A strong influence of substrate crystallinity is observed for thin-film Co/Ge reactions. For the detected phases (CoGe, Co5Ge7, and CoGe2), the formation temperatures on amorphous Ge (a-Ge) are found to be the lowest, while the highest are on single-crystalline Ge(100). Moreover, while the phase sequence on Ge(100) and polycrystalline Ge (poly-Ge) was unaltered, the formation of intermediate Co5Ge7 was not observed on a-Ge. It is likely that this is due to a promoted CoGe2 formation on a-Ge, resulting in a similar to 200 degrees C decrease in formation temperature (depending on the ramp rate). These observations suggest a strong competition among the formation of these Ge-rich phases. (c) 2007 American Institute of Physics.