Using light to move data would make computers much more efficient than they are today, but that requires a practical way to add optical components to silicon chips. MIT researchers have taken a step toward that goal by demonstrating the first germanium laser that can emit light at wavelengths suitable for digital communication. Unlike the materials used in standard lasers, germanium is easy to incorporate into the chip-making process: most manufacturers are already adding it to their silicon chips because it makes them faster.
The new device is the first germanium laser to operate at room temperature; previous examples, which emitted radiation in the terahertz frequencies, had to be cooled to near absolute zero. It also demonstrates that a class of materials called indirect-band-gap semiconductors can yield practical lasers.
In a semiconductor crystal, adding energy to an electron will knock it out of its usual state and into the so-called conduction band, where it can move freely around the crystal. Such an electron can be in one of two states, which determine what happens to what’s left of its extra energy when it eventually falls out of the conduction band. If it’s in the first state, it releases that energy as a photon. In the second state, it releases it as heat.
In direct-band-gap materials, the first state is a lower energy state than the second; in indirect-band-gap materials like germanium, 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, those electrons tend to go into the photon-emitting state, and in indirect-band-gap materials, they don’t.
By adding phosphorus atoms to germanium, a team of researchers in the lab of materials science professor Lionel Kimerling ‘65, PhD ‘69, led by principal research associate Jurgen Michel and including postdoc Jifeng Liu, PhD ‘07, coaxed excited germanium electrons into the photon-emitting state. Whereas a phosphorus atom has five outer electrons, Kimerling explains, “germanium has only four outer electrons, so each phosphorus gives us an extra electron.” The extra electron fills up the lower energy state in the conduction band, effectively causing excited germanium electrons to spill over into the higher energy state. Previously, according to Michel, other scientists had thought “that indirect-band-gap semiconductors will never lase”–that is, produce a coherent beam of light.
The researchers’ theoretical work suggests that phosphorus doping works best at 1020 atoms per cubic centimeter of germanium, Kimerling says. So far, they have developed a technique that can add 1019 phosphorous atoms to each cubic centimeter of germanium, “and we already begin to see lasing,” he says.