Scientists from Forschungszentrum Jülich are one step closer to integrating lasers directly on silicon chips, allowing data to be transferred more quickly. Together with researchers from Centre de Nanosciences et de Nanotechnologies (C2N) in Paris, the French company STMicroelectronics, and CEA-LETI Grenoble, they have developed a compatible semiconductor laser made of germanium and tin, whose efficiency is comparable with conventional GaAs semiconductor lasers on Si. Optical data transfer permits much higher data rates and ranges than current electronic processes while also using less energy. Computation and data centers default to optical fiber whenever cables exceed a length of about 1 m. The researchers predict that in the future, optic solutions will be in demand for shorter and shorter distances due to increasing requirements, such as board-to-board or chip-to-chip data transfer. This applies particularly to AI systems where large data volumes must be transferred within a large network in order to train the chip and the algorithms. “The most crucial missing component is a cheap laser, which is necessary to achieve high data rates. An electrically pumped laser compatible with the silicon-based CMOS technology would be ideal,” said Detlev Grützmacher, director at Forschungszentrum Jülich’s Peter Grünberg Institute (PGI-9). “Such a laser could then simply be shaped during the chip manufacturing process since the entire chip production is ultimately based on this technology.” However, pure silicon is known as an indirect semiconductor and unsuitable as a laser material. III-V compound semiconductors are the most common material used instead. “Their crystal lattice, however, has a completely different structure than that of silicon, which is a group IV element,” Grützmacher said. “Laser components are currently manufactured externally and must be integrated subsequently, which makes the technology expensive.” The new laser is based on germanium and tin, two group IV elements like silicon. Back in 2015, Jülich researchers showed that laser emission can be obtained in a GeSn system. The decisive factor in this is the high tin content: In 2015 it amounted to 12%; the solubility limit is 1%. “Pure germanium is, by its nature, an indirect semiconductor like silicon,” said Dan Buca, the working group leader at PGI-9. “The high concentration of tin is what turns it into a direct semiconductor for a laser source.” The patented epitaxial growth process developed by Jülich is used by several research groups all over the world. By further increasing the tin concentration, lasers have already been made that work at 0°C. “A high tin content, however, decreases the laser efficiency,” Nils von den Driesch said. “The laser then requires a relatively high pumping power. At 12% to 14% tin, we already need 100 to 300 kW/cm2. We thus tried to reduce the concentration of tin and compensate this by additionally stressing the material, which considerably improves the optical properties.” For the new laser, the researchers reduced the tin content to approximately 5% and simultaneously decreased the necessary pumping power to 0.8 kW/cm2. “This produces so little waste heat that this laser is the first group IV semiconductor laser that can be operated not only in a pulsed regime but also in a continuous working regime, that is, as a continuous-wave laser,” Driesch said. “These values demonstrate that a germanium-tin laser is technologically feasible and that its efficiency matches that of conventional III-V semiconductor lasers grown on Si,” Grützmacher said. “This also brings us much closer to an electrical pumped laser for industrial application that works at room temperature.” The new laser is currently limited to optical excitation and low temperatures of about −140 °C. Such a laser would be interesting not only for optical data transfer but also for a variety of other applications since there are hardly any cheap alternatives for the corresponding wavelengths. Potential applications range from infrared and night-vision systems all the way to gas sensors for monitoring the environment in climate research or even breath gases analyses for medical diagnosis. The research was published in Nature Photonics (www.doi.org/10.1038/s41566-020-0601-5).