Al-Ni-Ge ohmic contacts on n-GaAs(001) were prepared by sequential vapor deposition and subsequent annealing at 500 degrees C. Structural properties were studied as a function of contact composition, by transmission electron microscopy and scanning electron microscopy, in addition to other techniques. In all cases, Al was deposited as the top layer at a fixed thickness. While all metallizations exhibit a non-spiking interface with the GaAs substrate, it was found that the contact morphology varies strongly with the Ge:Ni thickness ratio. For a Ge:Ni thickness ratio of 3:4 or greater, annealing results in the formation of a thick Al3Ni layer adjacent to the GaAs substrate, as well as a non-uniform surface layer, characterized by dendritic Ge precipitates in an Al matrix. By lowering the Ge:Ni thickness ratio to 1:2, the surface morphology was greatly improved and the contact displays a stable layered structure of the type Al3Ni/(Ni-Ge)/GaAs. These results were accounted for on the basis of a recently developed Al-Ni-Ge ternary phase diagram.