Silicon-germanium single crystals with a Si content up to 10 at. % have been grown by the vertical Bridgman technique in a monoellipsoid mirror furnace. Single crystal regions up to 50 at. % have been realized by zone melting in a double ellipsoid mirror furnace. The typical high temperature gradients of radiation heated facilities (grad T up to 100 K/cm or even higher, depending on the sample geometry) reduce the occurrence of constitutional supercooling and stabilize the growth interface. Photoluminescence measurements as well as rocking curves and etch pit densities reveal a high crystal quality. Effective segregation coefficients of Al, Ga, In, P, As, and Sb in Ge1-xSx (x<13 at. %) were determined.