Analysis of blue and green chips used in LED displays
Analysis of blue and green chips used in LED displays
The working principle of LED is in the case of positive conduction, injected into the diode P / N section of the electron and hole meet composite, the potential energy is converted into light energy. The wavelength of the issued photons (that is, the color of light) is determined by the semiconductor band width, in layman's terms, the wider the width of the semiconductor band, the greater the energy of the emitted photons, the corresponding wavelength is shorter, the simple conversion relationship is: (nm). The current blue and green LED device material basis is the III nitride semiconductor, that is, GaN-based, InN, AlN as a supplement to the quadratic AlGaInN alloy system.
At present, the vast majority of blue and green light LED chip quantum well light-emitting layer material is composed of InxGa1-xN alloy and GaN, due to the InxGa1-xN alloy energy bandwidth with the proportion of InN x changes, can be adjusted in the 3.4eV (corresponding to the energy bandwidth of GaN) and 0.7eV (corresponding to the energy bandwidth of InN), so theoretically this material system can be cover the entire visible spectral region. However, the current material preparation technology is based on the epitaxial layer growth of GaN crystals, which can only grow alloys with a low InN content; the crystal quality of InxGa1-xN alloys decreases dramatically after the InN content x>15%. In fact, the current level of technology in the industry usually do blue light chip electro-optical conversion efficiency is about 2 times that of green light, is because the former's InN composition is much smaller than the latter, green light devices in the InN component is estimated to have been in more than 30% (InGaN alloy materials to determine the precise composition of the current academic community is still a difficult scientific problem). That is to say, the current technology is still very difficult to continue to increase the composition of InN, so that the InGaN alloy device can be highly efficient red light. But thankfully, as early as the 90s of last century, group III phosphide system (also commonly expressed as a quaternary system, AlGaInP) has become a mature material basis for red and yellow light LED devices. The basic physical characteristics of these two material systems and the position of the elements they contain in the periodic table.
Group III nitride semiconductor materials are currently prepared industrially by metal-organic chemical vapor deposition (metal-organic chemical vapor deposition, MOCVD) to achieve. The basic principle of this technique is to grow high-quality crystals on a heated substrate substrate (usually sapphire is chosen as the substrate) by introducing a high-purity metal-organic source (MO source) and ammonia gas (NH3) in a closed chemical reaction chamber. The basic chemical reaction formula is: Ga(CH3)3+NH3→GaN+CH4. Usually GaN crystals are hexagonal fibrillated zincite structures, and the basic physical properties are shown in Table 2. Need to point out two points in particular: (1) GaN's energy band width at room temperature 300K, equal to 3.39eV, is very rare wide-band semiconductor materials, if the light-emitting, corresponding photon wavelengths should be, belonging to the ultraviolet; (2) GaN's p-type doping is very difficult to achieve the carrier concentration than the n-type doping is nearly two orders of magnitude lower than that of the resistance is very large. This property imposes special requirements on its device design, which will be mentioned in the subsequent introduction of LED device structures.The differences in physical properties between GaN and its cousins AlN and InN are very significant, and specific comparisons are given in Table 3. The orientation of GaN crystals during crystal growth is closely related to the choice of crystal planes for the sapphire substrate. Currently, the industrialized growth of GaN crystals generally take the c-plane sapphire as the substrate substrate, GaN crystal growth and substrate crystal orientation will maintain a fixed coordination relationship (which is the meaning of “epitaxial”). The crystal growth is along the c-axis, layer by layer, atom by atom, that is, the c-axis direction of growth.
GaN-based LED epitaxial wafer basic structure is in the sapphire substrate in order to grow: (1) GaN crystalline layer; (2) n-type GaN (the actual production of a layer of unintentionally doped n-type GaN is generally the first long; (3) InGaN/GaN multi-quantum well light-emitting layer; (4) p-type GaN. In order to obtain high-performance devices, the whole epitaxial growth process of the various parameters have to get In order to obtain high-performance devices, the parameters of the entire epitaxial growth process should be optimized and precisely controlled, of which the structure that has the greatest impact on the luminous efficiency is the InGaN/GaN multi-quantum well light-emitting layer. p- and n-type materials are usually doped with Mg and Si, Mg by replacing Ga atoms in the GaN (Mg is less than Ga by one peripheral electron), to form a hole carrier, and Si by substituting for Ga atoms, to form an electron carrier (Si is more than Ga by one peripheral electron). (Si has one more peripheral electron than Ga). Generally, the epitaxial layer thickness of the whole device ranges from 4 to 8 μm, and the average growth rate is about 1 μm/hour, so it takes about 8 hours to complete the growth of a device.
