Since the invention of high-brightness blue LEDs by Nakamura Shuzo in the early 1990s, semiconductor lighting technology based on GaN-based blue LEDs and yellow phosphors combined to emit white light has received extensive attention and rapid development worldwide.
To date, the efficacy of commercial white LEDs has exceeded 150 lm/W, and the laboratory level has exceeded 200 lm/W, far higher than traditional incandescent lamps (15 lm/W) and fluorescent lamps (80 lm/W). s level.
From the market point of view, LED has been widely used in display screens, LCD backlights, traffic lights, outdoor lighting and other fields, and has begun to penetrate indoor lighting, automotive lights, stage lighting, special lighting, etc., and is expected to replace traditional light sources in the future. .
The quality of semiconductor illumination sources is closely related to the quality of LED chips. Further improving the light efficiency of LEDs (especially the light efficiency under high-power work), reliability, and lifetime are the goals of LED material and chip technology development. The key technologies of LED materials and chips and their future development trends are now sorted out as follows:
First, material extension
Epitaxial technology
Metal organic chemical vapor deposition (MOCVD) technology is the mainstream technology for growing LEDs. In recent years, thanks to advances in MOCVD equipment, the cost of epitaxial LED materials has dropped significantly. The main equipment suppliers on the market today 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-coupling sprinkler reaction chamber, which has the advantages of saving raw materials and growing LED epitaxial wafers with good uniformity. The latter device utilizes the high-speed rotation of the tray to create laminar flow, which has the advantages of simple maintenance and high productivity.
In addition, Japan's acid production is used by atmospheric pressure MOCVD for Japanese companies to obtain better crystal quality. American Applied Materials has created a multi-reaction cavity MOCVD device and has begun trials in the industry.
The future development direction of MOCVD equipment includes: further expanding the volume of the reaction chamber to increase the production capacity, further improving the utilization rate of raw materials such as MO source and ammonia gas, further improving the in-situ monitoring capability of the epitaxial wafer, and further optimizing the temperature field and the airflow field. The control is to enhance the support for the extension of large-sized substrates and the like.
2. Substrate (1) graphic substrate
The substrate is a substrate supporting the epitaxial film, and the GaN-based LED is generally grown on a hetero substrate such as sapphire, SiC, Si or the like due to the lack of a homogenous substrate. Since its inception, sapphire has become the most cost-effective substrate and is the most widely used. Since GaN has a higher refractive index than sapphire, in order to reduce the total emission of light emitted from the LED at the substrate interface, current off-chips generally perform material epitaxy on the pattern substrate to enhance light scattering.
A common pattern substrate pattern is generally a conical array of micrometer-sized cells arranged in a hexagonal shape, which can increase the light extraction efficiency of the LED to more than 60%. At the same time, some studies have shown that the use of a patterned substrate combined with a certain growth process can control the extension direction of dislocations in GaN and effectively reduce the dislocation density of the GaN epitaxial layer. In the future, the graphic substrate is still the main technical means adopted by the chip in the future.
The future direction of graphic substrates is to develop toward smaller sizes. At present, limited to the production cost, the sapphire pattern substrate is generally fabricated by contact exposure and ICP dry etching, and the size can only be on the order of micrometers. If the size can be further reduced to the order of 100 nm comparable to the wavelength of light, the ability to scatter light can be further improved. It can even be made into a periodic structure, which further improves the light extraction efficiency by utilizing the physical effects of the two-dimensional photonic crystal. Nano-patterning methods include electron beam exposure, nanoimprinting, nano-sphere self-assembly, etc. From the perspective of cost, the latter two are more suitable for substrate processing.
(2) Large size substrate
At present, the industry still uses 2 inch sapphire substrates as the mainstream, and some international manufacturers have already used 3 inch or even 4 inch substrates, and it is expected to expand to 6 inch substrates in the future. The enlargement of the substrate size is advantageous for reducing the edge effect of the epitaxial wafer and improving the yield of the LED. However, the current price of large-sized sapphire substrates is still expensive, and the expansion of the substrate size and the matching material epitaxial equipment and chip process equipment are faced with an upgrade, which is a small investment for manufacturers.
