Nitride Semiconductor Device Lattice Mismatch Reduction
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Solution Overview
Problem
Nitride semiconductor layers grown on silicon substrates suffer from poor crystallinity due to lattice mismatch, leading to high dislocation density, reduced electron mobility, and increased on-resistance in field-effect transistors.
Innovation Solution
A nitride semiconductor device structure is developed where the silicon substrate's crystal axis is inclined relative to the nitride semiconductor layer's axis, reducing lattice mismatch and improving crystallinity, with additional layers such as AlGaN and GaN to enhance electron mobility and withstand voltage.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Productivity
If a nitride semiconductor layer is grown on a silicon substrate, then mass production and cost reduction are achieved, but lattice mismatch causes high dislocation density and poor crystallinity
Solution Approach 1:
An aluminum nitride (AlN) buffer layer is introduced as an intermediary between the silicon substrate and the nitride semiconductor layer. This buffer layer mediates the lattice mismatch between silicon and GaN, reducing dislocation density and improving crystallinity while maintaining the advantage of using silicon substrates for mass production
2Area of stationary object
If a nitride semiconductor layer is grown on a silicon substrate, then large diameter substrates can be used, but thermal expansion difference generates tensile stress and defects
Solution Approach 1:
The aluminum nitride buffer layer serves as a stress buffer that absorbs and distributes the tensile stress generated by thermal expansion differences between silicon and nitride semiconductor during cooling, preventing crack formation and reducing defect density in large diameter substrates
Solution Approach 2:
The buffer layer changes the thermal and mechanical parameters of the substrate system, providing a transition zone that accommodates thermal expansion differences and reduces the impact of temperature changes from growth temperature to room temperature
3Ease of manufacture
If gallium nitride is formed on a silicon substrate, then silicon substrate advantages are utilized, but gallium combines with silicon reducing flatness
Solution Approach 1:
The aluminum nitride buffer layer acts as a barrier that prevents gallium from combining with silicon, maintaining surface flatness while still allowing the use of silicon substrates for manufacturing advantages
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 improves the crystallinity of nitride semiconductor layers, reducing on-resistance and enhancing electron mobility, thereby improving the performance of field-effect transistors.
Implementation Method 1
electric charges are generated by spontaneous polarization or piezoelectric polarization
Implementation Method 2
electric charges are generated by spontaneous polarization or piezoelectric polarization
Implementation Method 3
The sheet carrier concentration in the hetero-interface becomes 1×10^13 cm^-2 or more by these polarization effects even when this semiconductor is not particularly doped. It is therefore possible to make use of a two-dimensional electron gas (2DEG) in the hetero-interface
Implementation Method 4
it is necessary to form, by a vapor growth method, the nitride semiconductor on a substrate made of a material different in kind from the nitride semiconductor to be grown
Implementation Method 5
the nitride semiconductor is larger in thermal expansion coefficient than silicon, and a difference therebetween is also large. Moreover, crystal growth of the nitride semiconductor is generally conducted at a high temperature of about 1000° C.; thus, when a film of the nitride semiconductor is formed on the silicon substrate at a high temperature and subsequently the temperature of the substrate is lowered to room temperature, tensile stress is easily generated in the nitride semiconductor by a difference in thermal expansion coefficient
Data Source
AI summary
An object of the present invention is to provide a nitride semiconductor device and a nitride semiconductor substrate in each of which a nitride semiconductor layer formed on a silicon substrate is improved in crystallinity to realize a decrease in on-resistance of a field-effect transistor. The nitride semiconductor device includes a silicon substrate, and a first nitride semiconductor layer formed over the silicon substrate and including a nitride semiconductor, wherein a Si <111> axial direction of the silicon substrate is different from a <0001> axial direction of the first nitride semiconductor layer.