The choice of substrate materials is a primary consideration for the fabrication of LED chips . Which suitable substrate should be used depends on the requirements of the device and the LED device. There are currently three materials available on the market as substrates:
Sapphire (Al2O3)
2. Silicon (Si)
3. Silicon Carbide (SiC)
Sapphire Substrates Typically, epitaxial layers of GaN-based materials and devices are grown primarily on sapphire substrates. Sapphire substrates have many advantages: First, the production technology of sapphire substrate is mature and the device quality is good. Secondly, sapphire has good stability and can be used in high temperature growth process. 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.
There are also problems with the use of sapphire as a substrate, such as lattice mismatch and thermal stress mismatch, which can create a large number of defects in the epitaxial layer, while at the same time causing difficulties in subsequent device processing. Sapphire is an insulator with a resistivity of more than 1011 Ω·cm at room temperature. In this case, a device with a vertical structure cannot be fabricated; usually, n-type and p-type electrodes are formed only on the upper surface of the epitaxial layer (as shown in Fig. 1). The fabrication of two electrodes on the upper surface results in a reduction in the effective light-emitting area and increases the lithography and etching processes in device fabrication, resulting in reduced material utilization and increased cost. Due to the difficulty in doping P-type GaN, a method of preparing a metal transparent electrode on p-type GaN is generally used to diffuse current to achieve uniform illumination. However, the metal transparent electrode generally absorbs about 30% to 40% of the light, and the GaN-based material has stable chemical properties and high mechanical strength, and is not easily etched, so that a good device is needed in the etching process. This will increase production costs.
The hardness of sapphire is very high, and its hardness is second only to diamond in natural materials, but it needs to be thinned and cut (from 400nm to about 100nm) during the fabrication of LED devices. Adding equipment to complete the thinning and cutting process adds another large investment.
The thermal conductivity of sapphire is not very good (about 25W/(m·K) at 100 °C). Therefore, when using LED devices, a large amount of heat is conducted; especially for large-area devices with large area, thermal conductivity is a very important consideration. In order to overcome the above difficulties, many people have tried to grow GaN optoelectronic devices directly on a silicon substrate to improve thermal conductivity and electrical conductivity.
Silicon substrates currently have some LED chips using silicon substrates. The chip electrodes of the silicon substrate can adopt two kinds of contact modes, namely, L-contact (Horizontal-cONtact) and V-contact (Vertical-contact), which are hereinafter referred to as L-type electrode and V-type electrode. Through these two contact methods, the current inside the LED chip can be either lateral or longitudinal. Since the current can flow longitudinally, the light-emitting area of ​​the LED is increased, thereby improving the light-emitting 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.
The LED chip electrode of a silicon carbide substrate silicon carbide substrate ( CREE Corporation of the United States exclusively uses SiC material as a substrate) is an L-type electrode, and current is longitudinally flowing. Devices fabricated using such substrates have very good electrical and thermal conductivity properties, which facilitates the fabrication of large power devices with large areas.
The thermal conductivity of the silicon carbide substrate (the thermal conductivity of silicon carbide is 490 W/(m·K)) is more than 10 times higher than that of the sapphire substrate. Sapphire itself is a poor conductor of heat, and the bottom of the device needs to use silver glue to form a crystal. The silver paste has poor heat transfer performance. The chip electrode using the silicon carbide substrate is L-shaped, and the two electrodes are distributed on the surface and the bottom of the device, and the generated heat can be directly led out through the electrode; at the same time, the substrate does not require a current diffusion layer, so the light is not subjected to current. The material of the diffusion layer is absorbed, which in turn increases the light extraction efficiency. However, compared with sapphire substrates, silicon carbide is expensive to manufacture, and commercialization requires lowering the corresponding cost.
Performance Comparison of Three Substrates The foregoing describes the three substrate materials commonly used in the fabrication of LED chips. A comparison of the overall performance of these three substrate materials can be found 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 the substrate, and are usually selected according to the design requirements.
The price of the substrate material
Sapphire (Al2O3)
2. Silicon (Si)
3. Silicon Carbide (SiC)
Sapphire Substrates Typically, epitaxial layers of GaN-based materials and devices are grown primarily on sapphire substrates. Sapphire substrates have many advantages: First, the production technology of sapphire substrate is mature and the device quality is good. Secondly, sapphire has good stability and can be used in high temperature growth process. 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.