(3) SiC substrate
The lattice mismatch between the SiC substrate and the GaN-based material is smaller, and it has been proved that the quality of the GaN crystal grown on SiC is slightly better than that on the sapphire substrate. However, SiC substrates, especially high-quality SiC substrates, are expensive to manufacture, and there are few manufacturers for material extension of LEDs. However, Cree has become the only manufacturer in the industry to grow LEDs only on SiC substrates by virtue of its manufacturing advantages on high-quality SiC substrates, thus avoiding the patent barrier for growing GaN on sapphire substrates. At present, the mainstream size of SiC substrates is 3 inches, and it is expected to expand to 4 inches in the future. SiC substrates are more suitable for fabricating GaN-based electronic devices than sapphire substrates. In the future, with the development of wide-bandgap semiconductor power electronic devices, the cost of SiC substrates is expected to be further reduced.
(4) Si substrate
The Si substrate is considered to be an ideal choice for reducing the cost of the LED epitaxial wafer because its large (8-inch, 12-inch) substrate is the most mature. However, since the lattice mismatch and the thermal mismatch are too large to be controlled, the quality of the LED material based on the Si substrate is relatively poor, and the yield is low, so the LED products based on the Si substrate are currently very rare on the market. At present, LEDs grown on Si are mainly based on substrates of 6 inches or less. Considering the yield factor, the cost of actual LEDs is not superior to that of sapphire substrates. Like SiC substrates, most research institutes and manufacturers prefer to grow electronic devices instead of LEDs on Si substrates. LED epitaxial technology on future Si substrates should target larger-sized substrates such as 8-inch or 12-inch.
(5) Homogeneous substrate
As mentioned earlier, the epitaxial growth of LEDs is still dominated by the extension of heterogeneous substrates. However, lattice matching and thermally matched homogenous substrates are still seen as the ultimate solution for improving crystal quality and LED performance. In recent years, with the development of hydride vapor phase deposition (HVPE) epitaxy technology, large-area GaN-based thick substrate fabrication technology has been paid attention to, and its fabrication method generally uses HVPE to rapidly grow on a heterogeneous substrate to obtain tens of A GaN bulk material of several hundred microns thick is then peeled off from the substrate by mechanical, chemical or physical means, and the GaN thick layer is used as a substrate for LED epitaxy.
Japan's Mitsubishi Corporation and Sumitomo Corporation have been able to provide GaN-based substrates, but they are expensive and not cost-effective for general LED growth. Mainly used in the manufacture of lasers or in the study of non-polar/semi-polar surface LEDs. The UCC team at the University of California, Santa Barbara (UCSB) has done a lot of groundbreaking and representative work on the development of non-polar/semi-polar surface LEDs. Non-polar/semi-polar surface LEDs can circumvent the polarization effects present in conventional c-plane LEDs, further enhancing the efficiency of LEDs, especially long-wavelength visible LEDs. However, high-quality non-polar/semi-polar surface LEDs must rely on a homogeneous substrate, while non-polar/semi-polar surface GaN substrates are quite far from practical.
In addition, some schools and research institutes such as Japan, Poland, and the United States are also attempting to fabricate GaN bulk crystals under high-pressure and medium-temperature conditions using alkali metal melting and ammonia thermal methods, but they are still in the research stage.
3. Epitaxial structure and epitaxial technology
(1) Droop effect
After several years of development, the epitaxial layer structure and epitaxial technology of LED have been relatively mature, and the internal quantum efficiency is up to 90%. However, in recent years, with the rise of high-power LED chips, the quantum efficiency of LEDs under large injection has attracted widespread attention, and this phenomenon is aptly called the Droop effect. For the industry, solving the Droop effect can further reduce the chip size under the premise of ensuring power, thereby achieving the goal of reducing costs. For academics, the cause of the Droop effect is to attract scientists to research hotspots.
Different from traditional semiconductor optoelectronic materials, the cause of the Droop effect of GaN-based LEDs is very complicated, and there is no effective solution. Researchers have explored several reasons for the tendency to be: localization of carriers, leakage or overflow of carriers from the active area, and Auger recombination. Although the specific reasons are still unclear, experiments have found that using a wider quantum well to reduce the density of carriers and optimize the electron blocking layer of the p-type region are all ways to alleviate the Droop effect.
