Kinetics of Ni/a-Ge bilayer reactions

Thin film reactions between 100 nm of Ni and 400 nm of Ge deposited on thermally oxidized Si wafers have been studied as a function of annealing conditions, using X-ray diffraction (XRD), cross-sectional transmission electron microscopy, four-point kelvin resistance measurements and X-ray photoelectron spectroscopy (XPS); these last two techniques were carried out during reaction. The real-time resistance characteristic was nearly identical for temperature ramp rates between 3 and 50°C min^-1 over the temperature range 20-550 °C, suggesting that the same basic reactions path dominates for all these conditions. The initial room temperature sheet resistance of the bilayer structure is about ∼ 2 Ω/□ and, upon annealing, goes through a local maximum of ∼ 6 Ω/□ at 300 °C. XPS indicates that Ge first appears on the sample surface, while XRD indicates the presence of monoclinic Ni₅Ge₃ at this sheet resistance maximum. Orthorhombic NiGe nucleates and grows between 300 and 400 °C. Analysis of resistance vs. time during isothermal annealings at 200-300 °C suggests the formation of NiGe through a thermally activated process with activation energy E_a = 1.3 eV. At temperatures approaching 500 °C, XPS shows the disppearance of the Ni signal from the surface, indicating the onset agglomeration.