The epitaxial growth of (FeCo)(x)Ge1-x films on Ge and GaAs (001) substrates has been studied systematically with x in the range between 0 and 17 at. %, using combinatorial molecular beam epitaxy (MBE) techniques. Complementary doping using the two transition metal dopants into Ge (001) during MBE growth is shown to produce high quality coherent epitaxial films for transition metal concentrations as high as 11 at. %. As the doping level increases, rough growth occurs, which is accompanied by an increasing amount of stacking faults along the (111) directions. The crystal lattice that resulted from the rough growth exhibits a large out-of-plane tetragonal distortion. There are no detectable secondary phases up to a combined transition metal concentration of 17 at. %. The behaviors are shown to be invariant with respect to the choice of substrates. 0 2007 American Vacuum Society.