In the preparation process of LEDs, the upstream substrate material is the main factor determining the performance indicators such as LED color, brightness, and lifetime. The roughness of the surface of the substrate material, the coefficient of thermal expansion, the coefficient of thermal conductivity, the influence of polarity, the processing requirements of the surface, and the matching between the lattices of the epitaxial materials are closely related to the luminous efficiency and stability of the high-brightness LED. Therefore, the substrate material is the cornerstone of the technological development of the semiconductor lighting industry. The technical route of the substrate material will inevitably affect the technical route of the entire industry and is the key to the entire industrial chain.
At present, there are three main technical routes for semiconductor lighting, namely the sapphire substrate LED technology route represented by Japan Nichia Chemical, the silicon carbide substrate LED technology route represented by American CREE , and the silicon represented by China Crystal Energy Optoelectronics. Substrate LED technology route. On January 8, 2016, Nanchang University and Jingfeng Optoelectronics' Jiang Fengyi team's "High-efficiency GaN-based blue light-emitting diode on silicon substrate" technology project won the first prize of the 2015 National Science and Technology Award for technical inventions. LED Industry China The core dream was once again motivated. The award of the silicon substrate technology project indicates that the silicon substrate has been proven by the state and has been upgraded to the national strategic level. Silicon substrates will be ushered in commercial applications of scale.
Table 1 Comparison of three main substrate materials of GaN-based LED
Figure 1 LED market share of various substrate materials (Source: YOLE, CITIC Jiantou Securities Research and Development Department)
On October 7, 2014, the Royal Swedish Academy of Sciences announced that Akasaki, Amano and Nakamura had won the 2014 Nobel Prize in Physics for inventing the "High Efficiency Blue LED". The outstanding contributions of these three scientists were that in 1993 they broke through the core technology of preparing high-efficiency GaN-based blue LEDs on sapphire substrates. For more than 20 years, GaN-based blue LED technology and industry based on sapphire substrates have developed rapidly, occupying more than 90% of the market share in the substrate market and becoming the mainstream technology route on the market.
Sapphire has excellent optical properties, mechanical properties and chemical stability, high strength, high hardness and erosion resistance. As a substrate material, sapphire has the advantages of high chemical stability at high temperature (2000 ° C), difficulty in absorption of visible light, and low cost.
Table 2 Sapphire substrate material properties
Sapphire substrates also have disadvantages, such as: first, lattice mismatch and thermal stress mismatch can lead to a large number of defects in the epitaxial layer, and at the same time cause difficulties in the subsequent device processing; second, sapphire is an insulator with a large resistivity It is impossible to make a device with a vertical structure; thirdly, N-type and P-type electrodes are usually formed only on the upper surface of the epitaxial layer, resulting in a reduction in effective light-emitting area and a decrease in material utilization rate; and fourth, the hardness of sapphire is very high, only second. In diamond, it is difficult to thin and cut it. Fifth, the thermal conductivity of sapphire is not very good, so when using LED devices, it will conduct a lot of heat. For large-area high-power devices, the thermal conductivity is very Important considerations.
At present, the technical development of sapphire substrates is relatively mature. Although there are many defects, they are all overcome one by one. For example, the transition layer growth technology overcomes the large lattice mismatch problem; the same side P and N electrodes overcome the poor conductivity. The problem of not easy to cut can be solved by the laser dicing machine; the pressure problem on the epitaxial layer due to thermal mismatch can also be solved. However, Jiang Fengyi believes that sapphire substrates are difficult to achieve large-scale epitaxy of 8 inches to 12 inches, and because of the poor heat dissipation performance of sapphire, it is difficult to peel off the substrate, so it has performance limitations in high-power LEDs .
The sapphire substrate technology is represented by Nichia Chemical and Toyota Synthetic. In terms of sapphire crystal and wafer preparation, foreign countries are mainly concentrated in Japan, the United States, Russia and other countries. In 2010, Russia first exhibited 200 mm sapphire wafers. There is a big gap between China and foreign countries. Representative enterprises include Yuanliang Technology, Tongren Electronics, GCL Optoelectronics, Chongqing Silian Optoelectronics, China Gallium Semiconductor, Orient. In terms of extension, in addition to research units such as Tsinghua University, Peking University, Nanchang University, and Institute of Semiconductors of the Chinese Academy of Sciences, Sanan Optoelectronics, Huacan Optoelectronics, Shanghai Beida Blu-ray, Nanchang Xinlei Optoelectronics, and Jiangxi Lianchuang Optoelectronics are conducting epitaxial wafers. Production and research.