(2) quantum well active area
The InGaN/GaN quantum well active region is the core of the LED epitaxial material. The key to growing the InGaN quantum well is to control the stress of the quantum well and reduce the influence of the polarization effect. Conventional growth techniques include: growing a low In composition InGaN pre-well release stress before a multiple quantum well and acting as a carrier reservoir, heating the GaN barrier layer to increase the crystal quality of the barrier layer, and growing a lattice-matched InGaAlN barrier layer. Or growth stress complementary InGaN/AlGaN structure or the like. 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 at small injections, and the number of wells is higher. The LEDs are more efficient at large injections.
(3) p-type area
P-type doping of GaN is one of the important bottlenecks that have plagued LED production in the early days. This is because the unintentionally doped GaN is n-type and the electron concentration is above 1×10 16 cm −3 , and the implementation of p-type GaN is difficult. The most successful p-type dopant so far is Mg, but it still faces problems such as lattice damage caused by high concentration doping and acceptability of H element in the reaction chamber. The oxygen thermal annealing method invented by Nakamura Shuji in Nichia is simple and effective. It is a widely used method of acceptor activation, and some manufacturers directly activate it with nitrogen in situ annealing in a 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 reports on the use of p-AlGaN/GaN superlattices and p-InGaN/GaN superlattices to increase the hole concentration. Nevertheless, the hole concentration and hole mobility of p-GaN are still quite different from those of n-GaN, which causes asymmetry in LED carrier injection. It is generally necessary to insert an electron blocking layer of p-AlGaN on the side of the quantum well close to the p-GaN. However, the mismatch in polarity between AlGaN and the quantum well region is considered to be the main cause of carrier leakage. Therefore, some manufacturers have recently tried to replace it with p-InGaAlN.
4. No phosphor single-chip white LED
The existing white LEDs mainly use a combination of blue LEDs and yellow phosphors to emit white light. The typical color rendering index of such white light is not high, especially for red and green. In addition, phosphors also face problems such as poor reliability, loss of efficiency, and the like. It is theoretically feasible to rely solely on InGaN materials as the illuminating region to achieve white light in a single chip.
In recent years, some universities and research institutions at home and abroad have also carried out related research. More representative is the Chen Hong group of the Institute of Physics of the Chinese Academy of Sciences using the phase separation of In in the InGaN quantum well to achieve high In composition InGaN yellow light quantum dots, and the combination of blue quantum wells emit white light. However, the color rendering index of this white light is still relatively low. Non-phosphorescent single-chip white LEDs are an attractive development direction. If high efficiency and high color rendering index can be achieved, the technology chain of semiconductor lighting will be changed.
5. Other color LED
The external quantum efficiency of GaN-based blue LEDs has exceeded 60%, which means that blue LED devices are relatively mature. Therefore, people began to look at other bands that nitride materials can cover. Conventional III-V semiconductors have matured in the production of infrared and red-light-emitting devices, so it is more meaningful to develop green and ultraviolet LEDs for nitrides.
(1) Green LED
The green light band is currently the least efficient in the visible light band and is called "Green Gap". The reason why InGaN is inefficient in the green light band is because the polarization effect caused by the higher In composition and wider quantum well becomes stronger. The aforementioned growth of non-polar/semi-polar surface LEDs is an effective way to increase the efficiency of green LEDs, but limited by homogeneous substrates is currently not practical.
Recently, researchers at Osram, Germany, focused on LEDs for optical pump construction. They use blue LEDs as pump sources to excite green InGaN/GaN multiple quantum wells. The resulting green LEDs have a peak wavelength of 535 nm at 350 mA and a lumen efficiency of 127 lm/W, which is higher than direct injection of green carriers into the green. Light MQW LED.
(2) UV LED
Ultraviolet light has important applications in the fields of curing, sterilization, early warning, and covert communication. Traditional UV sources are vacuum devices. Nitride materials are the most suitable material for making UV LEDs, but because of the high dislocation density and the luminescence region of AlGaN (without In, the inductive efficiency of InGaN cannot be used to be insensitive to dislocations), GaN-based UV LEDs are especially The efficiency of deep ultraviolet LEDs (below wavelengths below 280 nm) is still very low. The Riken Institute in Japan and the Arif Khan group at the University of Southern California are pioneers in the study of deep-UV LEDs. Riken can achieve an external quantum efficiency of 3.8% for deep UV LEDs and an output power of 30 mW.
