With the continuous improvement of LED performance, the application market has also rapidly expanded. The hidden reason behind this is the use of high-gloss LEDs of GaN and AllnGaP luminescent materials, which have excellent longevity, power saving, shock resistance, and low voltage drive. Characteristics, and beyond light bulbs and halogens, and high luminous efficiency LED is more and more developed in recent years, therefore, the future development of high-brightness LED market will be more rapid and extensive growth.
The most obvious one is the white LED. In the late 1990s, it was expected by the market in the context of environmental protection and energy conservation. It also stimulated the rapid development of related technologies by the industry. At present, the main applications of white LEDs include backlighting for mobile phones and interior lighting for vehicles. These markets alone account for about 25% of the total sales of LEDs.
On the other hand, some of the lighting applications are in the beginning stage. The lighting of general buildings often accounts for 20% of the total power consumption. In Japan, it has exceeded 100 billion kWh per year in the 1990s. So the expectation for a new generation of energy-saving light sources is considerable, but unfortunately so far, white LEDs can only be used in a relatively small range. Because small white LEDs like 5mm can't be used like a light bulb or a fluorescent light, only one light can be used to get the amount of light needed for the environment. Therefore, if we want LED to be able to step into architectural lighting, we need a greater breakthrough in overall technology.
The basic structure of a high brightness white LED white LED <br> <br> basically two ways. One is a multi-chip type and the other is a single-chip type. The former is to encapsulate three kinds of red, green and blue LEDs together and emit light while generating white light. The latter uses blue or violet and ultraviolet LEDs as the light source, and emits white light in conjunction with phosphor powder. In the former method, the characteristics of various LEDs must be combined, and the driving circuit is relatively complicated. In the latter single-chip type, there is only one type of LED, and the circuit design is relatively easy. The single-chip type is further divided into two types, one being the use of blue LEDs for the light source and the other using near-ultraviolet and ultraviolet light. Now, most white LEDs on the market are blue LEDs with YAG phosphors.
In the past, only blue LEDs used GaN as a substrate material, but now LEDs that are used from the green field to near-ultraviolet light have also started to use GaN compounds as materials. Along with the expansion of white LED applications, the market's expectations for its performance have also gradually increased. From a purely point of view, the pursuit of high efficiency has always been expected by the market and the industry. However, on the other hand, color rendering will also be an important performance indicator. If it is just for display purposes, it may be sufficient that the luminescent color is white. However, in terms of lighting applications, in order to achieve higher efficiency, how to achieve The color close to natural light is very necessary.
GaN is becoming popular as a high-brightness LED substrate <br><br><br><br><br><br><br><br><br><br><br><br><br><br><br><br><br><br><br><br><br><br><br><br><br><br> Fierce competition started. Compared with the initial period, despite the dramatic brightness increase achieved today, the technology will soon face a more difficult threshold, so now both the academic community and the business community are concentrating on technology and research and development. Based on the current R&D direction of GaN LEDs, it is roughly divided into the development direction of large current, short wavelength, and high efficiency.
How to support larger LED current <br> <br> In recent years, the industry is very positive for only one can achieve a considerable brightness LED research and development, technology in this area and therefore it falls on how to support LED More current. Usually 30um2 LED can drive the current of 30mA at most, but this kind of result is still far unable to meet market's expectation, so the goal is to need to lead more than 10 times of the electric current, turn on LED element. Therefore, when the LED area size can be expanded to 1mm2, the next task is how to make the current value can reach 350500mA, because the driving voltage is more than 3V, so it can have 1W of power can be flowed into 1mm2 Chip area.
In terms of luminous color rendering, although there is such a large amount of power input into GaN LEDs, three-quarters of the input power cannot be converted into light to generate heat, so the LED will be overheated, which will also have a direct impact. LED color rendering results. Because the basic characteristics of the LED element is that if the temperature rises, the luminous efficiency will decrease and the color rendering deviation will occur. Therefore, how to effectively release a large amount of heat generating heat release technology has become the key, so the LED is installed in the thermal conductivity, heat capacity The material has become a very important issue, and most of the current use of valuable metals or ceramics.
