Selection of LED substrate materials

When making LED chips, the choice of substrate material is the primary consideration. The appropriate substrate should be selected according to the requirements of the equipment and LED devices. There are generally three types of materials available on the market that can be used as substrates:

Sapphire (Al2O3)

Silicon (Si)

Silicon Carbide (SiC)

Sapphire substrate

Usually, the epitaxial layers of GaN-based materials and devices 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; second, sapphire has good stability and can be used in high-temperature growth processes; finally, sapphire has high mechanical strength and is easy to handle and clean. Therefore, most processes generally use sapphire as the substrate. Figure 1 shows an example of an LED chip made using a sapphire substrate.

Figure 1 LED chip with sapphire as substrate

There are also some problems with using sapphire as a substrate, such as lattice mismatch and thermal stress mismatch, which will produce a large number of defects in the epitaxial layer and cause difficulties for subsequent device processing. Sapphire is an insulator with a resistivity greater than 1011Ω·cm at room temperature. In this case, it is impossible to make devices with vertical structures; usually only n-type and p-type electrodes are made on the upper surface of the epitaxial layer (as shown in Figure 1). Making two electrodes on the upper surface reduces the effective light-emitting area and increases the photolithography and etching processes in device manufacturing, resulting in reduced material utilization and increased costs. Due to the difficulty of doping p-type GaN, the method of preparing metal transparent electrodes on p-type GaN is currently widely used to diffuse the current to achieve uniform light emission. However, metal transparent electrodes generally absorb about 30% to 40% of light. At the same time, the chemical properties of GaN-based materials are stable and the mechanical strength is high, so it is not easy to etch them. Therefore, better equipment is required during the etching process, which will increase production costs.

Sapphire is very hard, second only to diamond in hardness among natural materials, but it needs to be thinned and cut (from 400nm to about 100nm) during the production of LED devices. Adding equipment to complete the thinning and cutting process requires a large investment.

Sapphire's thermal conductivity is not very good (about 25W/(m·K) at 100°C). Therefore, when using LED devices, a lot of heat will be conducted; especially for large-area high-power devices, 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 silicon substrates to improve thermal and electrical conductivity.

Silicon substrate

At present, some LED chips use silicon substrates. The chip electrodes of silicon substrates can adopt 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 vertically, 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.

Silicon carbide substrate

The LED chip electrode of silicon carbide substrate (CREE in the United States uses SiC material as substrate) is L-shaped electrode, and the current flows vertically. The device made of this substrate has very good electrical and thermal conductivity, which is conducive to making large-area high-power devices. The LED chip using silicon carbide substrate is shown in Figure 2.

Figure 2 LED chips using sapphire substrate and silicon carbide substrate

The thermal conductivity of silicon carbide substrates (the thermal conductivity of silicon carbide is 490W/(m·K)) is more than 10 times higher than that of sapphire substrates. Sapphire itself is a poor conductor of heat, and silver glue is required to solidify the bottom of the device when making the device. The heat transfer performance of this silver glue is also poor. The chip electrodes using silicon carbide substrates are L-shaped, and the two electrodes are distributed on the surface and bottom of the device. The heat generated can be directly discharged through the electrodes; at the same time, this substrate does not require a current diffusion layer, so the light will not be absorbed by the material of the current diffusion layer, which improves the light extraction efficiency. However, compared with sapphire substrates, the manufacturing cost of silicon carbide is relatively high, and its commercialization requires the corresponding cost to be reduced.

Performance comparison of three substrates

The previous content introduced the three substrate materials commonly used in making LED chips. The comprehensive performance comparison of these three substrate materials can be seen in Table 1.

Table 1 Performance comparison of three substrate materials

In addition to the three commonly used substrate materials mentioned above, GaAS, AlN, ZnO and other materials can also be used as substrates, and are usually selected according to design requirements.