Since the invention of high-brightness blue LEDs by Shuji Nakamura in the early 1990s, semiconductor lighting technology based on the combination of GaN-based blue LEDs and yellow phosphors to emit white light has received widespread attention and rapid development worldwide. So far, the luminous efficiency of commercial white LEDs has exceeded 150 lm / W, while the laboratory level has exceeded 200 lm / W, much higher than traditional incandescent lamps (15 lm / W) and fluorescent lamps (80 lm / W). s level. From the market perspective, LED has been widely used in display screens, LCD backlights, traffic lights, outdoor lighting and other fields, and has begun to penetrate into indoor lighting, car lights, stage lights, special lighting and other markets, and is expected to fully replace traditional light sources in the future . The quality of semiconductor lighting sources is closely related to the quality of LED chips. To further improve the light efficiency of LEDs (especially those under high-power operation), reliability, and lifespan are the development goals of LED materials and chip technology. The key technologies of LED materials and chips and their future development trends are now sorted out as follows: 1. Material Extension 1. Epitaxy Metal organic chemical vapor deposition (MOCVD) technology is the mainstream technology for growing LEDs. In recent years, thanks to the advancement of MOCVD equipment, the cost of LED material epitaxy has dropped significantly. The main equipment suppliers on the market are Aixtron in Germany and Veeco in the United States. The former can provide two types of equipment: a horizontal planetary reaction chamber and a near-coupled shower head type reaction chamber, which has the advantages of saving raw materials and growing LED epitaxial wafers with good uniformity. The latter equipment utilizes the high-speed rotation of the tray to generate laminar flow, which has the advantages of simple maintenance and large production capacity. In addition, the Japanese acid production of atmospheric pressure MOCVD exclusively for Japanese companies can obtain better crystal quality. American Applied Materials has created a multi-reaction chamber MOCVD equipment, and has begun to test it in the industry. The future development direction of MOCVD equipment includes: further expanding the volume of the reaction chamber to increase production capacity, further improving the utilization rate of MO sources, ammonia gas and other raw materials, further improving the on-site monitoring ability of the epitaxial wafer, and further optimizing the temperature field and air flow field Control to improve the ability to support large-scale substrate epitaxy. 2. Substrate (1) Graphic substrate The substrate is the base supporting the epitaxial thin film. Due to the lack of a homogeneous substrate, GaN-based LEDs are generally grown on heterogeneous substrates such as sapphire, SiC, and Si. So far, sapphire has become the most cost-effective substrate and is the most widely used. Since the refractive index of GaN is higher than that of sapphire, in order to reduce the total emission of the light emitted from the LED at the substrate interface, the currently installed chips generally perform material epitaxy on the graphic substrate to improve the light scattering. Common graphic substrate patterns are generally conical arrays in the order of micrometers that are closely packed in hexagons, which can increase the light extraction efficiency of LEDs to more than 60%. At the same time, some studies have shown that the use of a patterned substrate and a certain growth process can control the extension direction of dislocations in GaN to effectively reduce the dislocation density of the GaN epitaxial layer. For a considerable period of time in the future, graphic substrates will still be the main technical means adopted by formal chips. The development direction of the graphic substrate in the future is to develop to a smaller size. At present, limited by the manufacturing cost, the sapphire pattern substrate is generally manufactured by contact exposure and ICP dry etching, and the size can only be on the order of microns. If the size can be further reduced to the order of 100 nm comparable to the wavelength of light, the light scattering ability can be further improved. It can even be made into a periodic structure, using the physical effects of two-dimensional photonic crystals to further improve the light extraction efficiency. The production methods of nano-patterns include electron beam exposure, nano-imprinting, nano-ball self-assembly, etc. In terms of cost, the latter two are more suitable for substrate processing. (2) Large size substrate At present, 2-inch sapphire substrates are still the mainstream in the industry, and some large international companies are already using 3-inch or even 4-inch substrates, and are expected to expand to 6-inch substrates in the future. The expansion of the substrate size is beneficial to reduce the edge effect of the epitaxial wafer and improve the yield of the LED. However, the price of large-sized