The group IV optical platform based on Si CMOS technology offers powerful solutions in the field of sensing, data links from chip to chip and on-chip for data processing. Plentiful of passive optical elements are already demonstrated and are contained in the libraries of commercially operating manufacturing lines. However, the full monolithic integration of active laser sources with electronic Si is still under dispute and represents a major challenge for the research community. One of the most advanced avenues to realize active elements for the Si photonic platform consists of the implementation of direct bandgap group IV semiconductor materials .
Here, at PSI, we follow the two major concepts for achieving such a direct bandgap in group IV semiconductors which involve either tensile strained germanium and/or alloying of Ge with Sn.
Figure 1. The arrows in green and red indicate the current maximum values obtained by PSI - Leti/CEA and PSI - Jülich FZ, respectively. The point-dashed line indicates the cross-over between an indirect and fundamental direct bandgap semiconductor
Large tensile stress can be induced into Ge by micromechanical patterning and subsequent underetching of slightly biaxial stressed germanium on insulator substrates . Very large strain levels (figure 1) have been induced into Ge offering a remarkable impact on the optical and material properties such as a reduction of the fundamental direct bandgap by more than a factor of 3, from 0.8 to approximately 0.25eV (see figure 2(b)) [3,4]. 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.
This project is conducted in collaboration with the ETH Zürich and CEA LETI in Grenoble.
Figure 2 (a) Scanning electron microscopy image of Ge microbridge stressed along <100>. (b) Bandgap energy of strained Ge microbridges obtained from photoluminescence spectroscopy measurements.
Alternatively, GeSn alloys with Sn concentration of approximately 8% offer an indirect to direct bandgap crossover allowing the succesful fabrication of group IV lasers. Recently this allowed for the successful demonstration of lasing [5,6]. 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.
Figure 3 (a) Integrated photoluminescence intensity from various GeSn alloys. Coloured curves show the modeled intensity obtained from joint density of states calculations with the band offset ΔE between Γ and L valley. (b) Excitation power dependent photoluminescence showing