After completing the MOCVD epitaxial growth, GaN-based LED chips need to be prepared through a series of photomask graphics processing and physical etching or deposition process. Ordinary blue and green LED chip basic structure, need to do the following device processing on the epitaxial wafer in turn: (1) etching local area to expose n - type GaN conductive layer; (2) evaporation transparent conductive film NiAu or ITO; (3) evaporation welding line electrodes, including p electrodes and n electrodes; (4) evaporation passivation protection layer. The chip processing process requires strict quality management to avoid problems such as insufficient mechanical adhesion of the pads and contamination of the surface by foreign objects, which can easily lead to failure of the device in the encapsulation and use process. In addition, the chip then also need to do substrate thinning, physical cut separation, testing, sorting, and finally get the optoelectronic parameters consistent chip finished product. As GaN-based LED chip substrate sapphire is an insulator, the chip through the upper surface of the two + / - electrodes and metal welding line connection to conduct electricity. In contrast, the current common GaAs substrate red light chip or through the conductive adhesive so that the substrate and support the formation of conductive channels between the process of controlling the conductive adhesive physical bonding strength of the encapsulation circuit breakage failure control is particularly important.
Blue, green LED chip photoelectric parameter characteristics
1, I-V relationship curve
Blue, green LED chip is usually in the forward voltage 2.4V or so when the start of conduction, operating current 20mA voltage Vf range is generally 3.0 ~ 3.4V (for 14 × 14mil2 square chip size,), the higher operating voltage is determined by the forbidden bandwidth of the GaN semiconductor.
2.2 The effect of ambient temperature on the photoelectric properties
A common structure 14 × 14mil2 green LED chip in different ambient temperatures under the characteristic change curve. When the ambient temperature rises from 20 ℃ to 80 ℃, the display of the green LED light-emitting wavelength of the obvious drift, from 522nm red shift to 527nm; display light-emitting brightness reduced by 25%; display operating voltage from 3.23V down to 2.98V.
With the increase of ambient temperature, the red shift of the luminescence wavelength and the decrease of the operating voltage are due to the narrowing of the semiconductor forbidden bandwidth. However, due to the large material forbidden bandwidth of GaN system, the upper limit of ambient temperature that can be tolerated has a very obvious advantage over other materials. Experiments have found that in the 150 ℃ ambient temperature, GaN-based blue and green LED devices can still light, only the efficiency is greatly reduced. But, on the other hand, for such common structure of the chip, blue light electro-optical conversion efficiency between 20 ~ 30%; green light is significantly lower, generally only 10 ~ 20%. In addition to a small portion of the electrical energy into light energy, the other are generated heat, these heat for the tiny chip area is a great burden. Therefore, the use of chip packaging, need to pay special attention to the chip to do a good job in the design of the heat dissipation channel, so as to ensure that the chip can work stably and reliably.
2, the impact of the working current density on the wavelength
Ordinary 14 × 14mil2 green LED chip light-emitting wavelength with the operating current change curve. With the current density
Increase in current density, the green chip light-emitting wavelength from 534nm (2mA test) blue to 522nm (30mA test). In fact, the blue chip also has a similar blue shift trend, only the amplitude is smaller than the green chip, this characteristic is very important for the design of the operating conditions of the chip. In order to avoid color drift with brightness, the way to adjust the brightness is generally chosen to change the pulse width, rather than changing the current intensity.
