Nichias Blue LEDs - Essay Example

Nichia's blue LEDs. Luminescence is the result of electronic excitation of a material. The light-emitting diode is a p-n junction in which an appliedvoltage yields a flow of current, and the recombination of the carriers injected across the junction results in the emission of light. (The process involved here is in effect electroluminescence). LED emission is generally in the visible part of the spectrum with wavelengths from 0.4 to 0.7 m or in the near infrared with wavelengths between 2.0 and 0.7 m ("LED types"). For decades the blue laser was the ultimate dream in laser technology. 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 a phosphor that absorbs some of the blue light and fluoresces with a broad spectral output ranging from mid-green to mid-red. The overall luminous efficacy of Nichia's units in 1997 was approximately 7.5 lumens/watt but has since increased to 15-20 lumens per watt. Lumileds is now producing units achieving 25 lumens/watt and ones based on Cree blue chips may soon achieve 25-30 lumens/watt. The spectrum of these white LEDs consists of the LED band in the mid-blue plus the phosphor band from mid-green to mid-red. The spectrum runs low in far red and blue-green, high in mid-blue, low in violet-blue and really low in the violet. The color rendering index is 85 according to Nichia and Agilent claims a more conservative figure. With the exception of Cree, the manufacturers use a sapphire substrate to deposit the active semiconductor layers on. Since sapphire is nonconductive, two leads have to be attached to the chip. This gives non-Cree GaN LED chips an unusual appearance. Many have a light emitting area resembling a "square dumbbell" or "double diamond" or two squares merged together at the corners. Cree uses a silicon carbide substrate which is conductive. They only need to attach a top lead to the chip so their LEDs have a chip appearance like that of most non-GaN LEDs. InGaN LEDs have a maximum drive current of optimistically 30 mA. Typical voltage drop at 20 mA is about 3.4-3.8 volts. Today most efficient blue LEDs by Cree have 9 to optimistically 12 lumens/watt and soon it might be 20.6 and even 23. Toyoda Gosei claims approximately 5.5-plus. Wideband GaN Blue - This was the first ultrabright blue LED, pioneered by Nichia in the mid 1990's. It is not quite as efficient as the narrowband blue - maybe around 3 lumens/watt. Its efficiency seems nearly enough constant as current is varied from a few mA to 30 mA. The color is a slightly whitish blue. The spectrum has significant content through the violet, blue, and green regions. The peak wavelength is typically 450 nm. Maximum drive current is optimistically 30 mA ("LED types"). Today total sales of the blue LEDs produced by Chemical Industries Ltd are around $200 million a year. And the blue lasers are selling at around $2 million a year. Nichia's blue (peak wavelength 450 nm) LED's are about 100 times brighter than the previously available blue Silicon-Carbide based blue LED's. Nichia's blue LED's are now in commercial service in large outdoor television displays (for example at Hachiko-square in Tokyo-Shibuya) ("Blue LED's"). Works cited 1. Frank, M. "Nichia's Shuji Nakamura: Dream of the Blue Laser Diode", January/February 2000, Vol. 11, No. 1, < http://www.sciencewatch.com/jan-feb2000/sw_jan-feb2000_page3.htm>, 2. Kahaner, D.K., "Blue LED's: breakthroughs & implications", , 3. Klipstein, D. "LED types by Color, Brightness, and Chemistry", . Read More