Modulation Doped GaN Ohmic Contacts for Low Resistance
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Solution Overview
Problem
The existing methods for fabricating III-nitride high electron mobility transistors (HEMTs) face challenges in reducing contact resistance between the source and drain, particularly due to limitations in doping levels of GaN, which can lead to unsatisfactory ohmic contacts and surface morphology issues, affecting device performance.
Innovation Solution
A method involving modulation doping, where alternating high and low doping levels of n-type dopants like Ge are used to achieve a heavily doped n++ nitride layer, increasing carrier concentration without compromising material quality or surface morphology, thereby reducing contact resistance and enhancing RF performance.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If bulk doping with Ge is increased to achieve higher carrier concentration, then contact resistance is reduced, but surface pits form which are detrimental to ohmic contact reliability
Solution Approach 1:
The patent divides the doped layer into multiple thin sub-layers with alternating high and low doping concentrations. This segmentation allows the structure to achieve high overall carrier concentration while maintaining smooth surface morphology, as each thin sub-layer can be grown controllably without forming surface pits.
Solution Approach 2:
The patent applies local quality by creating regions of high doping concentration interspersed with regions of low doping concentration within the same doped layer. This local variation in doping quality enables high carrier concentration where needed while preserving surface integrity in other regions, ultimately achieving both low contact resistance and smooth surface morphology.
2Reliability
If lateral scaling of source-drain access regions is performed to reduce access resistance, then RF performance is improved, but breakdown voltage decreases
Solution Approach 1:
The patent changes the doping concentration parameter by introducing modulation doping with alternating high and low concentration regions. This parameter change enables the structure to achieve lower access resistance through high carrier concentration while the alternating structure maintains electrical field distribution that preserves breakdown voltage characteristics.
3Reliability
If Si doping concentration is increased beyond 6×10^19 cm^-3 to reduce contact resistance, then carrier concentration increases, but 3D epitaxial growth occurs resulting in un-coalesced surface
Solution Approach 1:
The patent segments the high-concentration doped layer into multiple thin sub-layers separated by low-doping regions. This segmentation allows each thin sub-layer to grow in a controlled 2D mode without transitioning to 3D growth, thereby maintaining surface coalescence while achieving high overall carrier concentration through the cumulative effect of multiple doped sub-layers.
Solution Approach 2:
The patent employs periodic action by alternating between high-doping and low-doping growth cycles. This periodic modulation of doping concentration during epitaxial growth allows the system to repeatedly achieve high carrier concentration in thin layers followed by recovery periods with low doping that maintain surface integrity, preventing 3D growth and un-coalesced surfaces.
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 significantly lower contact resistance and improved RF performance at high frequencies by achieving higher carrier concentrations in the n++ nitride layers, ensuring reliable ohmic contacts and enhanced device performance.
Implementation Method 1
modulation doping involves alternating epitaxy of high and low doped layers
Implementation Method 2
epitaxy of nitride materials on substrates (typically SiC, Si, Sapphire, or GaN, etc) with metalorganic chemical vapor deposition (MOCVD) or molecular beam epitaxy (MBE)
Data Source
AI summary
The invention relates to a method to reduce the contact resistance of ohmic contact in group III-nitride high-electron mobility transistor (HEMT). A heavily n-type doped nitride layer with modulation doping is epitaxially grown on selected contact regions for use as ohmic contact layer. The method for producing the n++ ohmic contact layer includes at least the following: deposition of nitride HEMT epitaxial structure on substrates (such as SiC, silicon, sapphire, GaN etc), deposition in-situ or ex-situ mask for selective growth of n-contact, selective etching to create of openings within the mask layer, deposition of modulation doped n++ nitride ohmic contact layer followed by ohmic metal deposition. The modulation doping involves alternating epitaxy of high and low doped nitride layers with common n-type dopant such as Ge, Si etc. The modulation doping significantly increases the range of n-type doping without detrimental effect on the material quality of the contact layer.