Green LED info wanted -- pure green, not yellow-green.
June 13, 2006 1:26 PM   Subscribe

Tell me about the development of green LEDs.

The blue LED emits a lovely wavelength, and it's become so common and trendy I think of it as the color of the 21st Century. Developmental history of the blue LED is easy to come by; my favored source is
this article in an early issue of Wired. But what about the green LED? We had green LEDs from just after the beginning, when there was only red; but those "greens" were really a yellow-green chartreuse. Actually, in addition to the familiar ruby red, there was also orange and yellow. The chartreuse was the only green available for decades, when of course there was no blue. But shortly after the blues became available, real green appeared also -- people in Silicon Valley (and everywhere, maybe) first saw it when the incandescent go traffic lights were replaced with brighter LED variants.

So how is the new, true green distinguished from the old, chartreuse kind? (I know those labeled "green" at Radio Shack are the older kind, and I imagine there's a great urge by wholesalers to unload the old, not-really-green ones.) And can you identify a source describing its development? Was real green related to blue?
posted by Rash to Technology (21 answers total) 1 user marked this as a favorite
Hmm, I know a girl who's dad had something to do with LEDs I'll see if she knows anything.
posted by Brainy at 1:30 PM on June 13, 2006

No idea, put here is a chart of all the LED colors, and materials used.
posted by zabuni at 1:35 PM on June 13, 2006

The "pure green" LEDs you are referring to are actually in the "blue-green" color range for LEDs (500nm wavelength). They are based on the same GaN (gallium nitride) technology that was used to develop blue LEDs.

It is also possible to produce other colors using the same basic GaN technology and growth processes. For example, a high brightness green (approximately 500nm) LED has been developed that is currently being evaluated for use as a replacement to the green bulb in traffic lights.

I can't find any documents explicitly saying it but I presume that after blue LEDs were developed, achieving bright "blue-green" LEDs was a no-brainer series of tweaks. Developing the GaN LEDs was the real hurdle.
posted by junesix at 2:03 PM on June 13, 2006

zabuni's LED color chart link clearly shows the 505nm - 525nm range of "blue-green" LEDs composed of the same SiC/GaN material as the blue LEDs whereas the older light green/yellow-green LEDs are composed of the earlier GaP and InGaAIP dye materials.
posted by junesix at 2:08 PM on June 13, 2006

when the incandescent go traffic lights were replaced with brighter LED variants.

Were you typing this using T9 on a cellphone? That's the only reason I can think of for replacing "in" with "go."

Sorry for the derail, just curious.
posted by disillusioned at 2:19 PM on June 13, 2006

Oh. Unless you meant "go" as in "green." Duh.
posted by disillusioned at 2:20 PM on June 13, 2006

One guy was responsible for all of this—Shuji Nakamura. He did it when he was at Nichia, a company in Japan—these days he's a professor in California, and says he's in the United States for good. I read a story about it years ago, but I can't find the article now. This was as close as I've gotten to the reference:

Did you beat the competition this time?

Nakamura: There was no competition. Suddenly we announced the production of blue LEDs. People working with zinc selenide announced that they had green LEDs, but their brightness was an order of magnitude lower than ours and their lifetime was very, very short. I made green LEDs in 1995 and also succeeded in increasing the brightness of my blue LEDs using a quantum-well structure. Then finally I switched to laser diodes.

posted by limeonaire at 2:28 PM on June 13, 2006

HERE! I'm excited. This is the Scientific American article I originally read. It was a very inspiring story—especially because after he moved to the U.S. to continue his research in an academic setting, Nichia tried to get him to agree not to research LEDs for three to five years, he balked and sued them, and was eventually awarded a judgment against the company for $8.1 million. (Down from the $184 million awarded in an original settlement, which a higher court reversed, but still a victory against the feudal Japanese business system.)
posted by limeonaire at 2:36 PM on June 13, 2006

Thank you, limeonaire. Speaking of Nakamura, his wikipedia entry discusses the judgement:
In 2001, Nakamura sued his former employer, Nichia Corporation of Japan, over his bonus for the discovery, which was originally ¥20,000. Although Nakamura originally won an appeal for 20 billion Yen, Nichia appealed the award and the parties settled in 2005 for 840 million Yen, at the time the largest bonus ever paid by a Japanese company.
Can you believe the original bonus Nichia awarded him for inventing this was just $200?
posted by Rash at 3:23 PM on June 13, 2006

zabuni's LED color chart link clearly shows the 505nm - 525nm range of "blue-green" LEDs composed of the same SiC/GaN material as the blue LEDs whereas the older light green/yellow-green LEDs are composed of the earlier GaP and InGaAIP dye materials.

