.
Solution Approach 2:
The patent uses parameter changes in the intermediate layers (aluminum content gradient from 0.8 to 0.2) to provide both crystalline quality improvement and strain compensation. The gradual parameter change allows the structure to maintain low dislocation density while simultaneously providing sufficient compressive strain to offset the tensile strain that develops during cooling.
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 method enables the production of thick, crack-free GaN layers with reduced dislocation density, suitable for electronic and optoelectronic devices, enhancing their performance and reliability by effectively compensating for thermal and lattice mismatches.
Implementation Method 1
there is a significant mismatch between the coefficients of thermal expansion of silicon and III-N materials... the divergent contraction of the silicon substrate (which is slow), and that of the III-N epitaxial layers (which is fast), on returning to ambient temperature after epitaxy, results in said layers being placed under tensile strain
Implementation Method 2
Method for manufacturing a thick epitaxial layer of gallium nitride on a silicon or similar substrate
Implementation Method 3
utilizing metalorganic vapour phase epitaxy (MOVPE)
Implementation Method 4
The first is a significant lattice mismatch with respect to III-N materials. Indeed, for the Si(111) face, the lattice mismatch between GaN (wherein the lattice parameter is 3.189 Å) and Si (the lattice parameter is 3.840 Å) is 16.9%
Data Source
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AI summary
The invention relates to a method for manufacturing, by means of epitaxy, a monocrystalline layer (3; 3', 3") of GaN on a substrate (1) wherein the coefficient of thermal expansion is less than the coefficient of thermal expansion of GaN, comprising the following steps: (b) three-dimensional epitaxial growth of a layer (3a) of GaN relaxed at the epitaxial temperature, (c1) growth of an intermediate layer (4a) of BwAlxGaylnzN, (c2) growth of a layer (3b) of BwAlxGaylnzN, (c3) growth of an intermediate layer (4b) of BwAlxGaylnzN, at least one of the layers (3b, 4a, 4b) formed in steps (c1) to (c3) being an at least ternary III-N alloy comprising aluminium and gallium, (d) growth of said layer (3; 3', 3") of GaN.