The GaN lattice mismatch grown on silicon carbide substrates is low. The low lattice mismatch of gallium nitride on SiC wafers indicates that the lattices of layer 1 and layer 2 match each other. The better the match, the fewer defects, and the better the performance and lifetime of the device. The higher the degree of lattice mismatch, the greater the possibility of defects.
Generally speaking, the mismatch less than 5% means it’s easy to grow, 5%-25% means it can grow, and more than 25% means it can’t grow. Vapor phase epitaxy generally requires a mismatch degree of less than 10%, liquid phase epitaxy requires a mismatch degree of less than 1%, and optoelectronic heterojunctions require less than 0.1%.
Why use layer 1 and layer 2? Because the concept is also applicable between the epitaxial layer and the epitaxial layer.
According to the optimal matching direction to match the film, the substrate needs to select the appropriate atomic period length.
The periodic arrangement of atoms in the trigonal and hexagonal crystals can be a, √3a, 2a (corresponding to 3.185, 5.517, 6.370 of the hexagonal GaN).
The arrangement period of cubic crystal atoms can be a/√2, a, √2a (corresponding to 3.21, 4.54, 6.42 of cubic GaN).
GaN Lattice mismatch=∣(epitaxial film atomic arrangement period-substrate atomic arrangement period)∣/epitaxial film atomic arrangement period.
Substrates for growing GaN are as follows:
|Substrates||Lattice constant aA||Lattice constant cA||Lattice mismatch %||Thermal expansion coefficient 10^-6/K||Thermal mismatch %|
- Among all the materials in the table, the GaN, 4H-SiC and 6H-SiC materials are from Xiamen Powerway Advanced Material Co., Ltd.
The method for determining the GaN(gallium nitride) lattice mismatch is very simple. Assuming that the Gan lattice mismatch on SiC substrate is zero, the interplanar spacing in the obtained XRD spectrum should be consistent. The interplanar spacing can be directly equivalent to the atomic arrangement period, but if the degree of mismatch exceeds 25%, lattice rotation should be considered and recalculated according to the matched atomic arrangement period. For example, the matching of GaN and Al2O3.
If the GaN lattice mismatch layer must be grown epitaxially, some epitaxial layers for matching are generally added in the middle. For example, after a layer of TiN or TiC is grown on a sapphire substrate, GaN is grown, and then the GaN layer can be peeled off, which is the self-supporting GaN.