The patterned 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 plane area between the substrate patterns will extend upward to the surface of the epitaxial layer, but the GaN material grown on the side of the substrate pattern will be bent by 90˚ in the direction of the dislocations, resulting in The dislocation line cannot 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 pattern sapphire substrate, the conical pattern sapphire substrate has a larger proportion of the sides, so it is more helpful to suppress the epitaxial layer surface. The line is misaligned. Similarly, increasing the density of the pattern also helps to increase the specific gravity of the side area, thereby reducing dislocations. Therefore, the patterned sapphire substrate facilitates the growth of high-quality GaN epitaxial layers.
The patterned sapphire substrate helps the LED to increase the light extraction rate. Due to the existence of the microstructure on the substrate surface, the photons that may have been confined to the active area are reflected multiple times by the sapphire substrate, changing the propagation path, and when the angle is in the escape When the angle is within the range, the photon successfully escapes. Therefore, the patterned sapphire substrate can increase the probability of photon escape and increase the LED light extraction rate.
In the initial stage of the development of patterned sapphire substrates, the research on patterns stayed at the micrometer scale. With the development of nanotechnology, the research on substrate patterns has also developed towards the nanometer level. A large number of studies have shown that nano-patterned sapphire substrates (NPSS) have advantages over micro-patterned sapphire substrates (MPSS) in terms of growth quality from the epitaxial layer or light escape efficiency. The reason is that as the size of the pattern shrinks, the gap between the substrate and the epitaxial layer is more likely to occur, which further reduces the contact area between the substrate and the epitaxial layer, so that there are fewer epitaxial layer dislocations caused by lattice mismatch and thermal mismatch. In summary, the reduction in the size of the pattern helps to improve the quality of the epitaxial layer and improve the efficiency of light extraction.
