Nature of non-D and non-G bands in Raman spectra of a-C:H(N) films grown by reactive sputtering

The Raman spectra of sputter-grown a-C:H(N) films are analyzed giving particular emphasis to the regions below and above that dominated by the well-known D and G bands, in order to deduce complementary information about the film physical properties and eventually clarify the nature of the features there detected. The conventionally studied evolution of the D and G bands evidences a sharp tendency toward graphitization, accompanied by an increasing level of structural disorder. The weak feature observed at about 700 cm-1 is assigned to the disorder-induced L band, originating from the formation, within the films, of graphitic microcrystallites, whose number and size are shown to change when the partial pressure of argon is varied. Thanks to the complementary infrared film characterization, indicating no appreciable hydrogen incorporation into the investigated samples, the broad asymmetrical Raman band centered at about 3000 cm-1 is suggested to represent the second order of the D and G bands, whose component detection and broadening are likely to represent a finite-crystal-size effect, due to the very small size of graphitic crystallites. A semiquantitative explanation of the observed dependence of the second- on the first-order integrated intensity is given.