GaN Epitaxial Growth on CTE-Matched AlN Substrates
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Solution Overview
Problem
Conventional epitaxial growth methods for gallium nitride (GaN) layers on sapphire substrates result in reduced uniformity and adverse effects on electronic/optical properties due to heteroepitaxial growth, leading to issues like cracking and peeling of the epitaxial layers.
Innovation Solution
The use of an engineered substrate structure with a polycrystalline ceramic core and engineered layers that match the coefficient of thermal expansion (CTE) of the GaN epitaxial layers, including adhesion, barrier, and bonding layers, to reduce thermal mismatch and stress during growth and cooling processes, enabling the growth of thick GaN layers without cracking or peeling.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If sapphire substrates are used for epitaxial growth of GaN layers, then the growth process can be performed, but the epitaxial layers exhibit reduced uniformity and adverse effects on electronic/optical properties due to thermal mismatch
Solution Approach 1:
The patent changes the thermal parameter (coefficient of thermal expansion) of the substrate by selecting AlN instead of sapphire, and further modifies it through doping with Ti or TiN to achieve CTE matching with GaN epitaxial layers, thereby reducing thermal mismatch stress and improving layer uniformity
Solution Approach 2:
The patent creates a polycrystalline AlN substrate that copies the desirable thermal expansion properties of single crystal GaN, allowing heteroepitaxial growth to proceed with minimal thermal mismatch while maintaining the benefits of polycrystalline material
2Length of stationary object
If thick GaN layers are grown on conventional substrates, then the desired thickness is achieved, but cracking and peeling occur due to thermal stress during growth and cooling
Solution Approach 1:
By doping AlN with Ti or TiN, the patent modifies the thermal and mechanical parameters of the substrate to better match GaN, enabling growth of thick layers (greater than 10 μm) without cracking or peeling that would otherwise occur due to thermal stress
Solution Approach 2:
The doped AlN substrate acts as an intermediary between the growth process and the final GaN structure, absorbing thermal stress through its modified properties and preventing stress transfer to the thick epitaxial layers, thereby maintaining structural integrity
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 allows for the growth of GaN layers up to 100 μm in thickness with reduced dislocation and peeling, suitable for various applications such as vertical power devices and large-diameter free-standing wafers, maintaining material properties across a wide temperature range.
Implementation Method 1
an engineered support structure that is characterized by a coefficient of thermal expansion (CTE) (e.g., a coefficient of linear thermal expansion) that is substantially matched to the thick GaN epitaxial layers grown thereon
Data Source
AI summary
An engineered substrate includes a support structure comprising a polycrystalline ceramic core, an adhesion layer coupled to the polycrystalline ceramic core, and a barrier layer coupled to the adhesion layer. The engineered substrate also includes an bonding layer coupled to the support structure, a substantially single crystal layer coupled to the bonding layer, and an epitaxial gallium nitride layer coupled to the substantially single crystal layer.