High-Efficiency Laser for Silicon Chips

The germanium–tin layer is only a few micrometres thick and is applied to a stressor layer made of silicon nitride and an aluminium base for improved heat dissipation and then coated with silicon nitride. Orienting the germanium–tin compound along the wider atomic distances in the crystal lattice of the silicon nitride leads to stresses in the embedded material, which ultimately cause optical amplification. (Source: N. Driesch, FZJ)

Transistors in computer chips work elec­trically, but data can be transmitted more quickly by using light. For this reason, researchers have long been looking for a way to integrate lasers directly in silicon chips. Scientists from Forschungs­zentrum Juelich have now come a step closer to achieving this. Together with researchers from Centre de Nanosciences et de Nano­technologies (C2N) in Paris and the French company STMicro­electronics as well as 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 centres, therefore, already default to optical fiber whenever cables exceed a length of about one metre. In future, optic solutions will be in demand for shorter and shorter distances due to increasing require­ments, for example board to board or chip to chip data transfer. This applies parti­cularly to arti­ficial intelli­gence 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,” explains Detlev Grützmacher, director at Forschungs­zentrum Jülich’s Peter Grünberg Institute. “Such a laser could then simply 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,” explains Grütz­macher.

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. Back in 2015, Jülich researchers showed that laser emission can be obtained in GeSn system. The decisive factor in this is the high tin content: back then, it amounted to 12 %, which is far above the solubility limit of 1 % . “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,” explains Dan Buca, working group leader at Jülich’s Peter Grünberg Institute.

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 not only at low tempera­tures but also at 0°C. “A high tin content, however, decreases the laser efficiency. The laser then requires a relatively high pumping power. At 12–14 % tin, we already need 100–300 kW/cm2,” explains Nils von den Driesch. “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, 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. This also brings much closer to an electrical pumped laser for industrial-appli­cation that works at room tempera­ture,” explains institute head Grützmacher. 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 in the infrared range of 2–4 µm. 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. (Source: FZJ)

Reference: A. Elbaz et al.: Ultra-low-threshold continuous-wave and pulsed lasing in tensile-strained GeSn alloys, Nat. Phot., online 16 March 2020; DOI: 10.1038/s41566-020-0601-5

Link: Semiconductor Nanoelectronics, Peter Grünberg Institute PGI 9, Forschungszentrum Juelich, Juelich, Germany

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