Germanium Diodes Mean Progress Toward Silicon-Chip Lasers

David Orenstein writes "Teams at Stanford and MIT have each reported getting strong light signals from germanium-based diodes on silicon at room temperature. Engineers have long sought to do this because, with further refinement into lasers, such diodes would allow for optical interconnects on chips. Optical interconnects could operate much faster and with less power than electrical (metal) ones that are becoming bottlenecks on current chips."

Comments

The 40-year old promise

[Laaserboy]

The promise of making a laser from indirect bandgap semiconductors, then gathering investors, then losing the investors' money goes back to the Sixties at least.

Some scientists showed off SiC blue LEDs in the '60s that shown brilliantly like laser light, but were not the read deal. The real blue room-temperature laser had to wait for Nakamura and a direct bandgap material.

Doping, adding nitrogen, and adding defects to the lattice to produce more light is nothing new. Look at your stop lights. It's working there, but don't count on these indirect materials suddenly turning into lasers. No need to hold your breath.

A quick scientific note. Photons have a lot of energy, but not much momentum. You get hot on a sunny day, but not blown over by the sun. Electrons fall almost directly down in the bandgap diagram to produce light. This makes direct-gap semiconductors useful for lasers. The trick one can use is to provide momentum-shifting impurities to the lattice of an indirect bandgap crystal. The electron creates a photon by dropping directly down, but some other mechanism shifts the electron momentum to create an overall diagonal transition. It's not efficient, but it works.

Vague impression?

[Kupfernigk]

Er...you do know that all the first transistors were germanium based and that early transistor computers used germanium? Before Schottky diodes, computer power supplies used germanium rectifiers because they were twice as efficient (half the heat) as silicon ones. And early audio amplifiers used germanium power transistors in the output stages because at the time they offered lower distortion than silicon, as they had better transfer characteristics in the crossover region. You could easily hear the difference between class AB tube amps, class B germanium amps and class B silicon into the early 70s. Germanium was initially seen as a low frequency technology because thin junctions were hard to form, but this is not necessarily true (Esaki (tunnel) diodes.)

Having said that you are entirely right in your main observation. The main problem for germanium has always been fabrication; no germanium ICs. This is because there is no germanium equivalent of planar technology. It has been known for a long time that if this could be overcome there would be a role for germanium. It's just that, as with so many apparently breakthrough technologies, making it happen turns out to be very hard.