After 57 years of development, visible LED has realized full-color, providing a colorful visual feast and efficient energy-saving lighting source for human beings. However, for a long time, the development of light efficiency (light power efficiency) of seven-color LED is very unbalanced, among which the light efficiency of yellow LED is much lower than that of other colors, resulting in high light efficiency yellow light has to be obtained by wavelength conversion of phosphors. This "electro-optic" conversion technology is the mainstream technology of LED lighting. However, in the process of light-to-light conversion, phosphors have some congenital shortcomings, such as high thermal loss, large thermal-to-light attenuation and slow response, which restrict the rapid development of LED to high-quality lighting and high-speed visible light communication.
Therefore, to solve the "Huang Guang Divide" problem has become an attractive goal in this field. What kind of substrate, what kind of material, what kind of device structure, what kind of chip structure and what kind of equipment can make high-efficiency yellow LED? There is no answer at home and abroad for a long time.
Fig. 1 Semiconductor yellow LED electroluminescence
In two kinds of yellow-light material systems, AlGaInP changes from red light to yellow light, and the band gap changes from direct to indirect, which is a physical bottleneck. Indium GaInP is a direct band gap, and its biggest difficulty is to grow high-quality, high-indium component InGaN quantum well materials, which is a technical bottleneck. The indium component in yellow-light LED quantum wells is about 30%, which is obviously higher than that in blue-light quantum wells about 15%. There are many difficulties in the growth of indium, gallium and nitrogen with high indium content: low growth temperature leads to more nitrogen vacancies in ammonia pyrolysis, slow atom migration and rough surface, blurred barrier interface with different thickness and inhomogeneous indium component, segregation of indium segregation phase in severe cases, and less overlap of carrier wavelet functions due to strong polarization of InGaN/GaN, etc.
Recently, Jiang Fengyi Group of Nanchang University published an article on Photonics Research: Efficient InGaN based Yellow Light-emitting Diodes, which shows the breakthroughs in the design and manufacturing technology of yellow-light-band LED materials. Based on the original GaN/Si-based blue-light LED material, device structure and chip structure, they designed new material structure, new growth equipment and new technology. They have transformed three major mismatches (large mismatch of thermal expansion coefficient, large mismatch of band gap width and large mismatch of lattice constant) into three major advantages.
The research team has invented a grid material growth method and technology. Regular grids are fabricated on substrates to replace irregular cracks, eliminating the GaN/Si stress accumulation effect, solving the problem that the material can not fabricate light-emitting chips due to cracks, and maintaining the tension stress of GaN, which has the advantage of growing high-performance indium-rich InGaN materials.
Researchers have invented a silicon-based LED chip structure with high light-collecting efficiency by adjusting the current direction, light-emitting position and light-emitting path. It not only solves the problems of substrate absorption and electrode blocking, but also has the advantages of single-side light-emitting and high beam quality. The new integrated transition layer not only solves the problem of high dislocation density, but also makes rational use of dislocations to form device structures with large V pits, which improves the hole transport path and improves the efficiency of LED.
On the basis of transforming the three mismatches into three advantages, they regulated the transport pathway and deposition mechanism of reaction gas, invented a dense coaxial tube structure reaction chamber, developed a device for growing indium-rich indium-GaN material, which was more conducive to indium incorporation, increased the growth temperature of the material, reduced the memory effect, and made the interface between quantum well and barrier steep; and the current density of 20A/cm2 and 3A/cm2 was high. Driven by D, the luminous efficiency of 565 nm silicon-based yellow LED is increased to 24.3% and 33.7%, corresponding to 149 lm/W and 192 lm/W, respectively, which effectively alleviates the yellow light gap and solves the problem of lack of high luminous efficiency in international LED.
The material structure is a three-dimensional P-N junction silicon substrate InGaN multi-quantum well structure with large V-pits, and the chip is single-sided and thin-film structure. Fig. 2 is a new type of LED street lamp which combines high-efficiency silicon-based InGaN yellow LED and silicon-based AlGaInP red LED. It has the characteristics of module color temperature 2190K, display finger 66, 143 lm/W warm tone and no phosphor.
Fig. 2 An application example of a new type of LED street lamp with red and yellow mixed light and warm tone without phosphor
This kind of street lamp not only has the advantages of warm tone of traditional high-voltage nanometer lamp, but also has the advantages of energy saving of fluorescent LED street lamp. It not only saves rare earth resources, speeds up the light response speed, but also avoids the light health risk of fluorescent blue-rich LED street lamp, reduces light pollution and creates a warm and healthy road lighting atmosphere.
Breakthroughs in manufacturing technology of high-efficiency yellow-light LED are of great significance to the development of fine-tunable high-quality LED lighting and high-speed visible light communication.