Cobalt-coated single-crystal Si-Ge layers grown epitaxially by ultrahigh vacuum chemical vapor deposition on silicon substrates were annealed by rapid thermal annealing in the temperature range from 450 degrees C to 800 degrees C for periods ranging from 1 to 3 min. The measured sheet resistivities of the films exhibit strong dependence on the annealing conditions. The Co-SiGe film annealed at 700 degrees C for 3 min had the lowest sheet resistivity (similar to 3 Ohm/square). Structural studies using cross-sectional transmission electron microscopy showed that the cobalt films reacted with the SiGe layer and the thickness of the resulting film increases with increasing annealing temperature or time. Electron diffraction and X-ray microanalysis using energy-dispersive spectrometry showed that CoSi2 was formed during initial annealing. The detection of germanium in the reacted layer and the deviation of the reacted layer’s lattice constant from that of CoSi2 indicated that germanium diffused into the CoSi2 and formed ternary compounds (CoxSiyGe2) during further annealing.