The energy of the electron-hole complex to generate photons determines the luminescence wavelength, which is determined by the potential energy of the electron-hole pairs bound in the quantum well. In fact, the chip from 2mA to 30mA current increase in the process, the potential energy of the electron-hole pairs in the quantum well has undergone two very important changes: firstly, the shielding of the built-in electric field in the quantum well, so that the conduction band and the valence band distance increases; and then the carrier filling effect makes the electron-hole pairs of the potential energy between the potential energy to increase further, and increase the potential energy of the electron-hole pairs of the photon corresponding to the photon will become shorter, the wavelength of light. This can be deduced from the aforementioned conversion relationship between wavelength and energy.
Key stages in the development of the technology
1, p-n junction GaN diode key technology breakthrough stage (1970 ~ 1993)
As early as the 1970s, American scientists J. Pankove and others have found that GaN is a good wide-band semiconductor light-emitting materials, and successfully produced a GaN Schottky tube that can emit blue light. However, the subsequent decade, scientists have not been able to break through the preparation of p-type GaN materials in the research efforts. Until the late 1980s, Japanese scientists Akasaki and Amano found that can first be deposited on a heterogeneous substrate AlN crystalline layer, and then be able to realize the MOCVD epitaxial growth of surface flat GaN single-crystal thin film materials. On this basis, they also found that can be activated by electron beam Mg doped GaN material in the hole carriers, the realization of p - type GaN material preparation, which is GaN-based p - n junction light-emitting diode the most critical basic technology breakthrough. Subsequently, GaN-based LED technology from the laboratory of the Institute into the factory. Nichia (Nichia) scientists Nakamura [15,16] realized the use of GaN crystalline layer to achieve high-quality epitaxial layer MOCVD growth, and soon found that you can activate Mg-doped GaN by thermal annealing to achieve p-type conductivity. As a result of this series of breakthroughs, in 1993 Nichia successfully commercialized the production of GaN blue LED.
2, internal quantum efficiency improvement stage (1993 ~ 2000)
After the successful commercial production of blue LEDs, academics and industry have invested a great deal of enthusiasm in researching many key physical issues in this field. One of the core issues is how to improve the quantum efficiency in the InGaN/GaN quantum well of blue LED chips, that is, how to improve the electro-optical conversion efficiency. The MOCVD equipment of many research units and enterprises was used to experiment with optimized growth conditions to improve the crystal quality of InGaN quantum wells; also many new device structure designs were tried to improve the carrier injection efficiency and the compounding efficiency. At this stage, the new research discoveries led to two major achievements: (1) the commercialization of green LEDs (1995 [17]); (2) the efficiency of blue LEDs was improved exponentially.
3. Stage of simultaneous improvement of internal and external quantum efficiency (2000 to present)
On the basis of the significant improvement in the performance of blue and green LEDs, they have been commercialized on a large scale, especially in cell phone backlighting
, full-color advertising signage and other application areas. Based on the stimulation of commercial interests, the improvement of luminous efficacy has become a life-and-death race among enterprises, which is particularly intense in Taiwan, South Korea and China. With many companies unable to significantly improve the internal quantum efficiency in a short period of time, these new entrants have begun to boldly try to do something about the luminous efficacy, that is, to improve the external quantum efficiency. The main breakthroughs are: (1) with ITO conductive film instead of metal semi-permeable film NiAu, transmittance increased by about 25%, that is, the brightness increased by 25%; (2) through the epitaxial layer surface growth of V-type pit defects, so that the surface of the total reflection is broken, thus significantly improving the efficiency of the extraction of light; (3) through the use of the surface of the coarsening of the sapphire substrate sheet, to break the GaN / sapphire all-reflective interface (3) By using surface roughened sapphire substrate sheet, the GaN/sapphire total reflection interface is broken, and the effect of significantly improving the light extraction efficiency is also realized. These methods in the introduction of the initial period have led to the device of other optoelectronic performance of the serious sacrifice, such as attenuation of serious, easy to produce leakage, electrostatic protection ability is weak, etc.. However, with the engineering advances made by corporate researchers, the various characteristics were gradually improved, while further understanding of the epitaxial material properties contributed to the continued improvement of the internal quantum efficiency. As a result, at this stage, blue and green LED light-emitting efficiency have been improved exponentially, and the latest research results show that the blue LED can achieve 50% electro-optical conversion efficiency under the optimization of internal and external quantum efficiency.