To date, the efficacy of commercial white LEDs has exceeded 150 lm/W, and the laboratory level has exceeded 200 lm/W, far higher than traditional incandescent lamps (15 lm/W) and fluorescent lamps (80 lm/W). s level.
From the market point of view, LED has been widely used in display screens, LCD backlights, traffic lights, outdoor lighting and other fields, and has begun to penetrate indoor lighting, automotive lights, stage lighting, special lighting, etc., and is expected to replace traditional light sources in the future. .
The quality of semiconductor illumination sources is closely related to the quality of LED chips. Further improving the light efficiency of LEDs (especially the light efficiency under high-power work), reliability, and lifetime are the goals of LED material and chip technology development. The key technologies of LED materials and chips and their future development trends are now sorted out as follows:
First, material extension
Epitaxial technology
Metal organic chemical vapor deposition (MOCVD) technology is the mainstream technology for growing LEDs. In recent years, thanks to advances in MOCVD equipment, the cost of epitaxial LED materials has dropped significantly. The main equipment suppliers on the market today 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-coupling sprinkler reaction chamber, which has the advantages of saving raw materials and growing LED epitaxial wafers with good uniformity. The latter device utilizes the high-speed rotation of the tray to create laminar flow, which has the advantages of simple maintenance and high productivity.
In addition, Japan's acid production is used by atmospheric pressure MOCVD for Japanese companies to obtain better crystal quality. American Applied Materials has created a multi-reaction cavity MOCVD device and has begun trials in the industry.
The future development direction of MOCVD equipment includes: further expanding the volume of the reaction chamber to increase the production capacity, further improving the utilization rate of raw materials such as MO source and ammonia gas, further improving the in-situ monitoring capability of the epitaxial wafer, and further optimizing the temperature field and the airflow field. The control is to enhance the support for the extension of large-sized substrates and the like.
2. Substrate (1) graphic substrate
The substrate is a substrate supporting the epitaxial film, and the GaN-based LED is generally grown on a hetero substrate such as sapphire, SiC, Si or the like due to the lack of a homogenous substrate. Since its inception, sapphire has become the most cost-effective substrate and is the most widely used. Since GaN has a higher refractive index than sapphire, in order to reduce the total emission of light emitted from the LED at the substrate interface, current off-chips generally perform material epitaxy on the pattern substrate to enhance light scattering.
A common pattern substrate pattern is generally a conical array of micrometer-sized cells arranged in a hexagonal shape, which can increase the light extraction efficiency of the LED to more than 60%. At the same time, some studies have shown that the use of a patterned substrate combined with a certain growth process can control the extension direction of dislocations in GaN and effectively reduce the dislocation density of the GaN epitaxial layer. In the future, the graphic substrate is still the main technical means adopted by the chip in the future.
The future direction of graphic substrates is to develop toward smaller sizes. At present, limited to the production cost, the sapphire pattern substrate is generally fabricated by contact exposure and ICP dry etching, and the size can only be on the order of micrometers. If the size can be further reduced to the order of 100 nm comparable to the wavelength of light, the ability to scatter light can be further improved. It can even be made into a periodic structure, which further improves the light extraction efficiency by utilizing the physical effects of the two-dimensional photonic crystal. Nano-patterning methods include electron beam exposure, nanoimprinting, nano-sphere self-assembly, etc. From the perspective of cost, the latter two are more suitable for substrate processing.
(2) Large size substrate
At present, the industry still uses 2 inch sapphire substrates as the mainstream, and some international manufacturers have already used 3 inch or even 4 inch substrates, and it is expected to expand to 6 inch substrates in the future. The enlargement of the substrate size is advantageous for reducing the edge effect of the epitaxial wafer and improving the yield of the LED. However, the current price of large-sized sapphire substrates is still expensive, and the expansion of the substrate size and the matching material epitaxial equipment and chip process equipment are faced with an upgrade, which is a small investment for manufacturers.
