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

TL;DR

This paper theoretically examines how defect-limited carrier lifetime affects the efficiency of strained germanium light sources, highlighting the importance of material quality improvements over strain and doping techniques.

Contribution

It demonstrates that enhancing the defect-limited carrier lifetime in germanium significantly boosts LED and laser performance, surpassing traditional strain and doping methods.

Findings

Improving carrier lifetime can increase LED quantum efficiency to over 90%.

Defect-limited lifetime limits laser performance as strain increases.

Material quality improvements are more effective than doping for Ge lasers.

Abstract

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.