Strained Germanium laser
The entire digital world surrounding us is based on silicon CMOS. The constant improvement of the transistor performances observed in the last fifty years is nevertheless approaching the end , encouraging the development of new concepts to supply the continuously increasing demand for information technologies. Si-photonics, one of the most promising concepts, envisages the integration of photonics into the current electronical platforms. Although integration of many photonic elements – such as modulator splitter etc. is possible on a large scale, unfortunately silicon, by fundamental reasons, lacks to be an efficient light emitter and thus limits Si-photonic to finally skyrocket. A visionary way out of this dilemma is to use germanium instead of Si, which, thanks to its CMOS compatibility and favourable band structure, may become the workhorse of the next generation of fully Si-compatible group IV lasers materials.
Here, at PSI, we follow the two major concepts for achieving a direct bandgap in group IV semiconductors, which involve either tensile strained germanium and/or alloying Ge with Sn. Large tensile stress can be induced into Ge by micromechanical patterning and subsequent under etching of slightly biaxial stressed germanium on insulator substrates , offering a remarkable impact on the optical and material properties such as a strong reduction of the fundamental direct bandgap . Furthermore, the simplicity of this wafer-based technology enables the investigation of a vast amount of physical properties at unprecedented strain levels in a variety of materials.
Based on above strain approach, in collaboration with CEA Grenoble and ETH Zürich, we could now demonstrate the first interband laser from elemental Ge . When the Ge microbridges are loaded up to 6 % of strain, and integrated into an optical cavity  a highly efficient lasing is observed up to 100 K . By making use of the unique set of tools available at PSI, we are currently investigating paths towards the room temperature electrical injected Ge laser, by exploring n-doping , biaxial versus uniaxial strain geometries, as well as the combination of strain and alloying with Sn.
Alternatively, GeSn alloys with Sn concentration of approximately 8% offer an indirect to direct bandgap crossover allowing the fabrication of group IV lasers. Recently this allowed for the successful demonstration of lasing [5,6,7]. This now offers the potential of a purely group IV epitaxially defined laser with similar fabrication methods as mature state of the art III-V semiconductor lasers.
This project is conducted in collaboration with the Forschungszentrum Jülich and CEA Grenoble.
Funding: Ultra highly strained semiconductors: materials for new applications - SNF 162658
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 V. Reboud et al. Optically pumped GeSn micro-disks with 16% Sn lasing at 3.1 μm up to 180 K. Appl. Phys. Lett. 111 (2017).