This article explores the use of Si1-xGex quantum-well layers with a coherent-wave or island-growth morphology in Si1-xGex based telecommunications photodetectors, The structural properties of such heterostructures have been determined by transmission electron microscopy, atomic force microscopy, x-ray diffraction, and Raman scattering. Photoluminescence and photocurrent spectroscopy measurements establish that strained, dislocation free Si0.5Ge0.5/Si superlattices were produced with band gaps as low as 0.745 eV, corresponding to an absorption edge near lambda = 1650 nm. These anomalously low band gaps result from a combination of reduced quantum confinement and higher local Ge concentration at the quantum-well thickness maxims. Waveguide photodetectors based on a coherent-wave Si/Si0.50Ge0.50 superlattice have achieved a photoresponse of 0.16 A/W at 1550 nm.