Valence band spectra of nitrogen incorporated amorphous carbon films

Abstract

Tetrahedral amorphous carbon (ta-C) films were deposited in a filtered cathodic vacuum arc chamber. Nitrogen, of atomic concentration up to 30%, was introduced in the films during deposition by a Kaufmann-ion source. Change of the film structure and the valence band (VB) spectra of ta-C film due to nitrogen incorporation was studied by ultraviolet photoelectron spectroscopy (UPS) using He I and He II excitations as well as x-ray photoelectron spectroscopy (XPS). A comparative study of the electronic structure between ta-C and the nitrogenated films was demonstrated by decomposition of their VB spectra into several bands and from the intensity difference of these spectra. An additional density of states close to the Fermi level (EF), representing the nitrogen lone pair state, has been detected from both UPS and XPS VB spectra of nitrogenated samples. From the shift of the VB relative to the EF nitrogen doping of ta-C is demonstrated. The change of the density of states at the edge of VB and especially the C 2s and N 2s states is thoroughly explained. The modification of the structure of nitrogenated films prepared by applying the substrate bias and temperature was also studied through comparison of the VB spectra.