The surface structure of Si(100) after carbon deposition has been studied by scanning tunnelling microscopy (STM) at a substrate temperature of 600 degrees C. At carbon coverages of about 1/3 monolayer (ML) a c(4 x 4) structure covering the entire surface is obtained. Higher coverages give rise to island formation and to a 2 x 1 structure of the surrounding substrate surface. The surface morphology after subsequent Si epitaxy is documented and depends on the initial carbon concentration. The c(4 x 4) structure (without islands) can be observed even after additional 3-nm thick epitaxial Si is deposited. The island formation of Ge on Si(100) at 550 degrees C - after carbon (0.1-0.3 ML) predeposition leads to smaller islands of about 5 nm in size than without carbon. The islands are al-ranged on a 2xn structure with missing dimers and missing dimer rows and the island density is about 1-3.6 x 10(11) cm(-2) (0.3 ML C predeposition). Subsequent Si deposition reveals, that the strain field around the islands slows down the growth rate in the island neighbourhood.