MIT researchers have demonstrated the first room temperatur laser built from germanium that can produce wavelengths of light useful for optical communication. 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.
Onchip photonics is a key to increasing the speed and lowering the power usage of computer chips to enable zettaflop computing (one million times faster than petaflop supercomputers that exist now).
The researchers describe how they coaxed excited germanium electrons into the higher-energy, photon-emitting state.
Their first strategy is a technique, common in chip manufacturing, called “doping,” in which atoms of some other element are added to a semiconductor crystal. The group doped its germanium with phosphorous, which has five outer electrons. Germanium has only four outer electrons, “so each phosphorous gives us an extra electron,” Kimerling says. 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 10^20 atoms per cubic centimeter” of germanium, Kimerling explains. So far, the group has developed a technique that can add 10^19 phosphorous atoms to each cubic centimeter of germanium, “and we already begin to see lasing,” Kimerling says.
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. 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. “The ability to grow germanium on silicon is a discovery of this group,” says Kimerling, “and the ability to control the strain of those germanium films on silicon is a discovery of this group.”
“High-speed optical circuits like germanium in general,” says Miao. “That’s a good marriage and a good combination. So their laser research is very, very promising.” Miao points 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,” he says, “the promise is exciting, and the fact that they got germanium to lase at all is very exciting.”