PHOTOELECTRON-SPECTROSCOPY STUDY OF AMORPHOUS A-CNXH

We report in situ x-ray photoemission, ultraviolet photoemission (UPS), and electron-energy-loss photoelectron-spectroscopy studies of the electronic structure of nitrogen-containing amorphous hydrogenated carbon (a-CN(x):H). Films with different nitrogen concentrations are prepared by the ion-beam-deposition technique. The photoemission measurements do not reveal a shift in the Fermi level with respect to the top of the valence band, as would be expected from an n-type nitrogen-doping mechanism proceeding via substitution. The C 1s core-level shifts observed as a function of nitrogen content are identified as chemical in nature, and the variation in line shape and peak position is explained in terms of a two-phase superposition model. A simple model fit to the He I UPS valence-band spectra reveals the effects of nitrogen on the spectral shape. It illustrates, in particular, the sensitivity of the partial p-pi density of states (DOS) to the presence of nitrogen, and confirms the role played by the pi bonds in controlling the electronic structure of a-C:H. The observed increases in area and width of the p-pi DOS with increasing nitrogen content are in line with an existing model which favors a defect density increase and a size increase of the graphitelike islands formed by the clustering of pi states in aromatic rings.