GaN Substrate Impurity Control for Ohmic Reliability
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Solution Overview
Problem
Group III-V nitride-based semiconductor substrates face challenges such as high dislocation density and periodic impurity concentration variations, leading to ohmic failure and reliability issues in devices, particularly in light-emitting diodes and ultraviolet LEDs, due to lattice mismatch and striation patterns during crystal growth.
Innovation Solution
A group III-V nitride-based semiconductor substrate with a periodic change in n-type impurity concentration, ensuring a minimum carrier concentration of not less than 5×10^17 cm^-3 and an amplitude of not more than 2×10^18 cm^-3, is developed to control carrier distribution and reduce striation effects, using techniques like HVPE and VAS methods for crystal growth and substrate separation.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Manufacturing precision
If a low temperature buffer layer technique is used to grow GaN on sapphire substrate, then single crystal epitaxial growth can be achieved, but the dislocation density becomes extremely high (10^9 to 10^10 cm^-2)
Solution Approach 1:
The invention segments the GaN crystal growth process into two distinct stages: first growing a thick GaN layer (5-50 μm) on the sapphire substrate to establish a single crystal structure, then separating this layer to create a freestanding substrate. This segmentation allows the initial growth to occur on the substrate while eliminating the substrate's harmful influence on dislocation density in the final device structure.
Solution Approach 2:
The invention extracts the GaN crystal layer from its original sapphire substrate context by mechanically separating the grown layer. This extraction removes the source of lattice mismatch and thermal expansion differences, thereby eliminating the continuous generation of dislocations that would occur if devices were grown directly on sapphire. The separated GaN layer becomes a freestanding substrate with dramatically reduced dislocation density.
2Ease of manufacture
If GaN templates with only GaN single layer are produced on sapphire substrate, then the device structure can be grown in separate process, but the lattice mismatch causes dislocation density to remain unacceptably high
Solution Approach 1:
The invention performs preliminary action by growing a sufficiently thick GaN layer (5-50 μm) on the sapphire substrate before separation. This preliminary growth establishes a complete single crystal structure that can be independently handled and processed. The thick layer ensures mechanical integrity during separation and subsequent device fabrication, while the single crystal quality is established before the harmful substrate influence is removed.
3Reliability
If n-type impurity concentration is increased to prevent ohmic failure, then carrier concentration improves, but periodic variations cause striation patterns that reduce device reliability
Solution Approach 1:
The invention changes the parameter of n-type impurity concentration from a high level that causes striation patterns to a controlled range of 5×10^17 to 5×10^18 cm^-3. This parameter change ensures sufficient carrier concentration for reliable ohmic contacts while avoiding the periodic variations that lead to striation. The specific concentration range is optimized to balance electrical performance with structural uniformity.
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 prevents ohmic failure and enhances device reliability by ensuring uniform carrier concentration, improving the performance and production yield of semiconductor devices by minimizing the impact of striation patterns and dislocation density.
Implementation Method 1
GaN has been hetero-epitaxially grown thereon by a vapor phase growth method such as MOVPE (metal-organic vapor phase epitaxy), MBE (molecular beam epitaxy), HVPE (hydride vapor phase epitaxy) etc.
Implementation Method 2
a vapor phase growth method such as MOVPE (metal-organic vapor phase epitaxy), MBE (molecular beam epitaxy), HVPE (hydride vapor phase epitaxy)
Implementation Method 3
a GaN growth method is devised that uses a low temperature buffer layer technique which has been developed for hetero-growing Si etc. on the sapphire substrate. The technique is conducted such that the buffer layer of AlN or GaN is formed on the sapphire substrate at a low temperature of about 500° C., and then the GaN is grown thereon while reducing lattice distortions by the low temperature growth buffer layer.
Data Source
AI summary
A group III-V nitride-based semiconductor substrate is formed of a group III-V nitride-based semiconductor single crystal containing an n-type impurity. The single crystal has a periodical change in concentration of the n-type impurity in a thickness direction of the substrate. The periodical change has a minimum value in concentration of the n-type impurity not less than 5×1017 cm−3 at an arbitrary point in plane of the substrate.