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GeSn alloys are the most promising semiconductors for light emitters entirelybased on group IV elements. Alloys containing more than 8 at.% Sn havefundamental direct band-gaps, similar to conventional III-V semiconductors andthus can be employed for light emitting devices. Here, we report on GeSnmicrodisk lasers encapsulated with a SiNx stressor layer to produce tensilestrain. A 300nm GeSn layer with 5.4 at.% Sn, which is an indirect band-gapsemiconductor as-grown with a compressive strain of -0.32 %, is transformed viatensile strain engineering into a truly direct band-gap semiconductor. In thisapproach the low Sn concentration enables improved defect engineering and thetensile strain delivers a low density of states at the valence band edge, whichis the light hole band. Continuous wave (cw) as well as pulsed lasing areobserved at very low optical pump powers. Lasers with emission wavelength of2.5 um have thresholds as low as 0.8kWcm^-2 for ns-pulsed excitation, and1.1kWcm^-2 under cw excitation. These thresholds are more than two orders ofmagnitude lower than those previously reported for bulk GeSn lasers,approaching these values obtained for III-V lasers on Si. The present resultsdemonstrate the feasabiliy and are the guideline for monolithically integratedSi-based laser sources on Si photonics platform.

Ultra-low threshold cw and pulsed lasing in tensile strained GeSn alloys