803T diode gives green light

I decanned one of my DELD's, and to my surprise I saw some green light being emitted from the top of the chip. It is only visible at extremely low currents. The greens increases in intensity until the current is about 70[ch956][ch65313][ch12288](approx 1/1500th of nominal current), and beyond that the violet starts glowing and the green is quickly completely overpowered by the violet glow (and we're still at 0.1mA).

My guess is that these lights are submicroscopic sparks. I happen to know that sparks of pure silver (Ag) are bright green. Furthermore, I decanned another diode, which is 55mW, and I did not see these sparks, and at the same input voltages there was only a current of 4[ch956][ch65313] flowing. When increasing this working diode to 70[ch956][ch65313], the voltage was almost 1.5 times higher and violet was blinding under the microscope.

Here are some photos I managed to take.








This last image is three-dimensional. I found two images which were slightly displaced among the photo shoot. Aim your eyes at infinity too see the depth (look through the screen until the images overlap)

Crossed-eye variation:

You say the top of the die? Can you be any more specific about where the light is coming from? Any from the middle of the die, or only the very top surface? And it looks like it's only coming from a couple of specific places on top, are they right around where the wire is bonded to the contact?

Very interesting....

very interesting!

My computer isn't displaying your characters correctly for some reason that I will have to investigate, but you're feeding these things only micro-amps, right? Another curiosity question, is the top of the diode the negative or positive contact, with respect to the driver input/voltage supply?


I think there's another junction hiding in the diode stealing some power, I'm just trying to guess where/what it is.

I got similar results from two different LDs. The part that interested me was that at (slightly) higher currents, with the diode in a module with an adjustable lens, I was able to 'project' a greenish-white image of the front surface of the die out through the lens and on to a piece of white paper in a darkened room. The image was very crisp, having a perfectly rectangular outline and showing very clearly the unique surface defects on each die - one had a chip on the edge, the other had a crumbly spot on the face.

The focus for this was a little farther out than the focus required to bring the led/laser to a pinpoint. I could focus on the violet light (which was pretty clearly coming from the lower right corner of the die, where it met the substrate) and get a foggy outline around it, or dial the focus out further and get the image of the die with a now-fuzzy spot of violet in the lower-right corner.

On a semiconductor-physics level I don't know if what I saw was the same thing you are seeing here, but it was damn interesting. I was able to see details of the diode construction via this 'projection' that I couldn't see any other way due to the brightness of the diode and the limits of my magnifying optics.

pullbangdead said:

You say the top of the die? Can you be any more specific about where the light is coming from? Any from the middle of the die, or only the very top surface? And it looks like it's only coming from a couple of specific places on top, are they right around where the wire is bonded to the contact?

Very interesting....

Click to expand...

It was on the very top of the die, near the edges. It also looked like the top od the die was dark and slightly curved, almost like a blob of solder. The green emits from very specific, sharp points (in the photo it's smeared out by lens aberrations and bad focus.

pullbangdead said:

My computer isn't displaying your characters correctly for some reason that I will have to investigate, but you're feeding these things only micro-amps, right? Another curiosity question, is the top of the diode the negative or positive contact, with respect to the driver input/voltage supply?


I think there's another junction hiding in the diode stealing some power, I'm just trying to guess where/what it is.

Click to expand...

The top is the positive contact.

Foobario said:

I got similar results from two different LDs. The part that interested me was that at (slightly) higher currents, with the diode in a module with an adjustable lens, I was able to 'project' a greenish-white image of the front surface of the die out through the lens and on to a piece of white paper in a darkened room. The image was very crisp, having a perfectly rectangular outline and showing very clearly the unique surface defects on each die - one had a chip on the edge, the other had a crumbly spot on the face.

The focus for this was a little farther out than the focus required to bring the led/laser to a pinpoint. I could focus on the violet light (which was pretty clearly coming from the lower right corner of the die, where it met the substrate) and get a foggy outline around it, or dial the focus out further and get the image of the die with a now-fuzzy spot of violet in the lower-right corner.

On a semiconductor-physics level I don't know if what I saw was the same thing you are seeing here, but it was damn interesting. I was able to see details of the diode construction via this 'projection' that I couldn't see any other way due to the brightness of the diode and the limits of my magnifying optics.

Click to expand...

That's a totally different effect. All violet lasers have this. The green lights, however, were only found on one specific LEDified violet.

Totally Cool! I can see the 3D. (A good way to do it is cross your eyes at the images and then focus on the combined image)

At the physics level, there may be some impurities that are causing a different bandgap region to appear. Green would have lower energy that the UV and would start producing photons at a lower bandgap voltage. Because it is happening at such a small current you are not consistently conducting across the entire die, just the first path that the electrons find.

Fantastic Photos! Interesting find for sure.

How did you "Decan" it and keep the diode intact?

Jules

That might be one other plausible explanation.


I decanned them with a knife blade (those long ones that you break pieces off when they chip). I cut it at 12 different angles, two times, before I was through.




[edit]You shouldn't cross your eyes to see it stereo because this creates negative depth. You should make the line of your eyes more parallel instead (like looking at infinity). If you cannot achieve this easily, put a thumb directly between your eyes and the screen, such that the thumb blocks the left image for your right eye and vice versa. Then combine the images by looking further.

Zom-B said:

That's a totally different effect. All violet lasers have this. The green lights, however, were only found on one specific LEDified violet.

Click to expand...

What is it, then? Just beeing curious.

ArRaY

That 3d effect is cool! You see 3, the right and left are in the "background" and the center is the combined 3d picture. Nice! (trying to cross my eyes gave me a headache) As far as the green, who knows, maybe it's a phenomenon caused from moisture accumulation? flaked off copper?

I'm going to go wild and guess that you've got a little bit of a light emitting Schottky diode at the p-GaN/metal contact interface, or some other form of junction formed due to extra compound formation (oxidation of metal contact during processing, maybe some extra heat-activated diffusion during the event which caused COD, maybe something like that).  A new compound formed might explain the color of the light, I don't know off the top of my head what it could/would be though.  

Either way, I think somehow it formed a new junction there around the p-GaN contact area.  It is notoriously hard to make good contacts, especially p-contacts, to GaN, so it makes perfect sense that there would be a Schottky behavior there, but it emitting light would be interesting.

It theoretically might could maybe be due to impurities, but impurities don't change the bandgap.  If anything, some donor or acceptor levels or recombination states might be created in the bandgap that could cause lower-energy recombinations (2 steps down through the bandgap instead of 1, or a half-step down and then back up), but recombinations like that are likely to be non-radiative (ie non-light emitting).

Where would the copper come from?  Bits of dust from can removal?  Copper oxide is a semiconductor, but its bandgap is lower than what is necessary for green.  

So, it's still a mystery, I guess.

I'm still unsire about the bandgap theory. The voltage of this diode is not higher than for any other diode. If anything, it's lower.

ArRaY said:

[quote author=Zom-B link=1220631465/0#5 date=1220640277]
That's a totally different effect. All violet lasers have this. The green lights, however, were only found on one specific LEDified violet.

Click to expand...

What is it, then? Just beeing curious.

ArRaY[/quote]
Search for "Yellow glow" on the forums

That is due to ESD damage to the charge confinement layer of the LD die.

Well, I ESD'd the diode on purpose, and it still didn't show the green spots...