π bands and gap states from optical absorption and electron-spin-resonance studies on amorphous carbon and amorphous hydrogenated carbon films

Amorphous carbon a-C and amorphous hydrogenated carbon a-C:H films were prepared by rf sputtering of a graphite target in argon and argon-plus-hydrogen atmospheres, respectively. The optical-absorption coefficients of these films were measured by a spectrophotometer in the high-absorption range and by photothermal deflection spectroscopy in the low-absorption range. They were also studied by electron-spin-resonance (ESR) measurements. The optical-absorption spectrum is found to have a rather broad peak, in contrast to the sharp rise in absorption near the band gap observed for normal semiconductors. To explain this broad peak, a model density of states (DOS) for these materials is proposed, in accordance with their well-known microstructure. This consists of a pair of broad Gaussian-like distributions lying above and below the Fermi level and separated by about 4 eV, arising out of the π states of the aromatic sixfold rings that comprise the bulk of the graphitic sp2 regions, as well as pairs of discrete levels that are about 0.6 eV apart and are produced by fivefold and sevenfold rings present in these regions. The proposed DOS explains the essential features of the optical-absorption spectra and ESR data. The validity of the concept of the optical gap for these materials is also discussed.