  By independently engineering strain and composition, this work demonstrates
and investigates direct band gap emission in the mid-infrared range from GeSn
layers grown on silicon. We extend the room-temperature emission wavelength
above ~4.0 {\mu}m upon post-growth strain relaxation in layers with uniform Sn
content of 17 at.%. The fundamental mechanisms governing the optical emission
are discussed based on temperature-dependent photoluminescence, absorption
measurements, and theoretical simulations. Regardless of strain and
composition, these analyses confirm that single-peak emission is always
observed in the probed temperature range of 4-300 K, ruling out defect- and
impurity-related emission. Moreover, carrier losses into thermally-activated
non-radiative recombination channels are found to be greatly minimized as a
result of strain relaxation. Absorption measurements validate the direct band
gap absorption in strained and relaxed samples at energies closely matching
photoluminescence data. These results highlight the strong potential of GeSn
semiconductors as versatile building blocks for scalable, compact, and
silicon-compatible mid-infrared photonics and quantum opto-electronics.
