The first germanium laser of its kind

New findings by a research team from MIT University bring us another step closer to developing computers that use light instead of electricity to transfer information

Researchers from MIT University demonstrated the activity of the first laser ever composed of the element germanium and which is capable of producing light at wavelengths useful for optical communication. It is the first germanium-based laser ever to operate at room temperature. Unlike other materials common in lasers, germanium is easy to integrate into existing processes that produce Zorn chips. So the result may be an important step forward towards the development of computers that transmit information - and perhaps even perform calculations - using light instead of electricity. However, more significant consequences are the fact that the researchers showed for the first time, contrary to popular belief, that a family of semiconductor materials called "indirect-band-gap" can indeed be used in lasers.

While the capacity of computer chips increases over time, they also require more and more bandwidth (the maximum data rate that can be transmitted over a communication network). However, typical electrical connectors will soon become impractical since they will require much greater energy consumption to perform their tasks. Transmitting information by lasers - devices that focus light into a narrow and powerful beam - could be much more energetically efficient, but would require a cheap way to integrate optical and electrical components inside Zorn chips.

The production of a Zorn chip is a complex and precise process in which layers of different materials are embedded with a Zorn layer, and the desired patterns are etched into them by chemical processes. Adding a new material to this process is complex: it is necessary for the new material to be chemically bonded to the layers above and below it, and its assimilation must be carried out at a temperature and chemical conditions suitable for the other materials with it.

The materials used today in existing lasers, such as gallium arsenide, are all difficult to adapt and integrate in such processes, says one of the researchers. As a result, the lasers must be assembled separately and then integrated into the chips - a process that is more expensive and longer than producing them directly using the silicon layer. Beyond that, the gallium arsenide material itself is much more expensive than tzoran.

On the other hand, the assimilation of the germanium material into production processes is an operation that almost all major chip manufacturing companies have already begun to carry out, since the incorporation of germanium increases the speed of activity of the forging chips. "We, and many others, know how to do this today," notes the lead researcher.

Gallium Arsenide, Zorn and Germanium are all examples of semiconductors, the type of materials used today in virtually all advanced electronic components. Lasers consisting of semiconductors convert the energy of the electrons - the charge carriers - into photons, the particles of light. There are two types of semiconductors: those with direct band gaps, such as gallium arsenide, and those with indirect band gaps, such as germanium and soran. Explains one of the main researchers at MIT: "In the scientific community, the opinion has been widespread until now that semiconductors with an indirect band gap will never be able to radiate laser light. "This is exactly what scientists teach their students at universities."

In a semiconductor crystal, an excited electron - one to which a certain amount of energy has been added - will be released and reach the conduction band, where it can move freely within the crystal without resistance. In fact, an electron in the conduction band can be in one of two states: in the first state it may exit the conduction band and release its excess energy in the form of a photon. In its second state it may release the energy in other forms, such as heat.

In materials with a direct band gap, the first state - in which a photon is emitted - is the state with lower energy than the second state; In materials with an indirect bandgap, the situation is reversed. An excited electron will often be in the lowest energy state it can find. So that in materials with a direct band gap excited electrons will be in a state of photon emission, while in materials with an indirect band gap they will not.

In their article, published in the scientific journal Optics Letters, the researchers describe how they were able to make germanium's electrons settle precisely in the higher energy level, the one where photon emission occurs. Their first approach, which is common in chip manufacturing, is called "doping" in which small amounts of atoms of another material are inserted into the semiconductor crystal. In this case, the researchers inserted phosphorus atoms, which have a valence of five "outer" electrons. "Germanium itself has only four such outer electrons, so the phosphorus "contributes" an additional electron to the integrated array," explains the lead researcher. The excess electron fills the low energy level found in the conduction band, and as a result the excited electrons are pushed to the higher energy level, the one that eventually emits photons.

According to the researchers, phosphorus deposition is optimal at a concentration of 10^20 atoms per cm1019 of germanium. So far the researchers have managed to reach a level of XNUMX phosphorus atoms per XNUMX cmXNUMX of germanium, "and we are already starting to see laser radiation," explains the researcher. The second approach was to reduce the energy gap between the two energy levels of the conduction so that excited electrons would more easily succeed in reaching the photon-emitting state. The researchers did this using another method common in the chip industry: they "stretched" the germanium - caused the atoms to move away from each other a little more, relative to their original state - by binding them directly with a layer of zinc. However, zinc does not shrink to the same extent as germanium when the temperature drops. The atoms of the colder germanium try to preserve their position relative to the forge atoms, so they move further apart. The changes in the angle and distance of the bonds between the germanium atoms also change the energies required to move their electrons into the conduction band. "The ability to grow germanium by mold is a discovery of this research group," explains the researcher, "as well as the ability to control the nature of these germanium layers."

The researchers point out that despite the exciting discovery, it is still necessary to increase the energy efficiency of germanium-based lasers in order to turn them into a practical radiation source in optical communication systems.

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