We describe the fabrication of ternary SiGeSn photodiodes and compare the optical and electrical properties of heterostructure devices with the same target compositions (Si: 8-11 %, Sn: 1.7- 2.3 %) grown directly on Si(100) and Ge(100) substrates via UHVCVD reactions of SnD4, Si3H8 and Ge2H6. We show that the quantum efficiency and dark current levels of near lattice-matched alloys grown on Ge wafers are superior to those of comparable Ge devices. The corresponding optoelectronic properties of analogous SiGeSn devices grown on Si are significantly degraded due to the inferior microstructure resulting from the lattice mismatch between the substrate and epilayer. Finally, we also show that minor variations in composition produce significant shifts in the absorption edge of the devices grown on Ge. Collectively the SiGeSn diodes described here exhibit enhanced performance over any previous Sn based group-IV system, underscoring the potential of this 1 eV gap material as versatile candidates for next generations of Si-based optoelectronics, including multijunction photovoltaics.