Why are 720 nm LEDs so uncommon?

Question

Disclaimer: Potentially more of a physics question, but this area is muddy.

I'm trying to find 720 nm LEDs but they're proving to be very uncommon. In fact, there seems to be a gap between 660 nm up to ~730 nm. I've only been able to find one supplier of 720 nm LEDs (data sheet here). These LEDs also tend to be very low powered compared to their counterparts in the nearby range. For example, a typical 690 nm LED outputs 690 mW, whereas the 720 nm LED only ouputs 170 mW, and that's the strongest one I could find (which is no longer carried).

My gut feeling says this has to do with the material being used in the LED (in this case GaAlAs). Perhaps it's very difficult to dope the material in such a way that it emits 720 nm light? I've already noticed that these LEDs are prone to burning out. Is the material just not very good for this purpose? Hopefully someone with more insight into this topic could shed some light (heh) on the situation.

I think it is on topic here, too, but you still might have a better chance for an answer at physics.se. If you don't get replies, you should think about migrating it. — Martin - マーチン, Mar 18 '15 at 03:13
This isn't a real answer to the underlying problem, but it seems that EPIGAP has some, although I haven't seen anything with an output > 35 mW. — Klaus-Dieter Warzecha, Mar 18 '15 at 09:03
I like the question however, and a great read for you @wes34499 is why the blue LED got a nobel prize in physics in 2014. — John Snow, Apr 10 '15 at 19:43
+1 Muddy subject. ;-) This area of physics is so chemist-ry that most physicist despair. Of course you also don't find many chemists who know their way around semiconductors. Nobody does solid-state chemistry any more. I blame the soft-matter hype. :-D — Karl, Mar 19 '19 at 23:25

score 3 · Answer 1 · answered Mar 18 '15 at 17:27

I suspect the answer is that AlGaAs is an alloy with a bandgap between that of AlAs - 2.16 (which is indirect) and GaAs - 1.42 eV (which is direct). If there is enough aluminum, the resulting semiconductor is an indirect bandgap semiconductor, which means that to emit a photon, it must be coupled with a lattice phonon, greatly reducing the probability of photon emission, which would make a AlGaAs LED of short enough wavelength rather inefficient at emitting light, instead producing a lot of heat rather than light.

Why are 720 nm LEDs so uncommon?

1 Answers1