Technology development trend outlook
Through the improvement of epitaxial material preparation technology and the optimization of device physical structure design, blue and green LED technology has made remarkable development in the past 20 years. At the same time, thanks to the continuous improvement of performance and rapid cost reduction, the application areas and scale have also been greatly developed. However, looking ahead to new and more challenging areas of general lighting, further breakthroughs in LED technology are a must. This breakthrough will be more focused around how to reduce the use of LED cost, the key has three development directions: (1) reduce the manufacturing cost of the device; (2) improve the electro-optical conversion efficiency of the device; (3) improve the input power of the device.
1, reduce the manufacturing cost of the device
LED device manufacturing costs relative to silicon-based devices is still very high, mainly due to the scale of the industry and the degree of technological development is still far less than the silicon-based semiconductor industry. However, with reference to the development history of the mature semiconductor industry, we can expect the manufacturing cost of LED devices will continue to decline in the next 10 years. The main cost savings contribution will focus on relying on three parts: (1) the core equipment manufacturing technology advances will exponentially improve production efficiency, thereby significantly reducing depreciation costs, the most typical is the GaN epitaxial MOCVD equipment; (2) the size of the processing wafer exponentially increased from the current mainstream of the 2-inch wafer development to 4 inches, will greatly reduce the processing cost of the chip process; (3) the scale of industry The level of expansion will significantly reduce the cost of consuming raw materials and comprehensive management costs. Comprehensive these factors, can be expected in the next 10 years the cost of LED chips will continue to reduce, which will further stimulate the development of LED emerging applications.
2, improve the device's electro-optical conversion efficiency
LED device electro-optical conversion efficiency will also significantly reduce the cost of the final customer's use, here the cost savings are reflected in two aspects: on the one hand, the unit lumen brightness of the chip cost will be with the improvement of chip luminous efficiency and decline; on the other hand is the saving of electricity, such as from the energy efficiency of 25% of the development of the chip technology to 50% of the technology will achieve the effect of energy-saving half. And what is more meaningful is that the benefits of energy saving are not only reflected in the economy, but also in the social benefits. Therefore, research on conversion efficiency improvement will continue to receive substantial commercial and government R&D resources.
The improvement of electro-optical conversion efficiency will continue to advance along the two previously mentioned directions: (1) the improvement of internal quantum efficiency and (2) the improvement of light extraction efficiency. The improvement of internal quantum efficiency mainly relies on the progress of MOCVD epitaxial material preparation technology, by improving the crystal quality of the light-emitting layer quantum well (MQW), to improve the carrier injection efficiency and composite efficiency of the device, and the room for improvement in this regard has now become more limited. On the contrary, the improvement of light extraction efficiency there is a lot of room for development, the main work in this area will be: (1) further optimization of the interface roughness of the process, so as to improve the efficiency of light escape from the light-emitting layer; (2) improve the chip cutting process, reduce the transparent sapphire substrate side brightness absorption loss.
3, improve the input power of the device
In can keep the device electro-optical conversion efficiency under the premise of the same, by improving the input power per unit area of the chip, can also achieve the effect of reducing the cost of use. This direction of effort relies on two aspects of technological progress: on the one hand, the need to minimize the chip as well as the thermal resistance of the package structure, so that the input power can be increased within a certain upper limit of the operating temperature of the device; on the other hand, it is necessary to improve the design of the device MQW structure, so that it can be injected into the conditions of carrier density to maintain a certain level of electro-optical conversion efficiency. In the research direction of device thermal resistance control, there is still a lot of space to be developed in the field of LED products, especially in the welding solid crystal technology with low thermal resistance, welding materials with high thermal conductivity, and chip holder materials, which are worthy of serious research.