(3) SiC substrate
The lattice mismatch between the SiC substrate and the GaN-based material is smaller, and it has been proved that the quality of the GaN crystal grown on SiC is slightly better than that on the sapphire substrate. However, SiC substrates, especially high-quality SiC substrates, are expensive to manufacture, and there are few manufacturers for material extension of LEDs. However, Cree has become the only manufacturer in the industry to grow LEDs only on SiC substrates by virtue of its manufacturing advantages on high-quality SiC substrates, thus avoiding the patent barrier for growing GaN on sapphire substrates. At present, the mainstream size of SiC substrates is 3 inches, and it is expected to expand to 4 inches in the future. SiC substrates are more suitable for fabricating GaN-based electronic devices than sapphire substrates. In the future, with the development of wide-bandgap semiconductor power electronic devices, the cost of SiC substrates is expected to be further reduced.
(4) Si substrate
The Si substrate is considered to be an ideal choice for reducing the cost of the LED epitaxial wafer because its large (8-inch, 12-inch) substrate is the most mature. However, since the lattice mismatch and the thermal mismatch are too large to be controlled, the quality of the LED material based on the Si substrate is relatively poor, and the yield is low, so the LED products based on the Si substrate are currently very rare on the market. At present, LEDs grown on Si are mainly based on substrates of 6 inches or less. Considering the yield factor, the cost of actual LEDs is not superior to that of sapphire substrates. Like SiC substrates, most research institutes and manufacturers prefer to grow electronic devices instead of LEDs on Si substrates. LED epitaxial technology on future Si substrates should target larger-sized substrates such as 8-inch or 12-inch.
(5) Homogeneous substrate
As mentioned earlier, the epitaxial growth of LEDs is still dominated by the extension of heterogeneous substrates. However, lattice matching and thermally matched homogenous substrates are still seen as the ultimate solution for improving crystal quality and LED performance. In recent years, with the development of hydride vapor phase deposition (HVPE) epitaxy technology, large-area GaN-based thick substrate fabrication technology has been paid attention to, and its fabrication method generally uses HVPE to rapidly grow on a heterogeneous substrate to obtain tens of A GaN bulk material of several hundred microns thick is then peeled off from the substrate by mechanical, chemical or physical means, and the GaN thick layer is used as a substrate for LED epitaxy.
Japan's Mitsubishi Corporation and Sumitomo Corporation have been able to provide GaN-based substrates, but they are expensive and not cost-effective for general LED growth. Mainly used in the manufacture of lasers or in the study of non-polar/semi-polar surface LEDs. The UCC team at the University of California, Santa Barbara (UCSB) has done a lot of groundbreaking and representative work on the development of non-polar/semi-polar surface LEDs. Non-polar/semi-polar surface LEDs can circumvent the polarization effects present in conventional c-plane LEDs, further enhancing the efficiency of LEDs, especially long-wavelength visible LEDs. However, high-quality non-polar/semi-polar surface LEDs must rely on a homogeneous substrate, while non-polar/semi-polar surface GaN substrates are quite far from practical.
In addition, some schools and research institutes such as Japan, Poland, and the United States are also attempting to fabricate GaN bulk crystals under high-pressure and medium-temperature conditions using alkali metal melting and ammonia thermal methods, but they are still in the research stage.
3. Epitaxial structure and epitaxial technology
(1) Droop effect
After several years of development, the epitaxial layer structure and epitaxial technology of LED have been relatively mature, and the internal quantum efficiency is up to 90%. However, in recent years, with the rise of high-power LED chips, the quantum efficiency of LEDs under large injection has attracted widespread attention, and this phenomenon is aptly called the Droop effect. For the industry, solving the Droop effect can further reduce the chip size under the premise of ensuring power, thereby achieving the goal of reducing costs. For academics, the cause of the Droop effect is to attract scientists to research hotspots.
Different from traditional semiconductor optoelectronic materials, the cause of the Droop effect of GaN-based LEDs is very complicated, and there is no effective solution. Researchers have explored several reasons for the tendency to be: localization of carriers, leakage or overflow of carriers from the active area, and Auger recombination. Although the specific reasons are still unclear, experiments have found that using a wider quantum well to reduce the density of carriers and optimize the electron blocking layer of the p-type region are all ways to alleviate the Droop effect.
