We have demonstrated the formation of Ni(Ge1-ySny) layers on Ge1-xSnx layers by using solid-phase reaction for samples with Sn contents ranging from 2.0% to 6.5%. We have also investigated solid-phase reaction products in Ni/Ge1-xSnx/Ge samples after annealing and the crystalline properties of nickel-tin-germanide layer/Ge1-xSnx contact structures. After annealing at temperatures ranging from 350 to 550 degrees C, the formation of polycrystalline Ni(Ge1-ySny) layers has been observed on epitaxial Ge1-xSnx layers with Sn contents ranging from 2.0% to 6.5%. We also observed anisotropic crystal deformation of NiGe with the incorporation of Sn atoms into substitutional sites in NiGe. In the case of the Ni/Ge1-xSnx/Ge sample with a Sn content of 3.6%, the formation of an epitaxial Ni-2(Ge1-zSnz) layer on the Ge1-xSnx layer was found. The formation of beta-Sn crystallites was observed after annealing at above 450 degrees C in samples with a high Sn content of 6.5%. This beta-Sn formation is due to the precipitation of Sn atoms. In all samples annealed at 350 degrees C, the morphology of Ni-Ge-Sn layers is smooth and uniform. However, the surface roughness and interface roughness increase for an annealing temperature of 550 degrees C. In particular, in the sample with a Sn content of 6.5%, the temperature at which agglomeration noticeably occurs is as low as 450 degrees C. (C) 2011 Elsevier Ltd. All rights reserved.