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The development speed of semiconductor lighting technology exceeds the expectations of the industry. The global LED market is growing steadily. China is a big country in the production of traditional lighting sources and lamps, and a major exporter. It also has considerable advantages in the field of semiconductor lighting.
In February 1963, N. Holonyak, the inventor of LED, expressed his views in the American magazine Reader's Digest, expressing his firm belief that LEDs would develop into practical white light sources and made a clear prediction: future lights can be pencil-sized. An alloy that is practical and not easily broken and will never be burned. It is at least 10 times more efficient than today's versatile bulbs.
LED process technology is rapidly improving
Today, LED epitaxial technology, chip manufacturing technology and packaging technology have made great progress, and Holonyak's prediction is becoming a reality step by step.
The quality of the LED epitaxial material is largely related to the degree of lattice matching between the substrate and the epitaxial material. After years of efforts, the number of epitaxial layer dislocations per inch of InGaN/sapphire has dropped from 1010 to 108. Can drop to 105 or better, and there is still much room for improvement.
InGaN epitaxial substrates mainly include sapphire, silicon carbide, zinc oxide, gallium nitride and aluminum nitride. Its research and development and industrialization progress are as follows:
1. Sapphire epitaxial wafer: In 2007, the dosage was about 5 million pieces (2 inches). It is estimated that the amount will be 10 million pieces by 2010, of which 2/3 is 2 inch pieces and 1/3 is 3 inch pieces. At the end of 2007, Japan’s Showa Denko began production with 4-inch sheets.
2. Silicon Carbide Epitaxial Wafer: Cree, USA, demonstrated a 4-inch zero dislocation single wafer in May 2007. The 3-inch film of the Crystal Research Institute of Shandong University has also been successfully developed, and the 2-inch film has been opened for use. The 2 inch diameter crystal of the Institute of Physics of the Chinese Academy of Sciences has been successfully developed, and the dislocation density is less than 100 per square centimeter.
3. Zinc Oxide Epitaxial Wafer: It is generally prepared by hydrothermal method. Many units such as Shanghai Opto-mechanical Institute have been successfully developed, but it is not common for InGaN epitaxy.
4. GaN epitaxial wafer: TOPGaN Company of Warsaw Warsaw High Voltage Research Center obtained φ10mm single crystal under extreme growth conditions (15000 atmosphere, 1600 °C), sliced ​​20 pieces -30 pieces, dislocation density 100 per square centimeter, used A gallium nitride laser of 15 μm 500 μm 1.89 W was fabricated. The molten salt method used by the Institute of Physics of the Chinese Academy of Sciences grows into a practical GaN single crystal at less than 1000 ° C and atmospheric pressure. In April 2003, Sumitomo Electric Industries of Japan produced GaN single crystal substrates by HVPE (Hydride Vapor Phase Epitaxy). Nanjing University first produced a 2-inch low dislocation GaN substrate by HVPE. In April 2007, Hitachi Cable of Japan made a 3-inch GaN wafer by a gap-forming peeling method.
5. Aluminum nitride epitaxial wafer: A 2-inch aluminum nitride substrate was fabricated in May 2006 by Crys-talIS, Washington, USA.
In addition, Japan Showa Denko has developed a new process that combines conventional MOCVD (metal organic chemical vapor deposition) and PPD (plasma physical deposition) to improve crystal integrity and eliminate particles generated by MOCVD processes. The FWHM (full width at half maximum) of the X-ray back-swing curve is reduced from 150 arc seconds to 50 arc seconds, which improves the stability of production (between furnace and furnace and between sheets and sheets), greatly improving production efficiency.
In the field of LED chip manufacturing, process technology has also made breakthroughs. The laser stripping technology of the substrate was initiated by Osram, and the light extraction efficiency was increased to 75%, which is three times that of the traditional chip. At present, many units in China have mastered this technology and put it into production.
Improvements in surface roughening or texturing techniques can increase luminous efficiency by 30%-50%. Lumileds has developed flip-chip technology that increases heat dissipation efficiency and increases light output by a factor of 1.6. The use of indium tin oxide (ITO) material enables uniform current injection and increases light extraction efficiency by 60%. Matsushita Electric invented the two-dimensional photonic crystal, which increased the light output by 1.7 times to 2.7 times. Xuming Company uses a metal vertical photonic crystal structure (MVP) to achieve a luminous efficiency of 90 lm/W-100 lm/W.
For the LED package, the thermal resistance is low, the heat dissipation is good, the junction temperature is lowered, and the luminous efficiency is improved; at the same time, the package structure has a high light extraction efficiency. The thermal resistance of the device is the sum of the thermal resistance of each structural layer, which should meet the design principle of less layer number, large layer thickness, and high thermal conductivity of the material.
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