sapphire substrates is still expensive, and the matching of material epitaxy equipment and chip processing equipment after expanding the substrate size must face upgrades, which is not a small investment for manufacturers. (3) SiC substrate The lattice mismatch between the SiC substrate and the GaN-based material is smaller, and it turns out that the quality of the GaN crystal grown on SiC is slightly better than the result on the sapphire substrate. However, the manufacturing cost of SiC substrates, especially high-quality SiC substrates, is very high, so few manufacturers use materials for LED epitaxy. However, with its manufacturing advantages on high-quality SiC substrates, American Cree has become the only manufacturer in the industry that only grows LEDs on SiC substrates, thus avoiding the patent barriers to growing GaN on sapphire substrates. The current mainstream size of SiC substrate is 3 inches, and it is expected to expand to 4 inches in the future. SiC substrates are more suitable for making GaN-based electronic devices than sapphire substrates. With the development of wide band gap semiconductor power electronic devices in the future, the cost of SiC substrates is expected to be further reduced. (4) Si substrate The Si substrate is considered to be an ideal choice to reduce the cost of LED epitaxial wafers, because its large-size (8-inch, 12-inch) substrates have developed the most mature. However, because the lattice mismatch and thermal mismatch are too large to control, the quality of LED materials based on Si substrates is relatively poor, and the yield is low, so LED products based on Si substrates are currently very rare in the market. At present, LEDs grown on Si are mainly based on substrates under 6 inches. Considering the yield factor, the actual cost of LEDs is not superior to those based on sapphire substrates. Like SiC substrates, most research institutions and manufacturers prefer to grow electronic devices on Si substrates rather than LEDs. In the future, LED epitaxial technology on Si substrates should target larger substrates such as 8 inches or 12 inches. (5) Homogeneous substrate As mentioned earlier, the current epitaxial growth of LEDs is still dominated by heterogeneous substrates. However, lattice-matched and thermally-matched homogeneous substrates are still regarded as the ultimate solution to improve crystal quality and LED performance. In recent years, with the development of hydride vapor deposition (HVPE) epitaxy technology, large-area GaN-based thick substrate manufacturing technology has been paid attention to. Its manufacturing method is generally to use HVPE to rapidly grow on heterogeneous substrates A GaN bulk material with a thickness of hundreds of micrometers is then mechanically, chemically or physically removed from the substrate by using a thick layer of GaN film, and this thick layer of GaN is used as a substrate for LED epitaxy. Mitsubishi and Sumitomo of Japan can already provide GaN-based substrate products, but the price is expensive, and it is not cost-effective for the growth of general LEDs. It is mainly used for the manufacture of lasers or the research of non-polar / semi-polar LEDs. The University of California, Santa Barbara (UCSB) Nakamura team has done a lot of groundbreaking and representative work in the development of nonpolar / semipolar LEDs. Non-polar / semi-polar LEDs can circumvent the polarization effect problem in traditional c-plane LEDs, thereby further improving the efficiency of LEDs, especially long-wavelength visible light LEDs. However, high-quality non-polar / semi-polar plane LEDs must rely on a homogeneous substrate, and non-polar / semi-polar plane GaN substrates are still far from practical. In addition, some schools and research institutions in Japan, Poland, and the United States are also trying to use alkali metal fusion method, ammonia heating method and other methods to manufacture GaN bulk crystals under high pressure and medium temperature conditions, but they are still in the research stage. 3. Epitaxy structure and epitaxy technology (1) Droop effect After several years of development, the epitaxial layer structure and epitaxial technology of LED have been relatively mature, and its internal quantum efficiency can reach up to 90%. However, in recent years, with the rise of high-power LED chips, the decline in quantum efficiency of LEDs under large injections has caused widespread concern. This phenomenon is called the Droop effect. For the industry, solving the Droop effect can further reduce the chip size under the premise of ensuring power, and achieve the purpose of reducing costs. For academics, the cause of the Droop effect is a hot spot that attracts scientists to study. Unlike traditional semiconductor optoelectronic materials, the cause of the Droop effect of GaN-based LEDs is very complex, and there is also a lack of effective solutions. After exploring, the researchers tended to favor the following reasons: delocalization of carriers, leakage or overflow of carriers from the active region, and Auger recombination. Although the specific