The influence of a poly-Si intermediate layer on the crystallization behaviour of Ni-Ti SMA magnetron sputtered thin films

Shape memory alloy Ni-Ti thin films as sputtered are amorphous if the substrate is not intentionally heated during deposition. Therefore, these films have to be heat treated to induce crystallization in order to exhibit the shape-memory effect. Several films have been prepared by dc magnetron sputtering and then studied concerning the influence of the type of substrate (single-crystal Si, polycrystalline Si) on the crystallization kinetics and the final structure. The structural development of the films during crystallization (at a constant temperature of 430 °C) has been studied by X-ray diffraction in grazing incidence geometry off-plane (GIXD) at a synchrotron-radiation beamline. These experiments allow us to establish a correlation between the deposition conditions and the kinetics of crystallization. For films deposited at an electrode distance of 70 mm on a Si(100) substrate, a longer crystallization time is needed compared with films obtained at 40 mm, for otherwise fixed deposition parameters. The analysis of the nucleation kinetics by using the Johnson–Mehl–Avrami equation leads to exponents between 2.6 and 3. The presence of an intermediate layer of poly-Si drastically enhances the crystallization process. Additionally, ex situ annealing of identical samples at 500 °C during 1 h and complementary characterization of the structure and morphology of the films by cross-sectional transmission electron microscopy and selected-area electron diffraction were performed. The temperature dependence of the electrical resistivity was measured, identifying the phase transformation temperature ranges. An increase of the overall resistivity with the precipitation of Ni₄Ti₃ has been detected. Results obtained by X-ray reflectometry and GIXD suggest that during crystallization excess nickel is driven into an amorphous region ahead of the crystal/amorphous interface, thus leading to a higher concentration of Ni at the surface and further precipitation of Ni₄Ti₃. PACS 81.15.Cd; 61.10.Nz; 68.55.Jk