Piezoresistive Ni:a-C:H thin films containing hcp-Ni or Ni3C investigated by XRD, EXAFS, and wavelet analysis

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.

► Ni3C was identified in piezoresistive Ni:a-C:H thin films by means of EXAFS and wavelet analysis.
► A wavelet analysis of EXAFS data is required in order to identify the Ni3C phase upon a hcp-Ni phase.
► Distinction of hcp-Ni from Ni3C by XRD might be possible, however, the sample preparation and XRD setup become a challenge.