LED epitaxial wafers: a comprehensive comparison of five substrate materials

LED epitaxial wafers: a comprehensive comparison of five substrate materials

2021-11-09 17:00:16 5

Different substrate materials, requiring different epitaxial growth technology, chip processing technology and device packaging technology, substrate materials determine the development of semiconductor lighting technology route. The choice of substrate materials depends on the following nine aspects.


  [1] good structural properties, epitaxial materials and substrates with the same or similar crystal structure, lattice constant mismatch is small, good crystallization properties, small defect density;


  [2] Good interfacial properties, conducive to epitaxial material nucleation and strong adhesion;


  [3] Good chemical stability, not easy to decompose and corrode in the temperature and atmosphere of epitaxial growth;


  [4] Good thermal properties, including good thermal conductivity and low thermal mismatch;


  [5] Good electrical conductivity, capable of making up and down structures;


  [6] Good optical properties, the light emitted from the device is absorbed by the substrate;


  [7] Good mechanical properties, easy processing of the device, including thinning, polishing and cutting, etc;


  [8] Low price;


  [9] Large size, generally requiring a diameter of not less than 2 inches.


  It is very difficult to select substrates to satisfy all the above nine aspects at the same time. Therefore, at present, we can only adapt the development and production of semiconductor light-emitting devices on different substrates by changing the epitaxial growth technology and adjusting the device processing process. There are more substrate materials used for GaN research, but there are only two kinds of substrates that can be used for production, namely sapphire Al2O3 and silicon carbide SiC substrates. Table 2-4 provides a qualitative comparison of the performance of the five substrate materials used for GaN growth.


Substrate Materials


Al2O3


SiC


Si


ZnO


GaN


Lattice-mismatching 

degree


Poor


Medium


Poor


Good


Excellent


Interface Features


Good


Good


Good


Good


Excellent


Chemical stability


Excellent


Excellent


Good


Poor


Excellent


Thermal conductivity


Poor


Excellent


Excellent


Excellent


Excellent


Thermal mismatch


Poor


Medium


Poor


Poor


Excellent


Electrical Conductivity


Poor


Excellent


Excellent


Excellent


Excellent


Optical Performance


Excellent


Excellent


Poor


Excellent


Excellent


Mechanical properties


Poor


Poor


Excellent


Good


Medium


Price


Medium


High


Low 


High


High


Size


Medium


Medium


Big


Medium


Small

Table 2-4: Comparison of the performance of substrate materials for GaN growth

1) GaN substrate


  The most ideal substrate for GaN growth is naturally GaN single crystal material, which can greatly improve the crystal quality of epitaxial film, reduce dislocation density, improve device working life, improve luminous efficiency and increase device working current density. However, it is very difficult to prepare GaN bulk single crystal material, and there is no effective method so far. Some researchers grow GaN thick film on other substrates (such as Al2O3, SiC, LGO) by HVPE method, and then separate the substrate and GaN thick film by exfoliation technique, and the separated GaN thick film can be used as substrate for epitaxy. The advantages of GaN thick film obtained in this way are obvious, namely, the dislocation density of GaN film epitaxially grown on GaN as substrate is significantly lower than that of GaN film epitaxially grown on Al2O3 and SiC; however, it is expensive. Therefore, the use of GaN thick film as a substrate for semiconductor lighting is limited.


  GaN substrate production technology and equipment.


  Lack of GaN substrate is one of the main difficulties hindering the research of GaN, and is also the fundamental reason for the progress of GaN light-emitting devices to stop again at present! Although some people have obtained single crystal gallium nitride body material from high pressure melt, but the size is very small and unusable, and it is mainly grown on sapphire, silicon and silicon carbide substrates at present. Although medium and low-grade GaN LED products can be produced on sapphire substrates, high-grade products can only be produced on GaN substrates. At present, only a few Japanese companies can provide GaN substrates, the price is very expensive, a 2-inch substrate price of about 10,000 U.S. dollars, these substrates are all produced by HVPE (hydride vapor phase epitaxy).


  HVPE is the technology of the 1960s and 1970s, because of its fast growth rate (more than one micron a minute), it cannot grow quantum wells, superlattices and other structural materials, and was eliminated by MOCVD, MBE and other technologies in the 1980s. However, because of its fast growth rate and the ability to grow GaN substrates, this technology is making a comeback and gaining attention again. It can be concluded that GaN substrates will definitely continue to develop and industrialize, and HVPE technology will definitely be regained attention. Compared with the high pressure pulling method, HVPE method is more likely to produce practical GaN substrates. However, there is no commercialized equipment for sale in the international market yet.


  At present, domestic and international research on GaN substrates is carried out separately with MOCVD and HVPE equipment. That is, MOCVD is used to grow 0.1~1 micron crystalline layer first, then HVPE is used to grow about 300 micron GaN substrate layer, and finally the original substrate is stripped and polished. Since a substrate needs to be grown twice in two growth chambers, it needs to be cooled down, stopped and taken out, which will inevitably lead to the following problems: (1) the sample surface is sticky and dirty; (2) the surface is reconstructed due to the growth stop and cool down, which affects the next growth.


  The focus of future research and development is still to find a suitable growth method and significantly reduce its cost.


2) Al2O3 substrate


  Currently, the most common substrate used for GaN growth is Al2O3, whose advantages are good chemical stability, no visible light absorption, moderate price, and relatively mature manufacturing technology. The large thermal mismatch forms a compressive stress on the epitaxial layer and thus does not crack. However, the poor thermal conductivity does not reveal significant deficiencies under low-current operation of the device, but the problem is prominent under high-current operation of power-type devices.


  The future research and development tasks for domestic and foreign Al2O3 substrates are to grow large diameter Al2O3 single crystals in the direction of 4-6 inches, as well as to reduce impurity contamination and improve the quality of surface polishing.


3) SiC substrate


  In addition to Al2O3 substrate, SiC is currently used for GaN growth substrate, which has the second largest share in the market, and there is no third substrate for commercial production of GaN LED. It has many outstanding advantages, such as good chemical stability, good electrical conductivity, good thermal conductivity, no visible light absorption, etc. However, the shortcomings are also very prominent, such as the price is too high, the crystal quality is difficult to reach Al2O3 and Si so good, and the mechanical processing performance is relatively poor. In addition, SiC substrate absorbs UV light below 380 nm, which is not suitable for developing UV LEDs below 380 nm. Therefore, it occupies an important position in the developing field of semiconductor lighting technology.


  At present, the only manufacturer that can provide high quality SiC substrates for commercial use is CREE in the United States. The task of future research and development of SiC substrates at home and abroad is to significantly reduce manufacturing cost and improve crystal crystallization quality.


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