There are also problems with the use of sapphire as a substrate, such as lattice mismatch and thermal stress mismatch, which can create a large number of defects in the epitaxial layer, while at the same time causing difficulties in subsequent device processing. Sapphire is an insulator with a resistivity of more than 1011 Ω·cm at room temperature. In this case, a device with a vertical structure cannot be fabricated; usually, n-type and p-type electrodes are formed only on the upper surface of the epitaxial layer (as shown in Fig. 1). The fabrication of two electrodes on the upper surface results in a reduction in the effective light-emitting area and increases the lithography and etching processes in device fabrication, resulting in reduced material utilization and increased cost. Due to the difficulty in doping P-type GaN, a method of preparing a metal transparent electrode on p-type GaN is generally used to diffuse current to achieve uniform illumination. However, the metal transparent electrode generally absorbs about 30% to 40% of the light, and the GaN-based material has stable chemical properties and high mechanical strength, and is not easily etched, so that a good device is needed in the etching process. This will increase production costs.
The hardness of sapphire is very high, and its hardness is second only to diamond in natural materials, but it needs to be thinned and cut (from 400nm to about 100nm) during the fabrication of LED devices. Adding equipment to complete the thinning and cutting process adds another large investment.
The thermal conductivity of sapphire is not very good (about 25W/(m·K) at 100 °C). Therefore, when using LED devices, a large amount of heat is conducted; especially for large-area devices with large area, thermal conductivity is a very important consideration. In order to overcome the above difficulties, many people have tried to grow GaN optoelectronic devices directly on a silicon substrate to improve thermal conductivity and electrical conductivity.
Silicon substrates currently have some LED chips using silicon substrates. The chip electrodes of the silicon substrate can adopt two kinds of contact modes, namely, L-contact (Horizontal-cONtact) and V-contact (Vertical-contact), which are hereinafter referred to as L-type electrode and V-type electrode. Through these two contact methods, the current inside the LED chip can be either lateral or longitudinal. Since the current can flow longitudinally, the light-emitting area of ​​the LED is increased, thereby improving the light-emitting 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.
The LED chip electrode of a silicon carbide substrate silicon carbide substrate ( CREE Corporation of the United States exclusively uses SiC material as a substrate) is an L-type electrode, and current is longitudinally flowing. Devices fabricated using such substrates have very good electrical and thermal conductivity properties, which facilitates the fabrication of large power devices with large areas.
The thermal conductivity of the silicon carbide substrate (the thermal conductivity of silicon carbide is 490 W/(m·K)) is more than 10 times higher than that of the sapphire substrate. Sapphire itself is a poor conductor of heat, and the bottom of the device needs to use silver glue to form a crystal. The silver paste has poor heat transfer performance. The chip electrode using the silicon carbide substrate is L-shaped, and the two electrodes are distributed on the surface and the bottom of the device, and the generated heat can be directly led out through the electrode; at the same time, the substrate does not require a current diffusion layer, so the light is not subjected to current. The material of the diffusion layer is absorbed, which in turn increases the light extraction efficiency. However, compared with sapphire substrates, silicon carbide is expensive to manufacture, and commercialization requires lowering the corresponding cost.
Performance Comparison of Three Substrates The foregoing describes the three substrate materials commonly used in the fabrication of LED chips. A comparison of the overall performance of these three substrate materials can be found 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 the substrate, and are usually selected according to the design requirements.
The price of the substrate material
1. Structural matching of the substrate and the epitaxial film: the crystal structure of the epitaxial material and the substrate material are the same or similar, the lattice constant mismatch is small, the crystallization property is good, and the defect density is low;
2. Matching the thermal expansion coefficient 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 may not only reduce the quality of the epitaxial film, but also cause heat during the operation of the device. 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, and it is not easy to decompose and corrode in the temperature and atmosphere of epitaxial growth. The quality of the epitaxial film cannot be degraded due to the chemical reaction with the epitaxial film. ;
4. The difficulty of material preparation and the cost: In view of the needs of industrial development, the preparation of substrate materials is simple and the cost is not high. The substrate size is generally not less than 2 inches.
There are currently 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 there is still a long way to go from industrialization.
2. Matching the thermal expansion coefficient 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 may not only reduce the quality of the epitaxial film, but also cause heat during the operation of the device. 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, and it is not easy to decompose and corrode in the temperature and atmosphere of epitaxial growth. The quality of the epitaxial film cannot be degraded due to the chemical reaction with the epitaxial film. ;
4. The difficulty of material preparation and the cost: In view of the needs of industrial development, the preparation of substrate materials is simple and the cost is not high. The substrate size is generally not less than 2 inches.
There are currently 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 there is still a long way to go from industrialization.
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