Ammonothermal GaN Seed Selection for Crack-Free Ingots
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Solution Overview
Problem
The production of high-quality group III-nitride wafers is hindered by defective seed crystals, leading to strained and bowed growths on heteroepitaxial substrates, which results in cracked films and limited availability of large, high-quality GaN ingots suitable for devices.
Innovation Solution
The ammonothermal growth method is enhanced by using nitrogen-polar seeds to minimize bowing and stress, harvesting crack-free regions on Ga-polar surfaces, and growing a series of ingots with specific crystal orientations to produce improved seed crystals with reduced strain and enhanced crystallinity.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Ease of manufacture
If heteroepitaxial growth is used on sapphire or silicon carbide substrates, then device fabrication can proceed, but the grown films become highly defected or cracked due to lattice mismatch
Solution Approach 1:
The patent uses ammonothermal growth as an intermediary process between heteroepitaxial seed formation and final device fabrication. This intermediate step grows high-quality GaN ingots from defective seeds, effectively mediating the transition from low-quality heteroepitaxial films to high-quality bulk crystals suitable for devices.
Solution Approach 2:
The patent extracts and removes the defective regions and lattice mismatch problems by growing new high-quality crystal layers over the defective seeds. The ammonothermal process selectively grows pristine GaN that overrides the underlying defects, effectively taking out the harmful effects of the original defective films.
2Productivity
If conventional ammonothermal growth is used with defective seeds, then ingot growth can proceed, but the crystal quality remains limited due to strain and bowing from the defective seeds
Solution Approach 1:
The patent performs preliminary actions by carefully selecting and preparing seed crystals with specific orientations and characteristics before the ammonothermal growth process. This preliminary preparation includes choosing seeds that will minimize strain propagation and enable subsequent high-quality growth, thereby improving crystal quality while maintaining productivity.
Solution Approach 2:
The patent changes critical growth parameters including temperature gradients, pressure conditions, and ammonia flow rates to optimize crystal quality. By adjusting these parameters, the process compensates for the presence of defective seeds and produces high-quality ingots with reduced strain and bowing.
3Volume of moving object
If continuous growth is pursued on original defective seeds, then larger ingots can be produced, but strain accumulation leads to cracking and limits ingot size
Solution Approach 1:
The patent segments the growth process into multiple stages: initial growth on defective seeds, intermediate harvesting of high-quality regions, and subsequent growth on improved seeds. This segmentation allows the production of large ingots by concatenating multiple growth cycles, each producing crack-free sections that are then combined.
Solution Approach 2:
Instead of continuing growth on original defective seeds until failure, the patent inverts the approach by harvesting the high-quality regions grown on the defective seeds and using these harvested regions as new seeds for continued growth. This inversion transforms the defective seeds from a liability into a stepping stone for producing larger crack-resistant ingots.
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 results in GaN wafers with reduced bowing and strain, enabling continuous oriented growth and improved crystallinity, thus overcoming the limitations of existing seed crystals and enhancing the feasibility of producing large, high-quality ingots for device substrates.
Implementation Method 1
high-pressure NH3 provides high solubility of source materials, such as GaN polycrystals or metallic Ga, and high transport speed of dissolved precursors
Implementation Method 2
The ammonothermal growth method is used to produce group III-nitride ingots
Implementation Method 3
The III-nitride crystal structure of interest has a wurtzite crystal structure
Data Source
AI summary
The present invention discloses methods to create higher quality group III-nitride wafers that then generate improvements in the crystalline properties of ingots produced by ammonothermal growth from an initial defective seed. By obtaining future seeds from carefully chosen regions of an ingot produced on a bowed seed crystal, future ingot crystalline properties can be improved. Specifically the future seeds are optimized if chosen from an area of relieved stress on a cracked ingot or from a carefully chosen N-polar compressed area. When the seeds are sliced out, miscut of 3-10° helps to improve structural quality of successive growth. Additionally a method is proposed to improve crystal quality by using the ammonothermal method to produce a series of ingots, each using a specifically oriented seed from the previous ingot. When employed, these methods enhance the quality of Group III nitride wafers and thus improve the efficiency of any subsequent device.