Raman scattering and the π-orbitals in amorphous carbon films

We report a Raman study of amorphous carbon films grown by different techniques. We compare Raman line shapes, depolarization ratios, and excitation profiles obtained from samples grown by rf-plasma deposition or sputtering as opposed to arc-evaporated carbon films. We find that all features of the Raman spectra are closer to results from crystalline graphite for samples deposited with an electron cyclotron resonance plasma using either a hydrocarbon gas source or a graphite sputtering target. This is surprising in view of the much larger optical band gap of these samples and also in view of recent e-2e scattering results which suggest that the π orbitals are more graphitelike in arc-evaporated carbon films. We offer a possible explanation of this paradox based on a reinterpretation of the optical properties of amorphous carbon which takes into account the special characteristics of the electronic states derived from π orbitals.