Sapphire materials are currently overcapacity and the low-end market is highly competitive. In order to extend the LED industry chain, we should vigorously promote the development of sapphire graphic substrates from a high starting point. Focus on the development of nanoscale patterned sapphire substrates (PSS), hemispherical and tapered patterned substrates, as well as laser induced wet etching (LIBWE), dry etching and other technologies. At the same time, it promotes the development of lithography and etching related to sapphire pattern substrates and related detection equipment and materials.
SiC has excellent thermal, mechanical, chemical and electrical properties. It is not only one of the best materials for high temperature, high frequency and high power electronic devices, but also can be used as a substrate material for GaN-based blue light-emitting diodes. Sapphire dominates the world, especially in the field of street lights and outdoor lighting has huge market potential. The most commonly used SiC in the semiconductor field is 4H-SiC and 6H-SiC.
Table 3 SiC substrate material properties
Compared with sapphire, SiC is not a semiconductor but an insulator. It can only be used as a single-sided electrode. Silicon carbide is a conductive semiconductor that can be used as a vertical structure. The thermal conductivity of silicon carbide substrates is 10 times higher than that of sapphire; sapphire itself is a poor conductor of heat, and the bottom of the device needs to be solidified with silver glue, and the heat transfer performance of silver glue is also poor. 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 need a current diffusion layer, so the light is not The material of the current diffusion layer is absorbed, which in turn increases the light extraction efficiency.
In the field of silicon carbide substrates, Cree has almost monopolized the global supply of high-quality silicon carbide substrates, followed by Germany's SiCrystal, Japan's Nippon Steel, Showa Denko, and East Fiber-Dow Corning. China's enterprises are weak. The enterprises or institutions that can produce and process silicon carbide substrates in China include Beijing Tianke Heda, Shandong Tianyue, Shandong University, Institute of Physics of Chinese Academy of Sciences, Shanghai Institute of Ceramics of Chinese Academy of Sciences, and 46 Electronic Technology Group of China. Wait. In July 2015, Shandong Tianyue independently developed a 4-inch high-purity semi-insulating silicon carbide substrate product.
The main challenge of using silicon carbide as a photovoltaic device substrate is that the cost is still relatively high, the technical threshold is high and the patent technology is insufficient, facing the patent threat of the monopoly of the industry. However, the LED market has high, medium and low-end points, and the silicon carbide substrate LED is positioned at the high end. High-power LED market demand is huge, silicon carbide material has superior performance, and has significant advantages such as high power, low energy consumption and high luminous efficiency, which can well meet the demand of high-power LED.
Silicon wafers have many advantages as substrates for GaN materials, such as high crystal quality, large size, low cost, easy processing, good electrical conductivity, thermal conductivity and thermal stability.
Table 4 Si substrate material properties
Due to the high maturity of single crystal silicon material growth technology, it is easy to obtain a low cost, large size (6-12 inch), high quality substrate, which can greatly reduce the cost of LED. Moreover, since silicon single crystals have been widely used in the field of microelectronics, the use of single crystal silicon substrates is expected to achieve direct integration of LED chips and integrated circuits, which is conducive to the miniaturization of LED devices. Therefore, the use of single crystal silicon as an LED substrate has always been a dream of the industry. In addition, compared with sapphire, single crystal silicon has some advantages in performance: high thermal conductivity, good electrical conductivity, vertical structure, and more suitable for high-power LED preparation.
However, compared with sapphire and SiC, it is more difficult to grow GaN on Si substrate, mainly because: (1) thermal mismatch and lattice mismatch between the two are larger; (2) thermal expansion of Si and GaN Differences in coefficients will also cause cracks in the GaN film; (3) The difference in lattice constant will cause high dislocation density in the GaN epitaxial layer; (4) Si substrate LEDs may also have 0.5 V between Si and GaN. The heterogeneous barrier causes the turn-on voltage to rise and the poor crystal integrity to cause the p-type doping efficiency to be low, resulting in an increase in series resistance; (5) Si absorption of visible light reduces the external quantum efficiency of the LED.
From the advent of the first GaN/Si LED in 1999 to commercial GaN/Si LEDs in 2002, it has not been widely used due to the large difference in performance from LEDs made from sapphire and silicon carbide. In 2010, Azzurro of Germany authorized GaN-on-Si technology to Osram, Germany. In April 2013, Toshiba Corporation of Japan acquired the technology of Bridgelux and started the production of 8-inch GaN-on-Si epitaxial wafers. Domestic Jiangxi Jingneng Optoelectronics Co., Ltd. announced the mass production of HB-LED chips as early as 2012; Jiangxi Jingrui Optoelectronics has also launched LED products with similar performance.
The silicon substrate technology of Jingneng Optoelectronics has completely independent intellectual property rights, forming a three-pronged situation of sapphire, silicon carbide and silicon substrate semiconductor lighting technology. Only by breaking the international giant's technology and patent monopoly and mastering the core technology can China's LED industry truly realize the transition from "follow" to "crossover".
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