For the production of LED chips, the choice of substrate material is the primary consideration. Which suitable substrate should be used depends on the requirements of the equipment and LED device. At present, there are generally three materials on the market that can be used as substrates:
1. Sapphire (Al2O3)
2. Silicon (Si)
3. Silicon carbide (SiC)
Sapphire substrate
Generally, GaN-based materials and device epitaxial layers are mainly grown on sapphire substrates. Sapphire substrates have many advantages: first, the production technology of sapphire substrates is mature and the device quality is good; secondly, the stability of sapphire is very good and can be used in the process of high temperature growth; finally, sapphire has high mechanical strength and is easy to handle And cleaning. Therefore, most processes generally use sapphire as the substrate. Figure 1 illustrates an LED chip made using a sapphire substrate.
The use of sapphire as a substrate also has some problems, such as lattice mismatch and thermal stress mismatch, which will cause a large number of defects in the epitaxial layer, and at the same time cause difficulties for subsequent device processing processes. Sapphire is an insulator with a resistivity greater than 1011Ω at room temperature. cm, in this case, devices with a vertical structure cannot be fabricated; usually only n-type and p-type electrodes are fabricated on the upper surface of the epitaxial layer (as shown in Figure 1). Making two electrodes on the upper surface results in a reduction in the effective light-emitting area, and at the same time increases the photolithography and etching processes in device manufacturing, resulting in reduced material utilization and increased cost. Due to the difficulty in doping P-type GaN, the current method of preparing a metal transparent electrode on p-type GaN is generally used to diffuse current to achieve uniform light emission. However, metal transparent electrodes generally absorb about 30% to 40% of the light. At the same time, GaN-based materials have stable chemical properties and high mechanical strength, and it is not easy to etch them. Therefore, better equipment is needed during the etching process. This will increase production costs.
The hardness of sapphire is very high, which is second only to diamond in natural materials, but it needs to be thinned and cut (from 400nm to about 100nm) in the manufacturing process of LED devices. Adding equipment to complete the thinning and cutting process requires a larger investment.
The thermal conductivity of sapphire is not very good (about 25W / (mK) at 100 ° C). Therefore, when using LED devices, a large amount of heat will be conducted; especially for large-power devices with large areas, thermal conductivity is a very important consideration. In order to overcome the above difficulties, many people try to grow GaN optoelectronic devices directly on silicon substrates, thereby improving thermal and electrical conductivity.
Silicon substrate
At present, some LED chips use a silicon substrate. The chip electrode of the silicon substrate can use two contact methods, namely L contact (Laterial-cONtact, horizontal contact) and V contact (Vertical-contact, vertical contact), hereinafter referred to as L-type electrode and V-type electrode. Through these two contact methods, the current inside the LED chip can flow horizontally or vertically. Since the current can flow longitudinally, the light emitting area of ​​the LED is increased, thereby improving the light extraction efficiency of the LED. Because silicon is a good conductor of heat, the thermal conductivity of the device can be significantly improved, thereby extending the life of the device.
Silicon carbide substrate
The LED chip electrode of the silicon carbide substrate (cree company in the United States specially uses SiC material as the substrate) is an L-shaped electrode, and the current flows longitudinally. The devices made by using this kind of substrate have very good electrical and thermal conductivity, which is beneficial for making a large-area high-power device.
The thermal conductivity of silicon carbide substrate (the thermal conductivity of silicon carbide is 490W / (mK)) is more than 10 times higher than that of sapphire substrate. Sapphire itself is a poor conductor of heat, and it is necessary to use a silver glue to fix the bottom of the device when manufacturing the device. This silver glue also has poor heat transfer performance. The chip electrode using a silicon carbide substrate is L-shaped. The two electrodes are distributed on the surface and bottom of the device. The heat generated can be directly exported through the electrode; at the same time, this substrate does not require a current diffusion layer, so light is not affected by current. The material of the diffusion layer absorbs, which in turn improves the light extraction efficiency. However, compared with sapphire substrates, the manufacturing cost of silicon carbide is relatively high, and the corresponding cost needs to be reduced to realize its commercialization.
Performance comparison of three substrates
The previous content introduces three kinds of substrate materials commonly used in making LED chips. The comprehensive performance comparison of these three substrate materials can be seen in Table 1.
In addition to the above three commonly used substrate materials, materials such as GaAS, AlN, and ZnO can also be used as substrates, which are usually selected and used according to design needs.
The price of the substrate material 1. The structure matching of the substrate and the epitaxial film: the crystal structure of the epitaxial material and the substrate material is the same or similar, the lattice constant mismatch is small, the crystallization performance is good, and the defect density is low;
2. The thermal expansion coefficient matching of the substrate and the epitaxial film: the matching of the thermal expansion coefficient is very important. The difference between the thermal expansion coefficient of the epitaxial film and the substrate material is not only possible to reduce the quality of the epitaxial film, but also due to heat generation during device operation And cause damage to the device;
3. The chemical stability of the substrate and the epitaxial film: the substrate material should have good chemical stability, it is not easy to decompose and corrode in the temperature and atmosphere of epitaxial growth, and the quality of the epitaxial film cannot be reduced due to the chemical reaction with the epitaxial film ;
4. The difficulty of material preparation and the level of cost: Considering the needs of industrial development, the preparation requirements of substrate materials are simple, and the cost should not be very high. The substrate size is generally not less than 2 inches.
Currently, there are many substrate materials for GaN-based LEDs, but there are currently only two types of substrates that can be used for commercialization, namely sapphire and silicon carbide substrates. Other substrates such as GaN, Si, and ZnO are still in the research and development stage, and are still some distance away from industrialization.
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