Germanium Laser Redefines Physics

The first laser built from germanium that can emit wavelengths of light useful for optical communications has been demonstrated by researchers at the Massachusetts Institute of Technology (MIT).

Germanium, unlike materials typically used in lasers, is easy to incorporate into existing processes for manufacturing silicon chips – a trait these researchers believe could prove an important step toward computers that move date and 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,” said Lionel Kimerling, the Thomas Lord Professor of Materials Science and Engineering, whose Electronic Materials Research Group developed the groundbreaking laser.

(Image: Christine Daniloff)

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 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.

Unchanneled energies

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.

Bridging the gap

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.

According to the group’s theoretical work, phosphorous doping “works best at 1020 atoms per cubic centimeter” of germanium, Kimerling explained. So far, the group has developed a technique that can add 1019 phosphorous atoms to each cubic centimeter of germanium, “and we really begin to see lasing,” he added.

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 this 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. Both the silicon and the germanium were deposited at high temperatures, but silicon doesn’t contract as much as germanium when it cools. The atoms of the cooling germanium tried to maintain their alignment with the silicon atoms, so they ended up farther apart than they would ordinarily be. Changing the angle and length of the bonds between germanium atoms also changed the energies required to kick their electrons into the conduction band.

“High-speed optical circuits like germanium in general,” said Tremont Miao, a marketing director at Massachusetts-based Analog Devices Semiconductor. “That’s a good marriage and a good combination. SO their laser research is very, very promising.” He went on to point out that the germanium lasers need to become more power-efficient before they’re a practical source of light for optical communications systems. “But on the other hand,’ Miao said, “the promise is exciting and the fact that they got germanium to lase at all is very exciting.”

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 postdoctoral, Jifeng Liu. Kimerling and grad students Xiaochen Sun and Rodolfo Camacho-Aguilera are also coauthors.

The research was funded by the Si-Based-Laser Initiative of the Multidisciplinary University Research Initiative (MURI), sponsored by the Air Force Office of Scientific Research (AFOSR).

For more information, visit: www.mit.edu