Transistors in computer chips work electrically but data can be transmitted more quickly using light. For this reason, researchers have long been looking for a way to integrate lasers directly in silicon (Si) chips. Scientists now 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, therefore, already default to optical fiber whenever cables exceed a length of about one meter. 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 artificial intelligence (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 an inexpensive laser. An electrically pumped laser compatible with the silicon-based CMOS technology could be shaped during the chip manufacturing process since the entire chip production is ultimately based on this technology.
But there is one problem: pure silicon is an indirect semiconductor and therefore unsuitable as a laser material. Different materials are currently used for manufacturing lasers; generally, III–V compound semiconductors are used instead. Their crystal lattice, however, has a completely different structure than that of silicon, which is a group IV element. Laser components are currently manufactured externally and must be integrated subsequently, which makes the technology expensive.
In contrast, the new laser can be manufactured during the CMOS production process. It is based on germanium and tin, two group IV elements like silicon. Pure germanium is, by its nature, an indirect semiconductor like silicon. The high concentration of tin is what turns it into a direct semiconductor for a laser source. The patented epitaxial growth process is used by several research groups; by further increasing the tin concentration, lasers have already been made that work not only at low temperatures but also at 0 °C.
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, i.e. as a continuous-wave laser. 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. The new laser is currently limited to optical excitation and low temperatures of about -140 °C.