reason is not clear, the experiment found that using a wider quantum well to reduce the carrier density and optimizing the electron blocking layer of the p-type region are all means to alleviate the Droop effect. (2) Quantum well active area The active area of ​​InGaN / GaN quantum well is the core of LED epitaxial material. The key to growing InGaN quantum well is to control the stress of quantum well and reduce the influence of polarization effect. Conventional growth techniques include: growth of low-In InGaN pre-wells before multi-quantum wells to release stress and act as a carrier reservoir, temperature-growing GaN barrier layer to improve the crystal quality of the barrier layer, and lattice-matched InGaAlN barrier layer Or InGaN / AlGaN structure with complementary growth stress. There is no uniform standard for the number of quantum wells. The number of quantum wells used in the industry ranges from 5 to 15. The final effect is not much different. LEDs with fewer wells are more efficient under small injection, while the number of wells is more. LEDs are more efficient under large injections. (3) p-type area The p-type doping of GaN is one of the important bottlenecks that plagued LED manufacturing in the early days. This is because unintentionally doped GaN is n-type, and the electron concentration is above 1 & TImes; 1016 cm-3, and the realization of p-type GaN is difficult. The most successful p-type dopant so far is Mg, but it still faces the problems of lattice damage caused by high concentration doping, and the acceptor is easily passivated by H element in the reaction chamber. The oxygen thermal annealing method invented by Nakamura Shuji at Nichia is simple and effective, and is a widely used method of acceptor activation. Some manufacturers directly use nitrogen in-situ annealing activation in the MOCVD epitaxial furnace. Nichia's p-GaN quality is the best and may be related to the atmospheric pressure MOCVD growth process. In addition, there are some reports using p-AlGaN / GaN superlattice and p-InGaN / GaN superlattice to increase the hole concentration. Nevertheless, the hole concentration and hole mobility of p-GaN are still very different from those of n-GaN electrons, which causes the asymmetry of LED carrier injection. Generally, the electron blocking layer of p-AlGaN must be inserted on the side of the quantum well near the p-GaN. However, the polarity mismatch between AlGaN and the quantum well region is considered to be the main cause of carrier leakage, so some manufacturers have recently tried to use p-InGaAlN instead. 4. Single chip white LED without phosphor Existing white LEDs mainly use a combination of blue LEDs and yellow phosphors to emit white light. The typical color rendering index of this white light is not high, especially for red and green. In addition, phosphors also face problems such as poor reliability and lost efficiency. It is theoretically feasible to completely rely on InGaN materials as light-emitting regions to realize white light in a single chip. In recent years, some universities and research institutions at home and abroad have also carried out relevant research. More representative is that the Hong group of the Institute of Physics of the Chinese Academy of Sciences has used In phase separation in InGaN quantum wells to achieve high-In composition InGaN yellow light quantum dots, which emit white light in combination with blue light quantum wells. However, the color rendering index of this white light is still relatively low. Single chip white LED without phosphor is an attractive development direction. If high efficiency and high color rendering index can be achieved, it will change the technology chain of semiconductor lighting. 5. Other colors LED The external quantum efficiency of GaN-based blue LEDs has exceeded 60%, which means that blue LED devices have been relatively mature. Therefore, people began to look at other wave bands that nitride materials can cover. Traditional III-V semiconductors have been very mature in making infrared and red light-emitting devices, so it is more meaningful for nitrides to develop green and ultraviolet LEDs. (1) Green LED The green band is currently the least efficient in the visible band and is called "Green Gap". The reason why InGaN is inefficient in the green band is because the polarization effect caused by the higher In composition and the wider quantum well becomes stronger. The aforementioned growth of non-polar / semi-polar surface LEDs is an effective method to improve the efficiency of green LEDs, but it is currently not practical to be restricted to homogeneous substrates. Recently, researchers at Osram in Germany have focused on LEDs with light pump structures. They used a blue LED as a pump source to excite the green InGaN / GaN multi-quantum well. The green LED obtained had a peak wavelength of 535 nm at 350 mA and a lumen efficiency of 127 lm / W, which was higher than that of direct injection of carriers into the green. Light MQW LED. (2) UV LED Ultraviolet light has important applications in the fields of curing, sterilization, early