Comparative analysis of three kinds of substrate materials used in LED chip

Review: Currently there are generally three kinds of materials can be used as substrate: sapphire (Al2O3), silicon (Si), silicon carbide (SiC).

For the fabrication of LED chips, the choice of substrate materials is the primary consideration. What kind of substrate should be used requires the selection of equipment and LED devices. Currently there are generally three kinds of materials can be used as substrate:

1 sapphire (Al2O3)

2 silicon (Si)

3 silicon carbide (SiC)

Sapphire substrate

In general, the epitaxial layers of GaN based materials and devices are mainly grown on sapphire substrates. The sapphire substrate has many advantages: first, the sapphire substrate production technology is mature, the quality of the device is better; secondly, sapphire's stability is very good, can be used in high-temperature growth process; finally, the mechanical strength of sapphire is high, easy handling and cleaning. Therefore, most of the processes are generally sapphire substrate. There are some problems in the use of sapphire as substrate, such as lattice mismatch and thermal stress mismatch, which will produce a large number of defects in the epitaxial layer. Sapphire is an insulator, the resistivity at room temperature is greater than 1011 (CM), in this case can not produce vertical structure of the device; usually only on the surface of the epitaxial layer of N and P type electrode (see Figure 2). When the two electrodes are made on the upper surface, the effective light emitting area is reduced, and the photoetching and etching process in the manufacture of the device is increased. Because of the difficulty of P type GaN doping, it is widely used to prepare metal transparent electrode on P type GaN, so as to achieve the purpose of uniform luminescence. But the metal transparent electrode to absorb about 30%~40% light, and the chemical properties of GaN based material stability, high mechanical strength, not easy to be so good for the etching, etching equipment in the process, it will increase the cost of production.

Comparative analysis of three kinds of substrate materials used in LED chip

Sapphire hardness is very high, in natural materials, the hardness of the second only to diamond, but in the production process of LED devices, but it needs to be thinning and cutting (from 400nm to about 100nm). Purchase of equipment to complete the thinning and cutting process to increase a larger investment. The thermal conductivity of sapphire is not very good (at about 100 25W/ (M - K)). Therefore, in the use of LED devices, a large amount of heat will be derived, especially for large areas of large power devices, thermal conductivity is a very important consideration. In order to overcome these difficulties, a lot of people try to improve the thermal conductivity and electrical conductivity of GaN devices directly on silicon substrate.

Silicon substrate

At present, some LED chips are made of silicon substrate. The silicon substrate chip electrode can be used in two ways, namely L contact (Laterial-cONtact, horizontal contact) and V contact (Vertical-contact, vertical contact), hereinafter referred to as L type electrode and V electrode. Through these two methods, the current in the LED chip can be either transverse or longitudinal. Because the current can flow in a longitudinal direction, the luminous area of the LED is increased, thereby improving the luminous efficiency of the LED. Because silicon is a good conductor of heat, the thermal conductivity of the device can be significantly improved, thus prolonging the life of the device.

SiC substrate

The LED chip electrode of the silicon carbide substrate is a L type electrode. The electric conductivity and the thermal conductivity of the device made by the substrate are very good, and the utility model is favorable for the large power device with larger area. The thermal conductivity of silicon carbide (490W/) is 10 times higher than that of sapphire substrate (M = K). Sapphire itself is a poor conductor of heat, and at the bottom of the device need to use silver colloidal solid crystal, the heat transfer properties of silver glue is also very poor.

The chip electrode using silicon carbide substrate for L type two electrode distribution on the surface of the device and the bottom, the heat generated by directly derived electrode; while the substrate does not need the current diffusion layer, so the light absorption is not the current diffusion layer material, and high light efficiency. But compared with the sapphire substrate, the higher cost of silicon carbide manufacturing, to achieve its commercialization also need to reduce the cost.

Performance of three substrates

Comparison of the contents of the presentation is the production of LED chips commonly used three substrate materials. Table 1 Comparison of the performance of these three kinds of substrate materials. In addition to the above three commonly used substrate materials, as well as GaAS, AlN, ZnO and other materials can also be used as a substrate, usually designed according to the need to choose to use.