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International team constructs first Group IV semiconductor laser

GeSn laser can be applied directly onto a silicon chip

Scientists from Forschungszentrum Juelich and the Paul Scherrer Institute in Switzerland in cooperation with international partners have made the first GeSn semiconductor laser, which is also the first made solely of group IV elements.

The GeSn laser can be applied directly onto a silicon chip, creating a new basis for transmitting data on computer chips via light: this transfer is faster than is possible with copper wires and requires only a fraction of the energy. The results were published today in the journal Nature Photonics.

 "Signal transmission via copper wires limits the development of larger and faster computers due to the thermal load and the limited bandwidth of copper wires. The clock signal alone synchronising the circuits uses up to 30 percent of the energy - energy which can be saved through optical transmission," explains Detlev Gruetzmacher, director at Juelich's Peter Gruenberg Institute.

Suitable material for chip production

Typical semiconductor lasers consist of elements from groups III or V and their crystal properties are such that they cannot be directly integrated onto silicon. In contrast, group IV semiconductors - to which both silicon and germanium belong - can be integrated into the manufacturing process without any major difficulties, according to the researchers. 

However, neither element is very efficient as a light source. They are classed among the indirect semiconductors. In contrast to direct semiconductors, they emit mostly heat and only a little light when excited. That is why research groups all over the globe are intensively pursuing the objective of manipulating the material properties of germanium so that it would be able to amplify optical signals and thus make it a usable laser source.

Compound with a high tin content

The scientists at Juelich's Peter Gruenberg Institute have now for the first time succeeded in creating a 'real' direct group IV semiconductor laser by combining germanium and tin. "The high tin content is decisive for the optical properties. For the first time, we were able to introduce more than 10 percent tin into the crystal lattice without it losing its optical quality," reports PhD student Stephan Wirths. "The functioning of the laser is so far limited to low temperatures of up to -183 degC, however. This is mainly due to the fact that we worked with a test system that was not further optimised," adds Dan Buca.

In cooperation with his colleagues fromSiegfried Mantl's group at PGI-9, Stephan Wirths applied the laser directly onto a silicon wafer whose properties were subsequently measured at the Paul Scherrer Institute in Switzerland. PhD student Richard Geiger fabricated the laser structures there. "That way, we were able to demonstrate that the GeSn compound can amplify optical signals, as well as generate laser light," reports Hans Sigg from the Laboratory for Micro and Nanotechnology.

The laser was excited optically for the demonstration. Currently, the scientists in Dan Buca's group at Juelich are working on linking optics and electronics even more closely. The next big step forward will be generating laser light with electricity instead, and without the need for cooling if possible. The aim is to create an electrically pumped laser that functions at room temperature.

New wavelength for new applications

The laser beam is not visible to the naked eye. GeSn absorbs and emits light in a wavelength range of about 3µm. Many carbon compounds, such as greenhouse gases or biomolecules, also display strong absorption lines at this boundary between near and mid-wavelength infrared. Hence, sensors made of GeSn promise a new possibility of detecting these compounds.

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