warning, concealed communication and so on. Traditional ultraviolet light sources are vacuum devices. Nitride materials are the most suitable material system for making ultraviolet LEDs, but due to the high dislocation density and the light emitting region is AlGaN (without In, the advantage of InGaN luminous efficiency is not sensitive to dislocations), GaN-based ultraviolet LEDs The efficiency of the deep ultraviolet LED (wavelength below 280 nm) is still very low. The Riken Institute in Japan and the Arif Khan group at the University of Southern California in the United States are pioneers in deep UV LED research. Riken can achieve the external quantum efficiency of the deep ultraviolet LED to 3.8%, and the output power reaches 30 mW. Second, the chip process 1. Formal chip Formal mounted chips are currently the most used chips on the market, and Japan Nichia is a typical representative of this technology route. It generally grows LED material on a sapphire pattern substrate, emits light from the surface p-GaN, and vapor-deposits a reflective film on the back of the sapphire. A part of the chip needs to be dry etched to n-GaN to make coplanar electrodes. The structure of the front mounted chip is simple, the production cost is low, and it is suitable for low power work. Due to the low heat dissipation capacity of the sapphire substrate, the high-power operation of the front-mounted chip will be subject to some restrictions, but the Nichia company has achieved considerable efficiency at high junction temperatures by virtue of its material quality advantages. The white LED with the external quantum efficiency 84.3% blue LED front-mounted chip package can achieve 249 lm / W light efficiency at 20 mA; the high-power white light LED has a light efficiency of 183 lm / W at 350 mA current. The key technologies of the chip are: (1) Transparent conductive film At present, the industry mainly uses indium tin oxide (ITO) electrodes as transparent ohmic electrodes on the surface of p-GaN. ITO is a transparent conductive film widely used in the field of solar cells and liquid crystals, and has good light transmittance in the blue light region. On the other hand, In element is not rich in reserves on earth and belongs to rare metals. Therefore, people began to look for new transparent conductive materials to replace ITO, the more representative is the ZnO transparent film. ZnO is also a wide band gap semiconductor and transparent to blue light. However, there is still a gap between its stability and contact characteristics compared with ITO, so the industry has not yet begun to use it. (2) Surface roughening As mentioned earlier, the use of a sapphire pattern substrate enhances the scattering of light at the interface between GaN and sapphire, and greatly improves the light extraction efficiency of the LED. Corresponding roughening structures can also be made on the p-GaN surface or ITO electrode surface to enhance light scattering. One of Nichia's representative technologies is to make ITO transparent electrodes into a mesh structure to facilitate light emission. Some institutions have also begun to study the formation of roughened structures on the surface of LEDs by the method of self-assembly growth of ITO nanowires. In addition, some people have tried to use dry etching to fabricate a two-dimensional photonic crystal structure on p-GaN, using the forbidden band of the photonic crystal to realize the full emission of blue light. However, it is very difficult to manufacture a large-area uniform photonic crystal, the cost is very high, and it will cause certain damage to the electrical characteristics, so it is not widely used in the industry. (3) DBR reflector The DBR reflector is mainly used for vapor deposition on the back of the thinned sapphire substrate to reflect the light originally emitted from the back of the sapphire to the surface of the LED. Early reflective coatings used Al, Au and other metals, but the cost was too high. At present, DBR reflectors composed of SiO2 / TIO2 dielectric films are mostly used. 2. Vertical structure chip Vertical structure chips are the mainstream technology route currently adopted by high-end LED chips. It is to vapor-deposit a high-reflectivity metal ohmic electrode on the surface of p-GaN and solder the LED to Si or metal heat sink, then peel off the substrate to expose the rough n-GaN, and make an ohmic electrode on the n-GaN surface , The current flows vertically through the chip when the device is working. This design does not lose the part of the light-emitting area that is eroded at the time of making the coplanar electrode, and the current flows vertically through the chip to avoid the congestion effect of lateral flow, and at the same time the heat dissipation capacity becomes very strong, so the performance of the chip under high power Very high. However, there are many process steps, and the manufacturing cost is higher than that of the chip. American Cree Company is the representative of the technology route, and has started mass production of white LED devices (non-traditional 1 & TImes; 1 mm2 size chips) with