Author: jiangwei68
The internal quantum efficiency (IQE) of the LED epitaxial structure has a decisive influence on its brightness, and many people misunderstand that IQE is determined by the MOCVD process. In fact, IQE should be determined by the design of the epitaxial material. What is lacking in China is precisely the design talent of the extension structure. Only those who use equipment may not be able to produce high-quality materials. The following briefly describes the impact of some material parameters and structure on IQE:
1. Double heterojunction structure: The CL (Cladding Layer) on both sides has a larger band gap than AL (Active Layer). AL uses a quantum well structure to better limit carriers, thereby improving IQE. On the other hand, using quantum wells AL, Barriers hinder the movement of carriers in adjacent wells, so with multiple quantum well structures, Barrier needs to be sufficiently transparent (low and thin) to prevent carrier non-uniformity in each well. distribution. The thickness of AL also has a great influence on IQE, it should not be too thick or too thin, and each material has its optimum range.
2, AL mixed: AL can not be heavy mixed, or lightly mixed, lower than the concentration of CL, each material has its best range, AL is often not mixed. AL uses p-type doping more than n-type doping, and p-type doping ensures uniform distribution of carriers within AL. There are advantages and disadvantages of AL doping, increased doping concentration, shortened lifetime of radiating carriers, and increased radiation efficiency. However, high concentrations of impurities also introduce defects. Interestingly, MOCVD growth is sometimes dependent on doping, and impurities can act as surface activators, increasing the surface diffusion coefficient and thus improving crystal quality. For example, InGaN uses Si doping to improve crystal quality.
3, CL doping: CL resistivity is an important parameter to determine the CL concentration, the concentration must be low enough to produce a thermal effect in CL, but the CL doping must be higher than AL to define the position of the PN junction. Each material has its best range. However, the p-type concentration is typically higher than the n-type. Too low a p-type impurity concentration in CL will cause electrons to escape from the AL, thereby reducing IQE.
4. Lattice matching: Cathode fluorescence shows bright LEDs, and the intersecting dislocation lines are black. It can be inferred that the lattice mismatch increases and the IQE decreases. Although lattice matching and IQE display are strongly related in GaAs and InP, the relationship in GaN is not obvious. This is mainly due to the low electrical activity of dislocations in GaN. In addition, the diffusion length of carriers in GaN is very high. Short, if the average distance between the dislocations is greater than the diffusion length, especially the diffusion length of the holes, the non-radiative recombination on the dislocations is not serious. Another explanation is that InGaN is highly efficient because composition fluctuations in the compound limit carrier diffusion to dislocation lines.
5. PN junction shift: Generally, CL is p-type and lower CL is n-type. The PN junction shift affects IQE, especially small atoms such as Zn and Be, which can easily diffuse through AL to the lower CL. At the same time, the diffusion coefficients of Zn and Be are obviously concentration-dependent. When the concentration exceeds the limit, the diffusion rate is greatly improved, so care must be taken.
6, non-radiative composite: the surface must leave the distance of several diffusion lengths of AL, this is the MESA type LED exposure AL, its IQE is far lower than the reason of Planar's RCLED. It should be noted that if the exposed surface has only one type of carrier, it does not affect IQE.
It can be seen that MOCVD is like art multi-image technology. Many parameters are the best range in dynamic balance, and this is still a single effect. If the composite effect is more complicated, sometimes only experience, and no theory and formula, There is no feasible basis for even changing equipment. Therefore, we must look at the technical background of the technology and the technical atmosphere and foundation of the environment in which MOCVD talents are introduced. If anyone can boast that MOCVD technology can be brought up in China, it is Bragging, everyone should not be deceived. The above is just a brief description of the relationship, the specifics of everyone to explore according to practice. (Editor: Yu Zhantao)
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