Conclusion
GaN-based blue and green LED technology over the past two decades of progress, has begun to open up a new solid-state new light source era in the world, this technology not only brings colorful, energy-saving and environmentally friendly new light source, but also is breeding a broader market space - solid-state general lighting market. Due to the technology's huge energy-saving benefits and the environmental characteristics of its materials, many strategic research projects have received a high degree of attention from major countries, and, at the same time, has attracted a large number of companies to participate in product development and promotion. It is reasonable to believe that in the next 10 years, the development of GaN-based blue and green LED technology will certainly lead to a thriving new solid-state lighting market!
At present, the vast majority of blue and green light LED chip quantum well light-emitting layer material is composed of InxGa1-xN alloy and GaN, due to the InxGa1-xN alloy energy bandwidth with the proportion of InN x changes, can be adjusted in the 3.4eV (corresponding to the energy bandwidth of GaN) and 0.7eV (corresponding to the energy bandwidth of InN), so theoretically this material system can be cover the entire visible spectral region. However, the current material preparation technology is based on the epitaxial layer growth of GaN crystals, which can only grow alloys with a low InN content; the crystal quality of InxGa1-xN alloys decreases dramatically after the InN content x>15%. In fact, the current level of technology in the industry usually do blue light chip electro-optical conversion efficiency is about 2 times that of green light, is because the former's InN composition is much smaller than the latter, green light devices in the InN component is estimated to have been in more than 30% (InGaN alloy materials to determine the precise composition of the current academic community is still a difficult scientific problem). That is to say, the current technology is still very difficult to continue to increase the composition of InN, so that the InGaN alloy device can be highly efficient red light. But thankfully, as early as the 90s of last century, group III phosphide system (also commonly expressed as a quaternary system, AlGaInP) has become a mature material basis for red and yellow light LED devices. The basic physical characteristics of these two material systems and the position of the elements they contain in the periodic table.
Group III nitride semiconductor materials are currently prepared industrially by metal-organic chemical vapor deposition (metal-organic chemical vapor deposition, MOCVD) to achieve. The basic principle of this technique is to grow high-quality crystals on a heated substrate substrate (usually sapphire is chosen as the substrate) by introducing a high-purity metal-organic source (MO source) and ammonia gas (NH3) in a closed chemical reaction chamber. The basic chemical reaction formula is: Ga(CH3)3+NH3→GaN+CH4. Usually GaN crystals are hexagonal fibrillated zincite structures, and the basic physical properties are shown in Table 2. Need to point out two points in particular: (1) GaN's energy band width at room temperature 300K, equal to 3.39eV, is very rare wide-band semiconductor materials, if the light-emitting, corresponding photon wavelengths should be, belonging to the ultraviolet; (2) GaN's p-type doping is very difficult to achieve the carrier concentration than the n-type doping is nearly two orders of magnitude lower than that of the resistance is very large. This property imposes special requirements on its device design, which will be mentioned in the subsequent introduction of LED device structures.The differences in physical properties between GaN and its cousins AlN and InN are very significant, and specific comparisons are given in Table 3. The orientation of GaN crystals during crystal growth is closely related to the choice of crystal planes for the sapphire substrate. Currently, the industrialized growth of GaN crystals generally take the c-plane sapphire as the substrate substrate, GaN crystal growth and substrate crystal orientation will maintain a fixed coordination relationship (which is the meaning of “epitaxial”). The crystal growth is along the c-axis, layer by layer, atom by atom, that is, the c-axis direction of growth.
GaN-based LED epitaxial wafer basic structure is in the sapphire substrate in order to grow: (1) GaN crystalline layer; (2) n-type GaN (the actual production of a layer of unintentionally doped n-type GaN is generally the first long; (3) InGaN/GaN multi-quantum well light-emitting layer; (4) p-type GaN. In order to obtain high-performance devices, the whole epitaxial growth process of the various parameters have to get In order to obtain high-performance devices, the parameters of the entire epitaxial growth process should be optimized and precisely controlled, of which the structure that has the greatest impact on the luminous efficiency is the InGaN/GaN multi-quantum well light-emitting layer. p- and n-type materials are usually doped with Mg and Si, Mg by replacing Ga atoms in the GaN (Mg is less than Ga by one peripheral electron), to form a hole carrier, and Si by substituting for Ga atoms, to form an electron carrier (Si is more than Ga by one peripheral electron). (Si has one more peripheral electron than Ga). Generally, the epitaxial layer thickness of the whole device ranges from 4 to 8 μm, and the average growth rate is about 1 μm/hour, so it takes about 8 hours to complete the growth of a device.