a luminous efficiency of 200 lm / W at 1W electric power. Its key technologies include: (1) Substrate peeling For Si substrates, the substrate is generally removed by wet etching. For sapphire or SiC substrates, laser ablation technology is generally used for separation. It focuses the ultraviolet laser at the interface between the substrate and the LED, allowing GaN to absorb the energy of the laser ultraviolet to generate liquid Ga and N2 to epitaxial the substrate and GaN Layer separation. This technology can peel off the entire substrate at once, which is very efficient, but it is necessary to avoid the damage of the LED epitaxial layer caused by the laser as much as possible. (2) Surface roughening The n-GaN surface after laser stripping is a rough N-polar surface, soak it in a heated KOH solution, KOH can etch the GaN surface to form a randomly arranged pyramid structure, which is very conducive to light scattering. The patent for this technology is in the hands of the UCSB Nakamura team, but many manufacturers are actually using the same technology. 3. Flip chip The sapphire substrate is the main factor restricting the heat dissipation of the LED chips. The American Lumileds Company took the lead in developing the flip chip structure based on the Si-based heat sink in the industry. It first prepares a large-sized LED chip with eutectic welding electrodes, and at the same time prepares a silicon substrate of corresponding size and produces a gold conductive layer and a drawn conductive layer for eutectic welding on it, and then uses eutectic welding equipment to The large-size LED chip is soldered together with the silicon substrate after being reversed, light is emitted from the back of the sapphire substrate, and heat is conducted away from the Si-based heat sink through the electrode solder. This structure is more reasonable, that is, considering the light emission problem and the heat dissipation problem, it is suitable for making high-power LEDs. After the development of laser glass technology on sapphire substrates, flip chip was once considered to be a transitional technology between front mounted chips and vertical structure chips. When most companies abandon the flip structure, Lumileds still adheres to this technical route, even if it can peel off the sapphire substrate, it still retains the design of the coplanar electrode. After the chip on board (COB) technology is developed, this flip-chip structure returns to people's vision. COB technology is to prepare a number of chip electrode solder joints of a series-parallel circuit that have been designed on a ceramic substrate by using a printed circuit, and solder LED flip chips one by one to a board in order to achieve high-power devices. This design simplifies packaging, realizes the miniaturization of high-power devices, and provides convenience for the design of lighting fixtures. 4. High voltage AC / DC drive LED A single LED chip works under low-voltage DC state. In order to apply to 220 V mains power, the LED lighting source needs to be matched with the corresponding driving power supply. However, the power conversion efficiency from 220 V high voltage to about 3 V low voltage is not high enough, and the life is limited by the electrolytic capacitor. There are many problems in practical use. Implementing series and parallel connection of multiple LED chips at the chip level can make the LED work at a higher driving voltage. There are two main ideas. One is to use the rectification characteristics of LED as a diode, a plurality of LED chips to form a bridge structure, and directly use 220 V AC to drive the LED. The advantage of this method is that the transformer is omitted, but only some LED points per half cycle Bright, so the efficiency is not high. The other is to connect multiple small LED chips in series and drive them with high-voltage direct current. This method still requires a power adapter, but because the voltage after the transformation is tens of volts, the driving power supply has high efficiency and high reliability, which is still improved compared to the traditional method. Therefore, high-voltage DC-driven LED chips are currently a hot spot for Korean and Taiwanese manufacturers. The key technologies and development status of the current LED material epitaxy and chip technology are summarized above. With the vigorous investment of enterprises and research institutions in various countries, LED materials and chip technology are relatively mature, and the light efficiency of the chip is no longer the main bottleneck restricting the application of LED lighting. The next development of semiconductor lighting technology is to provide better light color quality and human eye comfort than traditional lighting while reducing costs as much as possible. This puts new demands on LED materials and chips. If high-efficiency and high color rendering index phosphor-free single-chip white LEDs can be put into practical use, it will undoubtedly be a disruptive revolution in semiconductor lighting technology. Xinxiang Mina Import & Export Co., Ltd. , https://www.mina-motor.cn