  A true monolithic infrared photonics platform is within the reach if strain
and bandgap energy can be independently engineered in SiGeSn semiconductors.
However, this Si-compatible family of group-IV semiconductors is typically
strained and inherently metastable, making the epitaxial growth fraught with
extended defects and compositional gradients. Herein, we controlled the growth
kinetics to achieve epitaxial Si0.06Ge0.90Sn0.04 layers lattice-matched to a Ge
on Si substrate, with a uniform content and a thickness up to 1.5 {\mu}m.
Atomic-level studies demonstrated high crystalline quality and uniform
composition and confirmed the absence of short-range ordering and clusters.
Moreover, these layers exhibit n-type conductivity that is in striking
difference to the commonly observed p-type behavior in GeSn and SiGeSn alloys.
Room temperature spectroscopic ellipsometry and transmission measurements
showed the enhanced direct bandgap absorption at 0.83 eV and a reduced indirect
bandgap absorption at lower energies. Si0.06Ge0.90Sn0.04 photoconductive
devices exhibit a dark current similar to that of Ge devices and a slightly
higher room-temperature spectral responsivity reaching 1 A/W above 0.82 eV
(i.e. below 1.5 {\mu}m wavelengths). These results underline the enhanced
performance in lattice-matched devices and pave the way to introduce SiGeSn
semiconductors as building blocks to implement the long-sought-after
silicon-integrated infrared optoelectronics.
