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 substrate
Typically, epitaxial layers of GaN-based materials and devices are primarily grown 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.
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