After completing the MOCVD epitaxial growth, GaN-based LED chips need to be prepared through a series of photomask graphics processing and physical etching or deposition process. Ordinary blue and green LED chip basic structure, need to do the following device processing on the epitaxial wafer in turn: (1) etching local area to expose n - type GaN conductive layer; (2) evaporation transparent conductive film NiAu or ITO; (3) evaporation welding line electrodes, including p electrodes and n electrodes; (4) evaporation passivation protection layer. The chip processing process requires strict quality management to avoid problems such as insufficient mechanical adhesion of the pads and contamination of the surface by foreign objects, which can easily lead to failure of the device in the encapsulation and use process. In addition, the chip then also need to do substrate thinning, physical cut separation, testing, sorting, and finally get the optoelectronic parameters consistent chip finished product. As GaN-based LED chip substrate sapphire is an insulator, the chip through the upper surface of the two + / - electrodes and metal welding line connection to conduct electricity. In contrast, the current common GaAs substrate red light chip or through the conductive adhesive so that the substrate and support the formation of conductive channels between the process of controlling the conductive adhesive physical bonding strength of the encapsulation circuit breakage failure control is particularly important.
Blue, green LED chip photoelectric parameter characteristics
1, I-V relationship curve
Blue, green LED chip is usually in the forward voltage 2.4V or so when the start of conduction, operating current 20mA voltage Vf range is generally 3.0 ~ 3.4V (for 14 × 14mil2 square chip size,), the higher operating voltage is determined by the forbidden bandwidth of the GaN semiconductor.
2.2 The effect of ambient temperature on the photoelectric properties
A common structure 14 × 14mil2 green LED chip in different ambient temperatures under the characteristic change curve. When the ambient temperature rises from 20 ℃ to 80 ℃, the display of the green LED light-emitting wavelength of the obvious drift, from 522nm red shift to 527nm; display light-emitting brightness reduced by 25%; display operating voltage from 3.23V down to 2.98V.
With the increase of ambient temperature, the red shift of the luminescence wavelength and the decrease of the operating voltage are due to the narrowing of the semiconductor forbidden bandwidth. However, due to the large material forbidden bandwidth of GaN system, the upper limit of ambient temperature that can be tolerated has a very obvious advantage over other materials. Experiments have found that in the 150 ℃ ambient temperature, GaN-based blue and green LED devices can still light, only the efficiency is greatly reduced. But, on the other hand, for such common structure of the chip, blue light electro-optical conversion efficiency between 20 ~ 30%; green light is significantly lower, generally only 10 ~ 20%. In addition to a small portion of the electrical energy into light energy, the other are generated heat, these heat for the tiny chip area is a great burden. Therefore, the use of chip packaging, need to pay special attention to the chip to do a good job in the design of the heat dissipation channel, so as to ensure that the chip can work stably and reliably.
2, the impact of the working current density on the wavelength
Ordinary 14 × 14mil2 green LED chip light-emitting wavelength with the operating current change curve. With the current density
Increase in current density, the green chip light-emitting wavelength from 534nm (2mA test) blue to 522nm (30mA test). In fact, the blue chip also has a similar blue shift trend, only the amplitude is smaller than the green chip, this characteristic is very important for the design of the operating conditions of the chip. In order to avoid color drift with brightness, the way to adjust the brightness is generally chosen to change the pulse width, rather than changing the current intensity.
The energy of the electron-hole complex to generate photons determines the luminescence wavelength, which is determined by the potential energy of the electron-hole pairs bound in the quantum well. In fact, the chip from 2mA to 30mA current increase in the process, the potential energy of the electron-hole pairs in the quantum well has undergone two very important changes: firstly, the shielding of the built-in electric field in the quantum well, so that the conduction band and the valence band distance increases; and then the carrier filling effect makes the electron-hole pairs of the potential energy between the potential energy to increase further, and increase the potential energy of the electron-hole pairs of the photon corresponding to the photon will become shorter, the wavelength of light. This can be deduced from the aforementioned conversion relationship between wavelength and energy.
