Formation of Ni(Ge1−xSnx) layers with solid-phase reaction in Ni/Ge1−xSnx/Ge systems

We have demonstrated the formation of Ni(Ge_1?ySn_y) layers on Ge_1?xSn_x 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/Ge_1?xSn_x/Ge samples after annealing and the crystalline properties of nickel-tin-germanide layer/Ge_1?xSn_x contact structures. After annealing at temperatures ranging from 350 to 550 °C, the formation of polycrystalline Ni(Ge_1?ySn_y) layers has been observed on epitaxial Ge_1?xSn_x 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/Ge_1?xSn_x/Ge sample with a Sn content of 3.6%, the formation of an epitaxial Ni₂(Ge_1?zSn_z) layer on the Ge_1?xSn_x layer was found. The formation of β-Sn crystallites was observed after annealing at above 450 °C in samples with a high Sn content of 6.5%. This β-Sn formation is due to the precipitation of Sn atoms. In all samples annealed at 350 °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 °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 °C.