Compared with single crystal silicon and single crystal silicon carbide, sapphire has the advantages of good mechanical properties, high reliability, mature material preparation technology and easy control. However, there is about 17% lattice mismatch and about 26% thermal expansion coefficient mismatch between GaN and sapphire. Therefore, there will be a dislocation density of 109~1010 cm−2 in the GaN epitaxial layer grown on the LED sapphire substrate.
The patterned LED sapphire substrate can significantly reduce the linear dislocation density in the GaN epitaxial layer because the microstructure of the surface changes the growth process of the GaN material. The dislocations of the GaN material grown in the planar region between the substrate patterns will extend upward to the surface of the epitaxial layer. However, when the dislocation direction of the GaN material grown on the side surface of the substrate pattern is bent by 90˚, the dislocation line will not reach the surface of the epitaxial layer, thereby effectively reducing the dislocation density on the surface of the GaN epitaxial layer.
The lower dislocation density reduces the non-radiative recombination centers in the active region, alleviates carrier scattering, and improves the internal quantum efficiency. It is worth noting that the area of the side area of the pattern has a greater influence on the dislocation density. Compared with the trapezoidal patterned sapphire substrate, the conical patterned sapphire substrate has a larger proportion of the sides, so it is more helpful to suppress epitaxy. Line dislocations on the surface of the layer.
Increase light extraction efficiency
The patterned sapphire substrate helps the LED to increase the light extraction rate, and the rough GaN sapphire interface can scatter the photons emitted from the active area, so that the originally totally reflected photons have the opportunity to exit the device, thereby effectively improving the light extraction efficiency.