Search

Ni:a-C:H thin films obtained by reactive sputtering processes were investigated using EXAFS and subsequent wavelet analysis. Depending on overall thin film deposition conditions, the clusters of Ni in Ni:a-C:H can appear as fcc-Ni or as a non-fcc-Ni phase, such as hcp-Ni or Ni3C. While hcp-Ni and Ni3C are hardly distinguishable by XRD, the EXAFS analysis can reveal if carbon is the nearest neighbour of Ni or not, thus if Ni3C is present or hcp-Ni. Our study showed that wavelet transformation analysis of the EXAFS data is necessary to discriminate clearly between hcp-Ni and Ni3C regarding the presence or absence of carbon coordination shells around the X-ray absorbing Ni atoms. Furthermore, we are confident to have identified Ni3C in our thin films, however, we cannot exclude the possibility of the co-existence of hcp-Ni. Finally, calculations of XRD peaks of hcp-Ni and Ni3C showed that a discrimination of these two crystal phases might be feasible when the crystallite size is increased beyond 40 nm.

Nickel containing amorphous hydrogenated carbon (Ni:a-C:H) thin films prepared by reactive sputtering have a high potential for use as piezoresistive sensors. Investigations by means of X-ray diffraction (XRD), transmission electron microscopy, energy-dispersive X-ray spectroscopy, and magnetic characterizations indicate that sputtering parameters and heat treatment influence the film composition, the microscopic structure, and some relevant macroscopic physical properties. The films are heterogeneous in nature and consist of either nanometer sized hcp nickel, nickel carbide (these phases being indistinguishable by XRD), or fcc nickel clusters encapsulated by graphite-like carbon shells. The nature of the metal clusters in the thin films has a strong effect on its magnetic properties. For approximately 55 at.% Ni the electrical resistivity of the film is nearly temperature independent over a broad temperature range from 100 K to 400 K. The strain sensitivity, with a gauge factor of 20, is up to ten times higher than conventional temperature independent strain sensitive films. Compared to industry standard NiCr functional layers used for pressure sensors, Ni:a-C:H films provide a ten fold higher output signal.