Strain engineering for optical gain in germanium

Room temperature optical gain and lasing have been recently demonstrated in germanium. One key ingredient to obtain positive gain is to reduce the splitting between the conduction indirect and direct valleys by introducing tensile strain. We have investigated two distinct approaches to apply a large tensile strain in germanium. The first approach relies on the growth of germanium on InGaAs buffer templates. Biaxial tensile strains up to 0.8% have been achieved by this method. The second approach relies on stress transfer through silicon nitride layers. It offers the advantage of a full compatibility with CMOS processing. We have successfully fabricated tensile-strained photonic wires which exhibit optical gain under cw optical pumping.