Toward a Germanium Laser for Integrated Silicon Photonics

It has been demonstrated theoretically and experimentally that germanium, with proper strain engineering and n-type doping, can be an efficient light emitter and a gain medium at its direct bandgap within the third optical communication window (similar to 1520-1620 nm). In this paper, we systematically discuss the effect of strain, doping, and temperature on the direct-gap optical gain in germanium. For electrically pumped devices, properties and design guidelines of Ge/Si heterojunction are also analyzed and compared with the results from fabricated Ge/Si heterojunction LEDs.