MIT researchers have demonstrated the first laser built from germanium that can emit wavelengths of light useful for optical communications. It's also the first germanium laser to operate at room temperature. Unlike the materials typically used in lasers, germanium is easy to incorporate into existing processes for manufacturing silicon chips. So the result could prove an important step toward computers that move data — and maybe even perform calculations — using light instead of electricity.
More fundamentally, the researchers have shown that, contrary to prior belief, a class of materials called indirect-band-gap semiconductors can yield practical lasers. "The laser is just totally new physics," says Lionel Kimerling, the Thomas Lord Professor of Materials Science and Engineering, whose Electronic Materials Research Group developed the germanium laser.
As chips' computational capacity increases, they need higher-bandwidth connections to send data to memory. But conventional electrical connections will soon become impractical, because they'll require too much power to transport data at ever higher rates. Transmitting data with lasers — devices that concentrate light into a narrow, powerful beam — could be much more power efficient, but it requires a cheap way to integrate optical and electronic components on silicon chips.
The lasers used in today's communication systems are made from expensive materials such as gallium arsenide, and they have to be constructed separately and then grafted onto chips, which is more expensive and time consuming than building them directly on silicon. Integrating germanium into the manufacturing process, however, is something that almost all major chip manufacturers have already begun to do, since adding germanium increases the speed of silicon chips.
How they did it: In a semiconductor crystal, an excited electron — one that's had energy added to it — will break free and enter the so-called conduction band, where it can move freely around the crystal. But in fact, an electron in the conduction band can be in one of two states. If it's in the first state, and it falls out of the conduction band, it will release its extra energy as a photon. If it's in the second state, it will release its energy in other ways, such as heat.
In direct-band-gap materials, the first state — the photon-emitting state — is a lower-energy state than the second state; in indirect-band-gap materials, it's the other way around. An excited electron will naturally occupy the lowest-energy state it can find. So in direct-band-gap materials like gallium arsenide, excited electrons tend to go into the photon-emitting state; in indirect-band-gap materials like germanium, they don't.
Kimerling's group describes its results in a forthcoming paper in Optics Letters. The primary investigator on the project was Jurgen Michel, the principal research associate in the group, and the lead author was postdoc Jifeng Liu. Kimerling and grad students Xiaochen Sun and Rodolfo Camacho-Aguilera are also coauthors.
The researchers used two strategies to coax excited germanium electrons into the higher-energy, photon-emitting state. The first is a technique common in chip manufacture called "doping," in which atoms of some contaminant are added to a semiconductor crystal. The group doped its germanium with phosphorous, which has five outer electrons, where germanium has only four. The extra electron fills up the lower-energy state in the conduction band, causing excited electrons to, effectively, spill over into the higher-energy, photon-emitting state.
The second strategy was to lower the energy difference between the two conduction-band states, so that excited electrons would be more likely to spill over into the photon-emitting state. The researchers did that by adapting another technique common in the chip industry: they "strained" the germanium — or pried its atoms slightly farther apart than they would be naturally — by growing it directly on top of a layer of silicon.
Next steps: The researchers need to find a way to increase the concentration of phosphorus atoms in the doped germanium. That should increase the power efficiency of the lasers, making them more attractive as sources of light for optical data connections.