If that chart's correct, junesix, it means the true green are NOT related to the blue. It says "Super Pure Green" has the same basis as "Super Lime Yellow" (which is what I'm calling chartreuse).

(Adding to this confusion might possibly be the Japanases predilection for considering blue a shade of green: wiki.)

posted by Rash at 4:01 PM on June 13, 2006

What I was saying is that what you consider true green is actually blue-green in this LED lexicon. "Super Pure Green" is a slightly "greener" variant of the LED you see as chartreuse while the actual true green you want is actually referred to as blue-green.
posted by junesix at 4:21 PM on June 13, 2006

The green is actually InGaN, or indium gallium nitride, at a specific concentration. The technology is the same as the blue LEDs, but the wavelength of the emitted light is a bit shorter.

Shuji Nakamura is a professor in my department and a very interesting guy in person. From what I've seen, his discovery was part dumb luck, part hard work, and part pure stubbornness.

From a materials science point of view, it's actually quite amazing that GaN-based (blue/green/white) LEDs work at all. The defect density in the materials is much higher than what anyone predicted would produce light in any appreciable quantity.
posted by JMOZ at 10:10 PM on June 13, 2006

Further information for the layperson is available in the press release from when Nakamura came to UCSB in 2000.
posted by JMOZ at 10:20 PM on June 13, 2006

Oh, and one more comment- there has been talk that Shuji Nakamura has been nominated for a Nobel Prize in Physics this year. Of course, this is just talk so far, but it's talk from semi-informed sources. OK, I'm done unless anyone has any more questions.
posted by JMOZ at 10:23 PM on June 13, 2006

I know nothing on the subject - but I have a friend who's sent me this:

Pure green is generally accepted to be around 555nm. You can argue about
this as it is perception-based.

Led colours from violet to green can be created by different proportions
of indium and gallium in the emissive structures of InGaN leds.
Ultra-violet emissions can also be arranged.

Shuji Nakamura has a great claim to have invented the InGaN blue led.
But Nichia, his former employer, claims to have set him up to make this

Both 'sides' have pretty persuasive arguments.

posted by Dub at 7:29 AM on June 14, 2006

Dub- your friend's comments are mostly correct, but s/he might be a bit behind on the subject.

In the past 5(?) years, it was discovered that the previously measured bandgap of InN (~2eV) was actually incorrect and the actual bandgap is ~0.6eV. What that means is that InGaN LEDs (with high In content) can (in principle) actually extend through the entire visible region and into the infrared. Of course, it's currently better to make red LEDs out of arsenide phosphide-based materials, but this opens the possibility for multiple-active-region LEDs with different colors of active regions. It's fascinating stuff.
posted by JMOZ at 7:03 PM on June 14, 2006

JMOZ, in your last sentence are you saying that it'd be possible to have multiple areas of different color within a single LED using InGaN? Sounds like really cool stuff. Any citations or publications discussing this?
posted by junesix at 10:28 PM on June 15, 2006

junesix- yes, it would be possible, but the regions would be quite close together, so you would not see them as distinct regions. Instead, you'd see white light (without the phosphors used in current LEDs) or some other combination of colors.

The technology is quite similar (almost the same, actually) as that used in multijunction solar cells, which are sometimes also called multi-color solar cells in the popular press. For solar cells, these are the most efficient kind; Spectrolabs has reported efficiency of 40% in a 3 junction solar cell. I've actually considered putting together a Projects post on multijunction solar cells. (MeFi projects instead of FPP because 1) most of the references are quite technical and 2) I have a recent paper on a multijunction solar cell technology)

[/semi-non-sequitor] I don't have any references offhand for multi-color LEDs, but if you do a search for multiple active region LEDs, you might find something. If you get really stuck and are looking for technical publications, I can ask around.
posted by JMOZ at 12:00 PM on June 16, 2006

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