Key stages in the development of the technology
1, p-n junction GaN diode key technology breakthrough stage (1970 ~ 1993)
As early as the 1970s, American scientists J. Pankove and others have found that GaN is a good wide-band semiconductor light-emitting materials, and successfully produced a GaN Schottky tube that can emit blue light. However, the subsequent decade, scientists have not been able to break through the preparation of p-type GaN materials in the research efforts. Until the late 1980s, Japanese scientists Akasaki and Amano found that can first be deposited on a heterogeneous substrate AlN crystalline layer, and then be able to realize the MOCVD epitaxial growth of surface flat GaN single-crystal thin film materials. On this basis, they also found that can be activated by electron beam Mg doped GaN material in the hole carriers, the realization of p - type GaN material preparation, which is GaN-based p - n junction light-emitting diode the most critical basic technology breakthrough. Subsequently, GaN-based LED technology from the laboratory of the Institute into the factory. Nichia (Nichia) scientists Nakamura [15,16] realized the use of GaN crystalline layer to achieve high-quality epitaxial layer MOCVD growth, and soon found that you can activate Mg-doped GaN by thermal annealing to achieve p-type conductivity. As a result of this series of breakthroughs, in 1993 Nichia successfully commercialized the production of GaN blue LED.
2, internal quantum efficiency improvement stage (1993 ~ 2000)
After the successful commercial production of blue LEDs, academics and industry have invested a great deal of enthusiasm in researching many key physical issues in this field. One of the core issues is how to improve the quantum efficiency in the InGaN/GaN quantum well of blue LED chips, that is, how to improve the electro-optical conversion efficiency. The MOCVD equipment of many research units and enterprises was used to experiment with optimized growth conditions to improve the crystal quality of InGaN quantum wells; also many new device structure designs were tried to improve the carrier injection efficiency and the compounding efficiency. At this stage, the new research discoveries led to two major achievements: (1) the commercialization of green LEDs (1995 [17]); (2) the efficiency of blue LEDs was improved exponentially.
3. Stage of simultaneous improvement of internal and external quantum efficiency (2000 to present)
On the basis of the significant improvement in the performance of blue and green LEDs, they have been commercialized on a large scale, especially in cell phone backlighting
, full-color advertising signage and other application areas. Based on the stimulation of commercial interests, the improvement of luminous efficacy has become a life-and-death race among enterprises, which is particularly intense in Taiwan, South Korea and China. With many companies unable to significantly improve the internal quantum efficiency in a short period of time, these new entrants have begun to boldly try to do something about the luminous efficacy, that is, to improve the external quantum efficiency. The main breakthroughs are: (1) with ITO conductive film instead of metal semi-permeable film NiAu, transmittance increased by about 25%, that is, the brightness increased by 25%; (2) through the epitaxial layer surface growth of V-type pit defects, so that the surface of the total reflection is broken, thus significantly improving the efficiency of the extraction of light; (3) through the use of the surface of the coarsening of the sapphire substrate sheet, to break the GaN / sapphire all-reflective interface (3) By using surface roughened sapphire substrate sheet, the GaN/sapphire total reflection interface is broken, and the effect of significantly improving the light extraction efficiency is also realized. These methods in the introduction of the initial period have led to the device of other optoelectronic performance of the serious sacrifice, such as attenuation of serious, easy to produce leakage, electrostatic protection ability is weak, etc.. However, with the engineering advances made by corporate researchers, the various characteristics were gradually improved, while further understanding of the epitaxial material properties contributed to the continued improvement of the internal quantum efficiency. As a result, at this stage, blue and green LED light-emitting efficiency have been improved exponentially, and the latest research results show that the blue LED can achieve 50% electro-optical conversion efficiency under the optimization of internal and external quantum efficiency.
