Previous studies have shown that Ni-silicide phase formation in Ni contacts to SiNWs can differ from the phase formation sequence observed for thin Ni films on Si wafers. In the well-studied case of a thin film on a Si wafer, one often observes sequential phase formation beginning with orthorhombic Ni<sub>2</sub>Si, followed by NiSi upon increased time and temperature, and finally nucleation of NiSi<sub>2</sub> at temperatures in excess of 700°C. This sequential formation is not what has been observed in previous reports of Ni-silicidation of SiNWs. We have found that the phase formed is dependent on the growth direction of the SiNW. In the case of SiNWs with a  growth direction, a metastable high-temperature phase, hexagonal θ-Ni<sub>2</sub>Si, is formed and stabilized when annealed from 350 to 700°C for 2 min. We also observe that the Ni-silicide has an epitaxial orientation with respect to the SiNW of θ-Ni<sub>2</sub>Si||[11-1] and θ-Ni<sub>2</sub>Si(100)||Si(112). The second case is that of SiNWs with a  growth direction. We find NiSi<sub>2</sub> is the first phase to form after anneals of 350, 400, 450 and 550°C for 2 min, and it also forms with an epitaxial relationship to the SiNW: NiSi<sub>2</sub>[1-10]||Si[1-10] and NiSi2(111)||Si(111). After anneals of 600 and 700°C for 2 min, a conversion from NiSi<sub>2</sub> to NiSi is observed.
We have performed similar experiments, substituting GeNWs for the SiNWs and studying the solid state reaction between a Ni film and a GeNW to form Ni-germanides. We find that the reaction begins axially down the GeNW at a temperature of 350°C, and the phase that forms has a similar crystal structure to the hexagonal θ-Ni<sub>2</sub>Si phase identified to form in the reaction of Ni with SiNWs with a  growth direction. From indexing the diffraction patterns, the Ni-germanide phase could be either Ni<sub>2</sub>Ge or Ni<sub>3</sub>Ge<sub>2</sub>; however, these two phases cannot easily be differentiated because the space groups for the two crystal structures are identical, the only difference being the occupancy of sites by Ni or Ge atoms resulting in different stoichiometries. After annealing at 400°C for 2 min, the same hexagonal Ni-germanide phase is identified, with the exception that in some cases a thin layer (10 nm) of orthorhombic NiGe is formed at the reaction front between the hexagonal Ni-germanide and GeNW. Higher temperature annealing at 500°C results in a break in the GeNW away from the germanide/Ge interface. Further work is underway to determine if Ni-germanide formation in GeNWs has a dependence on the growth direction of the GeNWs, as was previously observed for the Ni/SiNW system.