How to improve the internal quantum efficiency of LED epitaxial structures

The internal quantum efficiency (IQE) of LED epitaxial structure has a decisive influence on its brightness . Many people misunderstand that IQE is determined by MOCVD process. In fact, IQE should be determined by the design of epitaxial materials. What is lacking in China is precisely the design talents of epitaxial structure. People who only know how to use equipment may not be able to grow high-quality materials. The following is a brief introduction to the influence of some material parameters and structures on IQE:

1. Double heterojunction structure: The bandgap width of the CL (Cladding Layer) on both sides should be larger than that of the AL (Active Layer). The quantum well structure of the AL can better limit the carriers, thereby improving the IQE. On the other hand, with the quantum well AL, barriers hinder the movement of carriers in adjacent wells , so a multi-quantum well structure is used, and the barriers need to be transparent enough (low and thin) to prevent uneven distribution of carriers in each well. The thickness of the AL also has a great influence on the IQE. It should not be too thick or too thin. Each material has its optimal range.

2. AL doping: AL must not be heavily doped, or lightly doped, with a doping concentration lower than that of CL. Each material has its optimal range, and AL is often not doped. AL uses p-type doping more than n-type doping. P-type doping can ensure uniform distribution of carriers in AL. AL doping has both advantages and disadvantages. As the doping concentration increases, the lifetime of radiated carriers is shortened, and the radiation efficiency is increased. However, high-concentration doping also introduces defects. Interestingly, MOCVD growth sometimes depends on doping. Impurities can act as surfactants to increase the surface diffusion coefficient, thereby improving crystal quality. For example, InGaN can improve crystal quality by using Si doping.

3. CL doping: The resistivity of CL is an important parameter that determines the CL concentration. The concentration must be low enough not to produce thermal effects in CL, but the CL doping must be higher than AL to define the position of the PN junction . Each material has its optimal 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 AL, thereby reducing IQE.

4. Lattice matching: The crossed dislocation lines of the LED that is lit by cathode fluorescence display appear black. It can be inferred that the lattice mismatch increases and the IQE decreases. Although there is a strong relationship between lattice matching and IQE display in GaAs and InP, this relationship is not obvious in GaN. This is mainly because the electrical activity of dislocations in GaN is very low. In addition, the diffusion length of carriers in GaN is very short. If the average distance between dislocations is greater than the diffusion length, especially the diffusion length of holes, then the non-radiative recombination on the dislocations is not serious. Another explanation is that the reason why InGaN has high efficiency is that the fluctuation of the composition of the compound limits the diffusion of carriers to the dislocation lines.

5. PN junction shift: Generally, the upper CL is p-type and the lower CL is n-type. PN junction shift will affect 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 increased, so you must be very careful.

6. Non-radiative recombination: The surface must be several diffusion lengths away from the AL. This is why the IQE of MESA-type LEDs exposed to the AL is much lower than that of Planar's RCLEDs. It should be pointed out that if the exposed surface has only one type of carrier, it will not affect the IQE.

It can be seen that MOCVD is more like art than technology. Many parameters are selected in the best range in a dynamic balance, and this is still based on a single effect. If the composite effect is more complicated, sometimes there is only experience, but no theory and formula, and even no feasible basis for changing equipment. Therefore, when introducing MOCVD talents, we must look at their technical skills and the technical atmosphere and heritage of their environment. If someone boasts that they can immediately develop MOCVD technology in China, that is bragging, and everyone must not be fooled. The above is just a brief description of the relationship, and everyone still needs to explore the details based on practice.