Luminescence is the result of electronic excitation of a material. The light-emitting diode is a p-n junction in which an applied voltage yields a flow of current, and the recombination of the carriers injected across the junction results in the emission of light…
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Blue light has the shortest wavelength of visible light. It means that if one builds a blue laser diode, one quadruples the amount of data that could be read and stored on a compact disc, a CD-ROM, or a DVD player. Before the task was accomplished, only red and green laser diodes were on sale. Researchers working for the biggest companies in the electronics industry tried to design the blue laser diode and failed. Only Shuji Nakamura, while working for Nichia Chemical Industries Ltd. in Tokushima, Japan, did it.
In 1989, when he started developing the technology, there were two materials for making blue LEDs: zinc selenide and gallium nitride. These had the right band gap energy for blue lasers. But everybody was working on zinc selenide because that was supposed to be much better, while Nakamura (as well as professor Akasaki, who worked independently) started working with gallium nitride.
The dislocation density of zinc selenide, which is a measure of the number of defects in the crystal, was less than 103 per cubic centimeter. Gallium nitride was more than 1010 per cubic centimeter. (The dislocation density has to be lower than 103 or even 102). First there was a need in MOCVD reactor, which stands for "metal organic chemical vapor deposition." A commercial reactor was used to grow gallium nitride crystals, but it was impossible to get them to grow on the substrate. So, the commercial reactor was modified and the Nichia's specialist succeeded in making the two-flow MOCVD reactor. Usually a MOCVD has only one gas flow. That's a reactive gas that blows parallel to the substrate. He added another subflow, with an inactive gas blowing perpendicular to the substrate. That suppressed the large thermal convection, and so, with the help of this two-flow MOCVD gallium nitride crystals of the highest quality in the world appeared. The dislocation density was still 1010. But there's another measure of crystal quality, which is hole mobility, and it was 200. That was a world record - the highest hole mobility ever achieved with gallium nitride was 100 ("Nachia's").
Thus it was possible to make any type of gallium nitride. In 1991, n-type gallium nitride was produced. The following year there was made p-type using a thermal annealing technique. Now all gallium nitride researchers use this technique. Finally at the end of 1993, the first commercial-based blue LEDs were made.
Among the main manufacturers of blue LEDs apart from Nichia are, for example: Toshiba, Cree Corporation, LEDReps, Oriol, Inc., Panasonic, ETG Inc., Data Display Products, Lumidrives, Red Line, Inc., Excel Technology Intl. Corp., LEDTronics, AXT Optoelectronics, Fiber Optic Products, Inc., Wilycon Co. Ltd., Toyoda Gosei Co., Ltd.; ISP Co. Ltd, Hewlett-Packard/Agilent, Lumileds.
At present Indium-Gallium-Nitride and Aluminum-Gallium-Nitride compounds are used for an emitting layer. Each type is described by color, brightness, and basic chemistry. Most of InGaN (indium gallium nitride) ultrabright blue LEDs are a slightly turquoisish blue (dominant wavelength around 470 nm), a slightly whitish green (dominant wavelength around 525-527 nm), or "traffic signal" bluish green. These are also made in deeper shades of blue, blue-violet and UV. Overall luminous efficacy of the Nichia blue ones was about 4 lumens per watt at 20 mA in 1997, in 2001 it improved to about 7-8 lumens/watt, today it is 7.5-8 to rarely 10 lumens/watt. White LEDs are blue LED chips covered with
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