Germanium, a semiconducting element, is not supposed to lase. That is, when it gets its electrons excited, they go flying off as heat – not light. So the conventional wisdom in microelectronic circles (and textbooks) is that germanium does not lase – and can’t be made to work in a laser. This was unfortunate, because germanium is a very useful semiconductor, already employed in making computer chips and other similar circuitry. Now a new paper from the Massachusetts Institute of Technology Electronic Materials Research Group (USA) describes how – theoretically and practically – germanium can be made to lase.

To get a handle on what has been accomplished, the starting point is the concept of a band gap. With semiconductor crystals an extra charge of energy will release an electron, which finds its way into a so-called ‘conduction band’, an area in the crystal where it can freely roam. Eventually however, the electron will fall out of the conduction band. When it does so, it can be in one of two states: It can become a particle of light, a photon; or it can be given off as heat. In general, semiconductors come in two types, in one – the direct-band-gap – the higher energy type, the energy is emitted as photons (lasing). In the other, lower energy type, the indirect-band-gap, it emits energy as heat (for example).

Germanium is an indirect-band-gap type material, which is why it was believed it could not be used for a laser. However, the research team devised a two-strategy process, using some techniques common in the computer industry, to ‘bridge the gap’ in germanium. First, the germanium is ‘doped’ with phosphorus (phosphorus atoms are mixed into the germanium). Because of the chemistry involved, the phosphorus adds an extra electron to the germanium, giving it enough energy when excited to spill over into the higher, light emitting, band. Second, the difference in energy between the lower and higher bands was reduced by ‘straining’ the germanium. This is done by making a wafer of silicon and bonding a germanium layer on top of it at high temperatures. When the wafer cools, the silicon contracts more slowly and ‘stretches’ the space between germanium atoms. So, one strategy increases the energy to be released by germanium, and the other makes it easier for that energy to release into the higher energy band – emitting photons of light. Presto – germanium lases.

Of course, the actual production techniques are a good deal more complicated than described here, but the general idea is that germanium can be made to behave like a direct-band-gap material and become a reasonably good laser. More importantly, the techniques for making a germanium laser are those already common in the semiconductor industry.

The materials used in today’s lasers, such as gallium arsenide, are “all tough fits,” says Tremont Miao, a marketing director at Massachusetts-based Analog Devices Semiconductor. “They’re all challenging integrations.” As a consequence, the lasers have to be constructed separately and then grafted onto the chips, which is more expensive and time-consuming than building them directly on silicon would be. Moreover, gallium arsenide is much more expensive than silicon in the first place.

“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.”

[Source: MIT Today]

There’s still a long way to go before germanium laser material can be used in communications and computing, but as they said – it’s an important step just to make germanium lase. It’s a natural material to find its way (eventually) into a lot of semiconductor laser designs, and much further on (perhaps) into optic (laser) computing.