Impact of Minority Carrier Lifetime on the Performance of Strained Ge Light Sources

We theoretically investigate the impact of the defect-limited carrier lifetime on the performance of germanium (Ge) light sources, specifically LEDs and lasers. For Ge LEDs, we show that improving the material quality can offer even greater enhancements than techniques such as tensile strain, the leading approach for enhancing Ge light emission. Even for Ge that is so heavily strained that it becomes a direct bandgap semiconductor, the ~1 ns defect-limited carrier lifetime of typical epitaxial Ge limits the LED internal quantum efficiency to less than 10%. In contrast, if the epitaxial Ge carrier lifetime can be increased to its bulk value, internal quantum efficiencies exceeding 90% become possible. For Ge lasers, we show that the defect-limited lifetime becomes increasing important as tensile strain is introduced, and that this defect-limited lifetime must be improved if the full benefits of strain are to be realized. We conversely show that improving the material quality supersedes much of the utility of n-type doping for Ge lasers.