Screening effects in the Ti 2p core level spectra of Ti-based ternary nitrides

The electronic structure of Ti-based ternary nitrides, Ti-X-N, with X = Si, Al or Cr, has been investigated by X-ray photoelectron spectroscopy and ultraviolet photoelectron spectroscopy. Striking screening effects associated with the decrease of the occupied states at the Fermi level have been observed in the Ti 2p core level spectra of the Ti-Si-N and Ti-Al-N systems with decreasing the Ti content, due to the reduction of the screening ability of conduction electrons. As a consequence of this effect, a strong increase of the ratio between the satellite or “poorly-screened” peak and the main line or “well-screened” peak in the Ti 2p spectra is observed. This behaviour strongly supports the explanation proposed by Porte et al. for the origin of the Ti 2p satellite structure in nearly stoichiometric TiNx films. Likewise, shifts of the Fermi edge have been observed in the Ti-Si-N and Ti-Al-N systems with decreasing the Ti content. These shifts could be associated with the formation of a nanocomposite structure in which metallic clusters are embedded in an insulator matrix, or just to reflect a metal-to-insulator transition in those ternary systems. Although, the core level and valence band spectra do not allow to univocally decide on this point. The comparative study of the evolution of Ti 2p spectra and of the occupied states at the Fermi level in Ti-Si-N and Ti-Al-N systems suggests that the disappearance of the well-screened peak upon decreasing the Ti content is related not only with the decrease of the occupied states at the Fermi level, but with the formation of isolated metallic clusters embedded in an insulating matrix. The formation of small isolated TiN clusters could change the degree of localization of screening orbitals, therefore promoting the satellite at the expense of the main line in the Ti 2p core level spectra.