In recent years, group IV SiGeSn-based optoelectronic devices including light emitting, detectors, modulators, and CMOS devices have been developed. The main advantage of SiGeSn is the bandgap tunability in the range between 1.5 to 5 µm via Si and Sn compositions. Remarkably, the GeSn binary alloy becomes a direct bandgap semiconductor at a relative low Sn content of about 7-8 at.% opening the quest for the realization of monolithically integrated laser sources on Si. In this direction, from the first laser emission proof in 2015 remarkable progress has been done. Supported by epitaxial growth success, processing technology, and cavity design, the laser emission of optically pumped was continuously increased from 80K to 270K. The main role here was played by increasing the Sn content up a concentration of 17 at.%.

The next major improvement came from layer transfer technology allowing defect engineering and heat management. Their combination made possible room temperature lasing. However, the real life application requires low laser threshold. Here the use of multiple quantum wells based on SiGeSn/GeSn heterostructures and strain engineering allow a remarkable threshold reduction to values of about 1 kA/cm². Interestingly, modeling shows that the very epitaxial challenging of having high Sn contents can be reduced by band structure engineering via introducing tensile strain and, in addition, larger gains are possible compared with high-Sn strain free layers. The optical laser spectrum at 0 °C for a GeSn layer under 1 % biaxial tensile strain is shown in Fig 1a.

The next step is the realization of an electrical pumped GeSn laser. The first data indicate that both waveguide and microdisk cavities can be electrically pumped leading to low temperature laser emission. The effect of the geometry and strain relaxation of under-etched cavities on the laser emission is under study. Figure 1b shows an exemplary spectrum and current-in light-out (LI) curve for a device with a SiGeSn/GeSn heterostructure taken at 50 K.