Short-wavelength brings high energy to the excitation light to increase the luminous efficiency of fluorescent powder
GaNLEDs starting from blue light have been successfully developed with high-luminance green LEDs. Although there has been a trend toward long-wavelength development, the crystallinity caused by the increase in the ratio of mixed crystals of InN has gradually been abandoned by the industry. . On the other hand, research and development for new applications such as becoming a laser substitute have begun to be considered, so the current industry is actively conducting research and development for short wavelengths. Recently, some universities in Japan have successfully developed 250nm LEDs in their laboratories, but their practicality remains to be considered because the human eye accepts wavelengths at about 380nm, so if the wavelength is shorter than 380nm, it cannot be produced. The LED in the domain may generate low output.
In order to avoid encountering the aforementioned problems, most of the following solutions are currently used:
1. The light emitting layer structure was changed: Instead of the GaInN structure used on the chip of the visible region LED, AlGaN or AlGaInN having a larger Eg was used.
2. Avoidance of light absorption loss: If there is a GaN or GaInM layer in the LED chip structure, light will not be emitted because it absorbs light itself, so using the AlGaN layer as a basis to construct the entire structure layer will yield better results. Or use GaN as an important n-type bottom layer.
3. Reducing Crystal Defects: The density of crystal defects in short-wavelength LEDs can have a large impact on light output and lifetime.
If we can successfully solve the three problems mentioned above, we believe that the practical distance for using LED as general lighting can be greatly shortened. At present, the efficiency of GaN white LEDs can exceed that of white-light bulbs and halogen lamps (1525 lm/W), but in order to be able to exceed the size of fluorescent lamps that have an overwhelming light output (above 5080 lm/W), it needs to be more drastic. The efficiency increase and the amount of light increase dramatically. In order to achieve the same light source characteristics as a fluorescent lamp, white light technology using the color mixture of fluorescent powder has become a key factor. If the efficiency of the LED is fully utilized and the wavelength can be shortened, the luminous efficiency of the fluorescent powder is believed to increase substantially due to the increased energy of the excitation light.
In the crystal growth face is the key to obtain a uniform mass <br> <br> so-called internal electron emission efficiency is the ratio of the light converted into. It can be said that the LED central part of the luminous efficiency. However, due to factors of crystal defects, the luminous efficiency of LEDs is seriously affected. When GaN grows, a very high density of migration defects occurs in the direction of the growth crystals because of the difference in lattice constant difference and thermal expansion coefficient between the sapphire substrate and the GaN single crystal on the substrate.
In general, the resulting density is above 109cm2. Such a density would be fatal if it occurs in short-wavelength LEDs and laser diodes. In order to reduce this kind of transposition density, there are roughly two methods. One is to prevent translocation from penetrating to the longitudinal direction, and the other is to suppress translocation. In the aspect of preventing the indexing from penetrating to the longitudinal direction, the substrate processed by the pattern may be used. When the vertical growth is made, the horizontal direction is formed, and the long side of the defect is bent toward the horizontal direction, and the vertical direction is realized. As a result, to reduce the translocation phenomenon, although this approach may be able to reach a low index below 107cm2, but the actual mass production, to obtain a uniform quality in the long crystal plane is the key. The latter method is to use a group III nitride substrate having a low density of crystal defects or a low-defect group III nitride on a substrate already formed.
Originally, there is no monocrystalline Bulk in the group III nitride. When using a sapphire substrate for hetero-epitaxial generation, the source of high density of the translocation is the use of this heterogeneous substrate. Of course, the Bulk substrate is the best solution. . As a result, R&D and mass production in various manufacturing methods are under active development, and some have begun to enter the sales stage. On the other hand, it is the Template substrate that is capable of performing a similar function as opposed to the final substrate Bulk substrate. At present, several companies are starting to produce small quantities. Although these are not as costly as Bulk substrates, the cost is not low, because they can only be used for lasers and electronic devices, UVLEDs, etc. because of the high cost and efficiency.
Although there are many crystal defects, why can GaN-based LED devices achieve high brightness and the chip does not deteriorate rapidly? These structural phenomena are still being studied by engineers and scholars, but there is no complete theory. Therefore, in order to achieve the maximum limit of the material and to exert the limit of GaN, it is necessary to determine the ideal layer structure and structural design of the light emitting structure.
If you can not achieve a good crystal everything is in vain <br> No matter how good the structural design, if you can not achieve a good crystal, everything is in vain. In the initial stage, mass-produced GaN LEDs were face-up type devices. The contact electrode on the p-side was a light-transmissive thin-film electrode that emitted light through this thin-film electrode, while the material used was an Au alloy electrode, but Translucent characteristics, but the actual transmittance and can not meet the needs of practical applications, because the optical coefficient of the electrode, or reflected light can not be emitted much, so that the luminous efficiency has not been improved. Therefore, R&D personnel will consider it later because face-up type LED elements have a high reflectivity. Therefore, a highly stable material must be used as an electrode to emit light from the sapphire substrate side to increase the luminous flux.
