  The reactive growth of cobalt germanide on Ge(001) was investigated by means
of in-situ x-ray absorption spectroscopy photoemission electron microscopy
(XAS-PEEM), micro-illumination low-energy electron diffraction ($\mu$-LEED),
and ex-situ atomic force microscopy (AFM). At a Co deposition temperature of
670{\deg}C, a rich morphology with different island shapes and dimensions is
observed, and a correlation between island morphology and stoichiometry is
found. Combining XAS-PEEM and $\mu$-LEED, we were able to identify a large part
of the islands to consist of CoGe$_2$, with many of them having an unusual
epitaxial relationship: CoGe$_2$[$\bar110$](111) $\parallel$
Ge[$\bar110$](001). Side facets with (112) and (113) orientation have been
found for such islands. However, two additional phases were observed, most
likely Co$_5$Ge$_7$ and CoGe. The occurrence of these intermediate phases is
promoted by defects, as revealed by comparing growth on Ge(001) single crystals
and on Ge(001)/Si(001) epilayer substrates.
