Haha, it's all good.
"You must spread some Reputation around before giving it to pullbangdead again." Bah! Great post. I have a question, though I am having trouble finding the right words.
So, in your post, you explained that the cavity is generally formed by etching a very very thin ridge out of the waveguide material at that point in construction. This ridge is obscured from view on the finished diode by other layers, but it's in there.
Yep, it's in there for most diodes. Not for a broad area diode, but it's generally there for the optical storage diodes we see most on here. And it may very well be visible in a microscope, it all just "depends".
From what I understand, solid state and dye lasers sort of amplify the energy of the pump by absorbing the energy and releasing the energy as a photon when another photon hits it. This occurs in the cavity, which is usually filled with, or made of a gain medium. It is this special material that amplifies the light.
So I was kinda curious if the ridge in a diode, the laser cavity, is filled with a gain medium too, like a classical laser? Would it be pure GaN deposited in that ridge?
It is a gain medium. The semiconductor itself IS the gain medium: the active material, in the active region. Touching on more below....
Also, I remember reading about the new process of growing GaN that can be cut along a different plane to provide better light extracting properties. Would that just be the big chunk of material on the bottom of the diode in the Wiki diagrams? Or is the entire diode made of GaN with various dopants? I don't understand how this new GaN would help unless it would be used in the active region, but that looks like it is grown on the substrate.
"Most" of the diode is made up of "
alloys" of GaN in the cases of violet, blue, and green laser diodes: GaN, InGaN, AlGaN, in some cases even InAlGaN. All of these are then doped as well, typically Si-doped for n-type and Mg-doped for p-type. With GaN especially, there is also what we call UID, "unintentionally doped", because it's pretty difficult or impossible to grow intrinsic/undoped GaN, it'll always come out a little bit n-type. Or you can just call it "undoped" because it's as close to undoped as possible.
So for laser diodes, you start with a piece of GaN. With LEDs that are a lot easier to make, they usually start with sapphire or silicon carbide. But yeah, that's the substrate. On top of that you grow layers of semiconductor to "build" the diode: GaN, AlGaN, InGaN, etc. With a laser diode, it could easily be dozens of layers, or even hundreds of layers with certain designs. The active region is a layer of lower bandgap material. With GaN laser diodes, the active region, where gain happens, is InGaN. For red and Ir laser diode,s the materials are different, but the ideas are the same. You trap the carriers there, allowing for more gain. The current you push in pumps the semiconductor, allowing for more gain and more light out.
In processing, you add additional materials like facets coatings and metal for contacts, but the semiconductors materials are what I'm talking about here. Also in processing, after you have grown all these semiconductor layers, is where you can, for instance, remove some material to make the waveguide, trapping/confining the laser light into a smaller area inside the semiconductor.
So yeah, umm...the "new GaN". That's all about crystal orientation. The subtrate matters because the orientation of the crystal substrate determines the orientation of the rest of the semiconductor layers you grow. GaN is a hexagonal prism, and think of the diode as always growing on a plane sliced out of that hexagonal prism. Growing on a slice that is differently oriented with respect to the hexagonal prism gives you different properties. So starting on a different crystal plane changes everything throughout the laser diode.
I tried to give this question some structure, but it all went to crap at the end. It also managed to multiply into 4 or 5 more questions.... Sorry 'bout that.
No problem at all.
Did all of that make sense? Any more explanations needed? This one was hard for me to turn my visualizations into words.