The most obvious one is the white LED. In the late 1990s, it was expected by the market in the context of environmental protection and energy conservation. It also stimulated the rapid development of related technologies by the industry. At present, the main applications of white LEDs include backlighting for mobile phones and interior lighting for vehicles. These markets alone account for about 25% of the total sales of LEDs.
On the other hand, some of the lighting applications are in the beginning stage. The lighting of general buildings often accounts for 20% of the total power consumption. In Japan, it has exceeded 100 billion kWh per year in the 1990s. So the expectation for a new generation of energy-saving light sources is considerable, but unfortunately so far, white LEDs can only be used in a relatively small range. Because small white LEDs like 5mm can't be used like a light bulb or a fluorescent light, only one light can be used to get the amount of light needed for the environment. Therefore, if we want LED to be able to step into architectural lighting, we need a greater breakthrough in overall technology.
The basic structure of a high brightness white LED white LED <br> <br> basically two ways. One is a multi-chip type and the other is a single-chip type. The former is to encapsulate three kinds of red, green and blue LEDs together and emit light while generating white light. The latter uses blue or violet and ultraviolet LEDs as the light source, and emits white light in conjunction with phosphor powder. In the former method, the characteristics of various LEDs must be combined, and the driving circuit is relatively complicated. In the latter single-chip type, there is only one type of LED, and the circuit design is relatively easy. The single-chip type is further divided into two types, one being the use of blue LEDs for the light source and the other using near-ultraviolet and ultraviolet light. Now, most white LEDs on the market are blue LEDs with YAG phosphors.
In the past, only blue LEDs used GaN as a substrate material, but now LEDs that are used from the green field to near-ultraviolet light have also started to use GaN compounds as materials. Along with the expansion of white LED applications, the market's expectations for its performance have also gradually increased. From a purely point of view, the pursuit of high efficiency has always been expected by the market and the industry. However, on the other hand, color rendering will also be an important performance indicator. If it is just for display purposes, it may be sufficient that the luminescent color is white. However, in terms of lighting applications, in order to achieve higher efficiency, how to achieve The color close to natural light is very necessary.
GaN is becoming popular as a high-brightness LED substrate <br><br><br><br><br><br><br><br><br><br><br><br><br><br><br><br><br><br><br><br><br><br><br><br><br><br> Fierce competition started. Compared with the initial period, despite the dramatic brightness increase achieved today, the technology will soon face a more difficult threshold, so now both the academic community and the business community are concentrating on technology and research and development. Based on the current R&D direction of GaN LEDs, it is roughly divided into the development direction of large current, short wavelength, and high efficiency.
How to support larger LED current <br> <br> In recent years, the industry is very positive for only one can achieve a considerable brightness LED research and development, technology in this area and therefore it falls on how to support LED More current. Usually 30um2 LED can drive the current of 30mA at most, but this kind of result is still far unable to meet market's expectation, so the goal is to need to lead more than 10 times of the electric current, turn on LED element. Therefore, when the LED area size can be expanded to 1mm2, the next task is how to make the current value can reach 350500mA, because the driving voltage is more than 3V, so it can have 1W of power can be flowed into 1mm2 Chip area.
In terms of luminous color rendering, although there is such a large amount of power input into GaN LEDs, three-quarters of the input power cannot be converted into light to generate heat, so the LED will be overheated, which will also have a direct impact. LED color rendering results. Because the basic characteristics of the LED element is that if the temperature rises, the luminous efficiency will decrease and the color rendering deviation will occur. Therefore, how to effectively release a large amount of heat generating heat release technology has become the key, so the LED is installed in the thermal conductivity, heat capacity The material has become a very important issue, and most of the current use of valuable metals or ceramics.
Short-wavelength brings high energy to the excitation light to increase the luminous efficiency of fluorescent powder
GaNLEDs starting from blue light have been successfully developed with high-luminance green LEDs. Although there has been a trend toward long-wavelength development, the crystallinity caused by the increase in the ratio of mixed crystals of InN has gradually been abandoned by the industry. . On the other hand, research and development for new applications such as becoming a laser substitute have begun to be considered, so the current industry is actively conducting research and development for short wavelengths. Recently, some universities in Japan have successfully developed 250nm LEDs in their laboratories, but their practicality remains to be considered because the human eye accepts wavelengths at about 380nm, so if the wavelength is shorter than 380nm, it cannot be produced. The LED in the domain may generate low output.
