Highly-Doped, Highly-Strained Germanium and Schottky Electroluminescent Diodes

Achieving a monolithic germanium laser source represents a major challenge for silicon photonics. Two key parameters have an impact on the laser performances including threshold, output power, reliability... The first key parameter is the germanium doping. Modeling indicates that an active electron carrier concentration of 3 - 4 x 10¹⁹ cm^-3 is requested to obtain a robust gain. We will show that such doping level can be achieved for germanium grown on silicon using a co-doping method followed by thermal anneal. The thermal annealing is necessary to minimize the non-radiative recombination processes. The second key parameter is the tensile strain present in germanium. The achievement of high or even record tensile strain values in small volumes is not sufficient for lasing. The homogeneity of the strain field within the germanium volume needs to be taken into account. We will show how it is possible to maximize the tensile strain homogeneity either for straight waveguides or germanium microdisks strained by a silicon nitride stressor. The trade-off between homogeneity and maximum strain value allows one to achieve a more robust modal gain. We will finally discuss the transfer of these concepts for electrical injection.