AlGaN Buffer Layer Composition Gradient for GaN Crystallinity
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Solution Overview
Problem
Conventional compound semiconductor devices face challenges in achieving good crystallinity for GaN and AlGaN layers due to thermal expansion coefficient differences between these layers and their substrates, leading to warping or cracking, and microdefects from abrupt composition changes in superlattice buffer layers.
Innovation Solution
A buffer layer with a composition of AlxGa1-xN(0≦x≦1) is formed, where the variation in x value across the thickness is 0.5 or less, reducing thermal strain and microdefects, thereby maintaining good crystallinity of the electron transit and supply layers.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Strength
If a superlattice buffer layer with abrupt composition changes is formed to suppress warping and cracking, then thermal strain is reduced, but microdefects are generated and crystallinity deteriorates
Solution Approach 1:
The buffer layer employs a gradual composition gradient where the Al composition ratio changes incrementally from the substrate interface toward the GaN layer. This local variation in composition allows each region to provide appropriate strain compensation while maintaining crystal quality, resolving the contradiction between thermal strain resistance and crystallinity
Solution Approach 2:
The invention changes the composition parameter (Al ratio x) gradually through the buffer layer thickness rather than using abrupt changes. By controlling the composition gradient so that x varies smoothly, the patent achieves both warping suppression and good crystallinity, eliminating microdefects that would otherwise form at sharp composition interfaces
2Manufacturing precision
If high temperature treatment is applied for epitaxial growth of GaN and AlGaN layers, then good crystallinity is achieved, but warping or cracking of the Si substrate occurs due to thermal expansion coefficient difference
Solution Approach 1:
The buffer layer is formed in advance between the Si substrate and the GaN layer to pre-compensate for thermal strain. This preliminary structure prepares the system to withstand the thermal stress that will occur during high-temperature epitaxial growth, allowing good crystallinity to be achieved without substrate warping or cracking
Solution Approach 2:
The buffer layer acts as an intermediary between the Si substrate and the GaN layer, mediating the thermal expansion coefficient mismatch. This intermediate layer absorbs and distributes the thermal stress, protecting the substrate from warping and cracking while enabling high-temperature growth for good crystallinity
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 approach effectively suppresses warping and cracking while ensuring high crystallinity of the GaN and AlGaN layers, enhancing the performance of compound semiconductor devices like GaN HEMTs by reducing thermal strain and microdefects.
Implementation Method 1
There is however a large difference in thermal expansion coefficient between the GaN layer/the AlGaN layer and the Si substrate. On the other hand, a high temperature treatment is required for epitaxial growth of GaN layer and AlGaN layer. Warping or cracking of the Si substrate may therefore occur due to the difference in thermal expansion coefficient during such high temperature treatment.
Implementation Method 2
In conventional compound semiconductor devices utilizing an superlattice structure as a buffer layer, it is difficult to obtain good crystallinity of the electron transit layer and the electron supply layer formed thereover
Implementation Method 3
In a GaN HEMT, in which a GaN layer is used as an electron transit layer and AlGaN as an electron supply layer, strain is produced on the AlGaN due to the difference in lattice constant between AlGaN and GaN. For this reason, piezoelectric polarization occurs and a highly concentrated 2-dimensional electron gas (2DEG) is obtained.
Data Source
AI summary
A compound semiconductor device includes: a substrate; an electron transit layer formed over the substrate; an electron supply layer formed over the electron transit layer; and a buffer layer formed between the substrate and the electron transit layer and including AlxGa1-xN(0≦x≦1), wherein the x value represents a plurality of maximums and a plurality of minimums in the direction of the thickness of the buffer layer, and the variation of x in any area having a 1 nm thickness in the buffer layer is 0.5 or less.