In order to avoid encountering the aforementioned problems, most of the following solutions are currently used:
1. The light emitting layer structure was changed: Instead of the GaInN structure used on the chip of the visible region LED, AlGaN or AlGaInN having a larger Eg was used.
2. Avoidance of light absorption loss: If there is a GaN or GaInM layer in the LED chip structure, light will not be emitted because it absorbs light itself, so using the AlGaN layer as a basis to construct the entire structure layer will yield better results. Or use GaN as an important n-type bottom layer.
3. Reducing Crystal Defects: The density of crystal defects in short-wavelength LEDs can have a large impact on light output and lifetime.
If we can successfully solve the three problems mentioned above, we believe that the practical distance for using LED as general lighting can be greatly shortened. At present, the efficiency of GaN white LEDs can exceed that of white-light bulbs and halogen lamps (1525 lm/W), but in order to be able to exceed the size of fluorescent lamps that have an overwhelming light output (above 5080 lm/W), it needs to be more drastic. The efficiency increase and the amount of light increase dramatically. In order to achieve the same light source characteristics as a fluorescent lamp, white light technology using the color mixture of fluorescent powder has become a key factor. If the efficiency of the LED is fully utilized and the wavelength can be shortened, the luminous efficiency of the fluorescent powder is believed to increase substantially due to the increased energy of the excitation light.
In the crystal growth face is the key to obtain a uniform mass <br> <br> so-called internal electron emission efficiency is the ratio of the light converted into. It can be said that the LED central part of the luminous efficiency. However, due to factors of crystal defects, the luminous efficiency of LEDs is seriously affected. When GaN grows, a very high density of migration defects occurs in the direction of the growth crystals because of the difference in lattice constant difference and thermal expansion coefficient between the sapphire substrate and the GaN single crystal on the substrate.
In general, the resulting density is above 109cm2. Such a density would be fatal if it occurs in short-wavelength LEDs and laser diodes. In order to reduce this kind of transposition density, there are roughly two methods. One is to prevent translocation from penetrating to the longitudinal direction, and the other is to suppress translocation. In the aspect of preventing the indexing from penetrating to the longitudinal direction, the substrate processed by the pattern may be used. When the vertical growth is made, the horizontal direction is formed, and the long side of the defect is bent toward the horizontal direction, and the vertical direction is realized. As a result, to reduce the translocation phenomenon, although this approach may be able to reach a low index below 107cm2, but the actual mass production, to obtain a uniform quality in the long crystal plane is the key. The latter method is to use a group III nitride substrate having a low density of crystal defects or a low-defect group III nitride on a substrate already formed.
Originally, there is no monocrystalline Bulk in the group III nitride. When using a sapphire substrate for hetero-epitaxial generation, the source of high density of the translocation is the use of this heterogeneous substrate. Of course, the Bulk substrate is the best solution. . As a result, R&D and mass production in various manufacturing methods are under active development, and some have begun to enter the sales stage. On the other hand, it is the Template substrate that is capable of performing a similar function as opposed to the final substrate Bulk substrate. At present, several companies are starting to produce small quantities. Although these are not as costly as Bulk substrates, the cost is not low, because they can only be used for lasers and electronic devices, UVLEDs, etc. because of the high cost and efficiency.
Although there are many crystal defects, why can GaN-based LED devices achieve high brightness and the chip does not deteriorate rapidly? These structural phenomena are still being studied by engineers and scholars, but there is no complete theory. Therefore, in order to achieve the maximum limit of the material and to exert the limit of GaN, it is necessary to determine the ideal layer structure and structural design of the light emitting structure.
If you can not achieve a good crystal everything is in vain <br> No matter how good the structural design, if you can not achieve a good crystal, everything is in vain. In the initial stage, mass-produced GaN LEDs were face-up type devices. The contact electrode on the p-side was a light-transmissive thin-film electrode that emitted light through this thin-film electrode, while the material used was an Au alloy electrode, but Translucent characteristics, but the actual transmittance and can not meet the needs of practical applications, because the optical coefficient of the electrode, or reflected light can not be emitted much, so that the luminous efficiency has not been improved. Therefore, R&D personnel will consider it later because face-up type LED elements have a high reflectivity. Therefore, a highly stable material must be used as an electrode to emit light from the sapphire substrate side to increase the luminous flux.
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