First germanium-tin semiconductor laser directly compatible with silicon chips

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A germanium-tin laser prototype has the potential for providing super-fast light-based data transmission on computer chips (Photo: Forschungszentrum Jülich)

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Swiss scientists have created the first semiconductor laser consisting solely of elements of main group IV (the carbon group) on the periodic table. Simply, this means that the new device is directly compatible with other elements in that group – such as silicon, carbon, and lead – and so can be directly incorporated in a silicon chip as it is manufactured. This presents new possibilities for transmitting data around computer chips using light, which could result in potential transfer speeds exponentially faster than possible with copper wire and using only a fraction of the energy of today’s integrated circuits.

Germanium lasers have been mooted for use in optical computer chips before, but the demonstration of a working prototype that has the potential for being embedded directly in a silicon-based integrated circuit brings this goal closer to reality.

Scientists in Switzerland working at Forschungszentrum Jülich’s Peter Grünberg Institute (PGI-9), and the Paul Scherrer Institute have used germanium (Ge) and tin (Sn) to produce a laser specifically for trial attachment and testing on a silicon wafer. The properties were subsequently measured at the Paul Scherrer Institute, showing that the GeSn compound was able to both generate laser light and amplify light signals, with the inclusion of tin being specifically important to the new device’s optical performance.

"… we were able to demonstrate that the germanium-tin compound can amplify optical signals, as well as generate laser light," said Dr. Hans Sigg from the Laboratory for Micro and Nanotechnology.

"The high tin content is decisive for the optical properties. For the first time, we were able to introduce more than 10% tin into the crystal lattice without it losing its optical quality," added PGI-9 PhD student Stephan Wirths.

Currently, semiconductor lasers used in electronic systems are made of elements from the periodic table main groups III or V, such as gallium arsenide. Because of this, lasers constructed from these materials are not directly compatible with the silicon used in other, silicon-based, semiconductors and it is difficult and time-consuming to make them adhere to such components. In addition, with different coefficients of expansion to the material on which they are attached, they also have reduced usable lifetimes.

Incorporating lasers constructed from the same periodic group, however, should mean that production costs could be reduced and material compatibility would no longer be an issue. All of which could result in a significant increase in component service life.

As it is still in prototype form, the laser was not powered directly for its first demonstration, rather it was excited optically. The researchers at Jülich are, however, working towards linking optics and electronics more intricately, with the next major goal to generate laser light by applying electric current to the device and without the need for cooling. In other words, an electrically operated laser that operates at room temperature.

"The functioning of the laser is so far limited to low temperatures of up to minus 183 degrees Celsius (-297.4° F), however. This is mainly due to the fact that we worked with a test system that was not further optimized," said Dr. Dan Buca, of PGI-9.

The laser beam emits light in a wavelength range of about 3 micrometers, making it invisible to the human eye and operating in the infra-red portion of the spectrum. Operating at this level, the researchers believe that the new device may also have potential to be used as a detector for carbon compounds, such as greenhouse gases or biomolecules which, like GeSN, display strong absorption lines in the near and mid-wavelength infrared electromagnetic boundaries.

A more pressing application for this new technology, however, would be in its employment for light-borne data transmission to increase transfer speeds between multiple cores in computer chips and other electronic components.

"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.," said Professor Detlev Grützmacher, Director at Jülich's Peter Grünberg Institute. "The clock signal alone synchronizing the circuits uses up to 30% of the energy – energy which can be saved through optical transmission."

The research was published in the journal Nature Photonics.

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