For a direct bandgap material, like III-V, an electron near the bottom of the conduction band can recombine with a hole is near the top of the valence band, annihilating the electron and releasing its energy as a photon. Unfortunately, in an indirect band gap material such a process would violate the conservation of crystal momentum.
That, of course, is one of the issues that IBM engineers are working on as we speak. And you can bet IBM is also working on process/material/archtectural innovations to overcome the hurdles to silicon emitters.
Yes, Intel has successfully grafted a flake of III-V onto a silicon chip as an emitter, but not in a production environment. I believe that instead of fabricating III-V materials on CMOS chips, that instead IBM is figuring on using a traditional discrete III-V emitter and just piping its emissions onto its CISN chips with fiber optics, where the silicon modulators will take over translating electrical data into optical data.
Good emitters need to have a direct bandgap so that electrons in the conduction band can annihilate a hole in the valence band, thereby releasing the excess energy as a photon. Silicon has an indirect bandgap that prevents it from being a good emitter.
I thought of this idea of using air-gaps as waveguides when I was interviewing IBM, but then forgot to ask about it. My best guess is that the people working on air-gaps are not in the same huddle as the guys working on silicon photonics. Maybe when both technologies are a little more mature, there will be some cross-fertilization.
Intel has many of the components for silicon photonics, including a hybrid emitter that uses a III-V flake, but IBM claims it is the only vendor that has downsized its photonic components enough to make them commercially feasible.
Thank you Colin. Yes, silicon emitter would have poor performance, carbon nanotube based or else (BTW, carbon nanotube laser sounds like a long shot)...and III-V laser would be lower cost, but only if manufactured in III-V process...I think it will be very expensive if manufactured in silicon process, imagine how many process steps will be require to add that laser, enormous complexity, hardly a manufacturable solution! Kris
IBM has a silicon emitter, but it is a hybrid that uses a carbon nanotube: htp://bit.ly/eHrLjj
However when I asked IBM about it, they said cost-wise no silicon emitter could yet compete with III-V emitters in performance or cost, and until they do no one should switch.