Design optimization of tensile-strained SiGeSn/GeSn quantum wells at room temperature

A direct bandgap can be engineered in Ge-rich group-IV alloys by increasing Sn content and by introducing tensile strain in GeSn. Here, we combine these two routes in quantum well (QW) structures and systematically analyze the properties of SiGeSn/GeSn quantum wells for a range of Sn content, strain, and well width values, within realistic boundaries. Using the k⋅p method, and including L-valley within the effective mass method, we find that 13–16 nm is a preferred range of well widths to achieve high gain for tensile-strained SiGeSn/GeSn quantum wells. Within the range of the well widths, a loss ridge caused by inter-valence band absorption and free carrier absorption is found in the region of parameter space where Sn content and strain in the well are related as Sn(%)≈−7.71εxx(%)+17.13. Limited by a practical strain boundary of 1.7%, for a 14 nm quantum well, we find that 7.5±1% Sn and 1±0.2% strain is a promising combination to get a good net gain for photon transition energy higher than ∼0.42 eV. A maximum utilization of strain is preferred to obtain the best gain with lower energies (<0.42 eV). By comparing these designs with a compressive strain example, an engineered tensile structure shows a better performance, with a low threshold current density (1.42 kA/cm2). Finally, the potential benefit of p-doping of the tensile-strained GeSn QW is also discussed.