  GeSn alloys are the most promising semiconductors for light emitters entirely
based on group IV elements. Alloys containing more than 8 at.% Sn have
fundamental direct band-gaps, similar to conventional III-V semiconductors and
thus can be employed for light emitting devices. Here, we report on GeSn
microdisk lasers encapsulated with a SiNx stressor layer to produce tensile
strain. A 300nm GeSn layer with 5.4 at.% Sn, which is an indirect band-gap
semiconductor as-grown with a compressive strain of -0.32 %, is transformed via
tensile strain engineering into a truly direct band-gap semiconductor. In this
approach the low Sn concentration enables improved defect engineering and the
tensile strain delivers a low density of states at the valence band edge, which
is the light hole band. Continuous wave (cw) as well as pulsed lasing are
observed at very low optical pump powers. Lasers with emission wavelength of
2.5 um have thresholds as low as 0.8kWcm^-2 for ns-pulsed excitation, and
1.1kWcm^-2 under cw excitation. These thresholds are more than two orders of
magnitude lower than those previously reported for bulk GeSn lasers,
approaching these values obtained for III-V lasers on Si. The present results
demonstrate the feasabiliy and are the guideline for monolithically integrated
Si-based laser sources on Si photonics platform.
