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

The Raman spectra of sputter-grown a-C:H(N) films are analyzed giving parti cular emphasis to the regions below and above that dominated by the well-kn own D and G bands, in order to deduce complementary information about the f ilm physical properties and eventually clarify the nature of the features t here detected. The conventionally studied evolution of the D and G bands ev idences a sharp tendency toward graphitization, accompanied by an increasin g level of structural disorder. The weak feature observed at about 700 cm(- 1) is assigned to the disorder-induced L band, originating from the formati on, within the films, of graphitic microcrystallites, whose number and size are shown to change when the partial pressure of argon is varied. Thanks t o the complementary infrared film characterization, indicating no appreciab le hydrogen incorporation into the investigated samples, the broad asymmetr ical 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 broadenin g are likely to represent a finite-crystal-size effect, due to the very sma ll size of graphitic crystallites. A semiquantitative explanation of the ob served dependence of the second- on the first-order integrated intensity is given. (C) 2001 American Institute of Physics.