Identification of carbon and tantalum chemical states in metal-doped a-C:H films

Abstract

Detailed X-ray photoelectron spectroscopy (XPS) measurements analysing the various chemical states of carbon and tantalum were carried out for a-C:H films doped with different contents of tantalum. The a-C:H(Ta) films were prepared in an r.f.-activated combined physical and chemical vapour deposition process; by varying the ratio of gas flow between argon and the reactive ethylene component, metal concentrations in the range of 2 – 100 at.% could be obtained. For small amounts of tantalum the dominant species was polymeric carbon with a C 1s binding energy of 284.6 eV. With increasing tantalum concentration a second C 1s state with a binding energy near 283.0 eV was detected, which was identified as carbidic carbon. The existence of a tantalum carbide is supported by the Ta 4f spectra, which show a chemical shift δ E of 1.0 eV to higher binding energy (compared with metallic tantalum). Further support for the build-up of carbidic tantalum clusters was obtained from X-ray diffraction measurements; at nearly stoichiometric 1:1 Ta:C concentrations, the diffraction angles and relative intensities were similar to that of polycrystalline г-TaC. From the line width of the X-ray diffraction peaks the size of the tantalum carbidic clusters in the a-C:H matrix could be determined. Both XPS and X-ray diffraction data suggest that for low tantalum concentrations a metastable TaC 2 − x (0 < x < 1) phase is present in a-C:H(Ta). Based on the XPS and X-ray diffraction measurements a structural model for the a-C:H(Ta) microstructure is proposed which is in qualitative agreement with electrical conductivity data.