In recent years, LED technology has developed rapidly, making breakthroughs in substrate, epitaxy and chip core technologies.
1. Graphical substrate
At present, LED epitaxy generally uses patterned substrate (PSS). PSS is currently divided into micro-scale PSS and nano-scale nPSS. Micro-scale PSS has various shape patterns. The height of the pattern is generally 1.1~1.6μm, and the diameter of the circle is 2.5~3μm. About 4μm, using light micro-projection and plasma dry etching technology, generally can improve the light efficiency by 30~40%. The nPSS generally adopts nanoimprint technology, and the pattern size is about 260 nm, and the period is about 460 nm, which generally improves the light efficiency by about 70%.
(1) nPSS substrate
The nano-template and substrate parallelism are demanding, nPSS advantages: LEDs have higher luminous efficiency, better uniformity and lower cost. For example, a hexagonal array with a period of 450 nm is obtained by nanoimprint lithography on a sapphire substrate, so that the output power of the green LED is three times that of the original.
(2) Nanopillar PSS
The new technology of the British company Seren uses a unique nanolithography technology on the sapphire substrate to form a nano-column on the surface. The epitaxial growth on the substrate can relieve the stress by 85%, thereby greatly reducing defects and improving the brightness of the light. Up to 80~120%, the industrialization level of LED light efficiency reaches 200lm/w, and the Droop effect is improved, and the attenuation is reduced by about 30%.
Summary: PSS can greatly improve the LED luminous efficiency, especially the nano-scale nPSS can greatly improve the LED luminous efficiency. PSS is the development trend of the current LED core technology. There are different views on PSS in terms of cost reduction.
2, homogenous substrate
The homogenous substrate is made of GaN as the substrate. There are various methods for growing the GaN substrate. Generally, HVPE (Hydride Vapor Phase Epitaxy) or sodium flow method is used. The GaN substrate is produced to solve the residual stress and surface roughness problem. The bottom thickness is about 400~500μm, and it can be industrialized. Advantages of GaN substrate: low dislocation density (105~106/cm2), internal quantum efficiency of over 80%, short growth time of about 2 hours, saving a lot of raw materials, and greatly reducing costs.
(1) Realize high brightness LED
Toyota Synthetic uses a c-plane GaN substrate to grow LED chips with an area of ​​1mm2, which can achieve 400lm luminous flux.
(2) HVPE growth GaN substrate industrialization
Mitsubishi Chemical , Sumitomo Electric, Hitachi Cable and other companies use HVPE to grow GaN substrates with a thickness of about 450μm and dislocation density (106~107pcs/cm2). Mitsubishi Chemical recently announced the availability of 6′′ GaN substrates, and plans to 2015. The cost will be reduced to one tenth of the current. Dongguan Zhongrong (PKU) can mass produce GaN substrates.
(3) Improve internal quantum efficiency
Japan Insulators uses a sodium flow method to grow GaN substrates with low defect density and internal quantum efficiency of 90%. At 200 mA, its luminous efficiency reaches 200 lm/w, which can provide 4′′ GaN substrates and is accelerating the development of low defects. "Substrate.
(4) Large size GaN substrate
Sumitomo Electric and Soitec have jointly developed 4" and 6" GaN substrates, using crystal garden manufacturing technology and intelligent lift-off layer transfer technology to produce ultra-thin high-quality GaN substrates with low defect density and announced the availability of GaN substrates.
(5) LiGaO2 substrate
South China University of Technology research and development on the LiGaO2 substrate by laser molecular beam epitaxy growth of non-polar GaN substrate, thickness 2μm, as a composite substrate growth GaN chip, required to achieve a dislocation density of 1 × 106 / cm2, internal quantum efficiency of 85% The conversion efficiency is 65%.
(6) Award-winning products
The US company Soraa used Nakamura Shuji's GaN-on-GaN technology to make LED replacement lamps, which was named "one of the most important achievements in semiconductor materials science in the past 30 years" by SVIPLA. Its LED crystal integrity has increased by more than 1,000 times, making it possible to use one LED device per lamp.
Summary: GaN-on-GaN homogeneous substrate growth LED, its defect density (105 ~ 106 / cm2), can greatly improve the LED luminous efficiency, and Droop is not obvious when increasing the current density, so that the general lighting is adopted The single-chip LED light source takes the LED core technology to a new level. Summary with Nakamura Shuji: We believe that with GaN-on-GaN LED, we have really written a new chapter in LED technology, namely LED version 2.0.
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