(2) quantum well active area
The InGaN/GaN quantum well active region is the core of the LED epitaxial material. The key to growing the InGaN quantum well is to control the stress of the quantum well and reduce the influence of the polarization effect. Conventional growth techniques include: growing a low In composition InGaN pre-well release stress before a multiple quantum well and acting as a carrier reservoir, heating the GaN barrier layer to increase the crystal quality of the barrier layer, and growing a lattice-matched InGaAlN barrier layer. Or growth stress complementary InGaN/AlGaN structure or the like. 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 at small injections, and the number of wells is higher. The LEDs are more efficient at large injections.
(3) p-type area
P-type doping of GaN is one of the important bottlenecks that have plagued LED production in the early days. This is because the unintentionally doped GaN is n-type and the electron concentration is above 1×10 16 cm −3 , and the implementation of p-type GaN is difficult. The most successful p-type dopant so far is Mg, but it still faces problems such as lattice damage caused by high concentration doping and acceptability of H element in the reaction chamber. The oxygen thermal annealing method invented by Nakamura Shuji in Nichia is simple and effective. It is a widely used method of acceptor activation, and some manufacturers directly activate it with nitrogen in situ annealing in a 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 reports on the use of p-AlGaN/GaN superlattices and p-InGaN/GaN superlattices to increase the hole concentration. Nevertheless, the hole concentration and hole mobility of p-GaN are still quite different from those of n-GaN, which causes asymmetry in LED carrier injection. It is generally necessary to insert an electron blocking layer of p-AlGaN on the side of the quantum well close to the p-GaN. However, the mismatch in polarity between AlGaN and the quantum well region is considered to be the main cause of carrier leakage. Therefore, some manufacturers have recently tried to replace it with p-InGaAlN.
4. No phosphor single-chip white LED
The existing white LEDs mainly use a combination of blue LEDs and yellow phosphors to emit white light. The typical color rendering index of such white light is not high, especially for red and green. In addition, phosphors also face problems such as poor reliability, loss of efficiency, and the like. It is theoretically feasible to rely solely on InGaN materials as the illuminating region to achieve white light in a single chip.
In recent years, some universities and research institutions at home and abroad have also carried out related research. More representative is the Chen Hong group of the Institute of Physics of the Chinese Academy of Sciences using the phase separation of In in the InGaN quantum well to achieve high In composition InGaN yellow light quantum dots, and the combination of blue quantum wells emit white light. However, the color rendering index of this white light is still relatively low. Non-phosphorescent single-chip white LEDs are an attractive development direction. If high efficiency and high color rendering index can be achieved, the technology chain of semiconductor lighting will be changed.
5. Other color LED
The external quantum efficiency of GaN-based blue LEDs has exceeded 60%, which means that blue LED devices are relatively mature. Therefore, people began to look at other bands that nitride materials can cover. Conventional III-V semiconductors have matured in the production of infrared and red-light-emitting devices, so it is more meaningful to develop green and ultraviolet LEDs for nitrides.
(1) Green LED
The green light band is currently the least efficient in the visible light band and is called "Green Gap". The reason why InGaN is inefficient in the green light band is because the polarization effect caused by the higher In composition and wider quantum well becomes stronger. The aforementioned growth of non-polar/semi-polar surface LEDs is an effective way to increase the efficiency of green LEDs, but limited by homogeneous substrates is currently not practical.
Recently, researchers at Osram, Germany, focused on LEDs for optical pump construction. They use blue LEDs as pump sources to excite green InGaN/GaN multiple quantum wells. The resulting green LEDs have a peak wavelength of 535 nm at 350 mA and a lumen efficiency of 127 lm/W, which is higher than direct injection of green carriers into the green. Light MQW LED.
(2) UV LED
Ultraviolet light has important applications in the fields of curing, sterilization, early warning, and covert communication. Traditional UV sources are vacuum devices. Nitride materials are the most suitable material for making UV LEDs, but because of the high dislocation density and the luminescence region of AlGaN (without In, the inductive efficiency of InGaN cannot be used to be insensitive to dislocations), GaN-based UV LEDs are especially The efficiency of deep ultraviolet LEDs (below wavelengths below 280 nm) is still very low. The Riken Institute in Japan and the Arif Khan group at the University of Southern California are pioneers in the study of deep-UV LEDs. Riken can achieve an external quantum efficiency of 3.8% for deep UV LEDs and an output power of 30 mW.
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