Microscopic gain analysis of modulation-doped GeSn/SiGeSn quantum wells: epitaxial design toward high-temperature lasing.

Threshold carrier densities of GeSn quantum well (QW) lasers and the physical reason of low-temperature lasing of current GeSn laser are investigated through the comparison of threshold carrier densities of conventional III-V QW lasers. Electrons distributed over L-band is the main cause of decreased gain for GeSn QWs. To increase the gain (and improve the laser characteristics), a modulation-doped GeSn QW is proposed and the material gain is analyzed based on many-body theory for both qualitative and quantitative simulation. Significant gain increase can be expected for n-type modulation doping QWs. The doping condition for elevated temperature lasing is discussed and it was found that material gain curve similar to III-V QW is obtained for GeSn QW with n-type modulation doping of 6 × 10 cm. It was also found that unlike III-V QW lasers, n-type modulation doping is more effective for high-speed operation in terms of differential gain than p-type modulation doping.