Technology development trend outlook
Through the improvement of epitaxial material preparation technology and the optimization of device physical structure design, blue and green LED technology has made remarkable development in the past 20 years. At the same time, thanks to the continuous improvement of performance and rapid cost reduction, the application areas and scale have also been greatly developed. However, looking ahead to new and more challenging areas of general lighting, further breakthroughs in LED technology are a must. This breakthrough will be more focused around how to reduce the use of LED cost, the key has three development directions: (1) reduce the manufacturing cost of the device; (2) improve the electro-optical conversion efficiency of the device; (3) improve the input power of the device.
1, reduce the manufacturing cost of the device
LED device manufacturing costs relative to silicon-based devices is still very high, mainly due to the scale of the industry and the degree of technological development is still far less than the silicon-based semiconductor industry. However, with reference to the development history of the mature semiconductor industry, we can expect the manufacturing cost of LED devices will continue to decline in the next 10 years. The main cost savings contribution will focus on relying on three parts: (1) the core equipment manufacturing technology advances will exponentially improve production efficiency, thereby significantly reducing depreciation costs, the most typical is the GaN epitaxial MOCVD equipment; (2) the size of the processing wafer exponentially increased from the current mainstream of the 2-inch wafer development to 4 inches, will greatly reduce the processing cost of the chip process; (3) the scale of industry The level of expansion will significantly reduce the cost of consuming raw materials and comprehensive management costs. Comprehensive these factors, can be expected in the next 10 years the cost of LED chips will continue to reduce, which will further stimulate the development of LED emerging applications.
2, improve the device's electro-optical conversion efficiency
LED device electro-optical conversion efficiency will also significantly reduce the cost of the final customer's use, here the cost savings are reflected in two aspects: on the one hand, the unit lumen brightness of the chip cost will be with the improvement of chip luminous efficiency and decline; on the other hand is the saving of electricity, such as from the energy efficiency of 25% of the development of the chip technology to 50% of the technology will achieve the effect of energy-saving half. And what is more meaningful is that the benefits of energy saving are not only reflected in the economy, but also in the social benefits. Therefore, research on conversion efficiency improvement will continue to receive substantial commercial and government R&D resources.
The improvement of electro-optical conversion efficiency will continue to advance along the two previously mentioned directions: (1) the improvement of internal quantum efficiency and (2) the improvement of light extraction efficiency. The improvement of internal quantum efficiency mainly relies on the progress of MOCVD epitaxial material preparation technology, by improving the crystal quality of the light-emitting layer quantum well (MQW), to improve the carrier injection efficiency and composite efficiency of the device, and the room for improvement in this regard has now become more limited. On the contrary, the improvement of light extraction efficiency there is a lot of room for development, the main work in this area will be: (1) further optimization of the interface roughness of the process, so as to improve the efficiency of light escape from the light-emitting layer; (2) improve the chip cutting process, reduce the transparent sapphire substrate side brightness absorption loss.
3, improve the input power of the device
In can keep the device electro-optical conversion efficiency under the premise of the same, by improving the input power per unit area of the chip, can also achieve the effect of reducing the cost of use. This direction of effort relies on two aspects of technological progress: on the one hand, the need to minimize the chip as well as the thermal resistance of the package structure, so that the input power can be increased within a certain upper limit of the operating temperature of the device; on the other hand, it is necessary to improve the design of the device MQW structure, so that it can be injected into the conditions of carrier density to maintain a certain level of electro-optical conversion efficiency. In the research direction of device thermal resistance control, there is still a lot of space to be developed in the field of LED products, especially in the welding solid crystal technology with low thermal resistance, welding materials with high thermal conductivity, and chip holder materials, which are worthy of serious research.
Conclusion
GaN-based blue and green LED technology over the past two decades of progress, has begun to open up a new solid-state new light source era in the world, this technology not only brings colorful, energy-saving and environmentally friendly new light source, but also is breeding a broader market space - solid-state general lighting market. Due to the technology's huge energy-saving benefits and the environmental characteristics of its materials, many strategic research projects have received a high degree of attention from major countries, and, at the same time, has attracted a large number of companies to participate in product development and promotion. It is reasonable to believe that in the next 10 years, the development of GaN-based blue and green LED technology will certainly lead to a thriving new solid-state lighting market!