An international team of scientists has developed a germanium-tin (GeSn) laser that can be applied directly onto a silicon chip and thus creates 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 development is the result of a collaborative effort between scientists from Forschungszentrum Jülich and the Paul Scherrer Institute in Switzerland, together with international partners.
The transfer of data between multiple cores, as well as between logic elements and memory cells, is a bottleneck in fast-developing computer technology. Data transmission via light could be the answer to the call for a faster and more energy efficient data flow on computer chips, as well as between different board 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. The clock signal alone synchronising the circuits uses up to 30% of the energy – energy which can be saved through optical transmission,” explains Professor Detlev Grützmacher, director at Jülich’s Peter Grünberg Institute.
Some long-distance telecommunication networks and computing centres have been making use of optical connections for decades. They allow very high bandwidths even over long distances. Through optical fibres, signal propagation is almost lossless and possible across various wavelengths simultaneously; a speed advantage which increasingly benefits both micro- and nano-electronics. Although the integration of optical components is already well advanced in many areas, a laser source that is compatible with the manufacturing of chips is not yet achievable.
Because silicon and germanium are both found in main group IV of the periodic table, they can be integrated into manufacturing processes without any major difficulties. Neither element is very efficient as a light source, however. 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.
The scientists at Jülich’s Peter Grünberg Institute have now succeeded in creating a ‘real’ direct main group IV semiconductor laser by combining germanium and tin, which is also classed in main group IV. “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,” reports PhD student Stephan Wirths. The functioning of the laser is so far limited to low temperatures of up to -183°C, however, but can be subjected to further optimisation.
The laser was excited optically for the demonstration. Currently, the scientists 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.
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