Raman scattering form extremely thin a-C-H films deposited on semiconductors

Hard amorphous carbon (a-C:H) films with thicknesses varying from 10 Angstroem to 2000 Angstroem were plasma-deposited on Si or Ge and studied by Raman spectroscopy. The main peak in the Raman spectrum, which is at approx. 1520 cm-1 for thick a-C:H films, shows a frequency down shift for layers thinner than approx. 100 Angstroem. This mode softening, which amounts to 70 cm-1 for 10 Angstroem a-C:H on Si, is substrate dependent and reflects the carbon-substrate bonding at the interface. For films with an optical thickness of approx. 1/4 of the exciting laser wavelength, a pronounced enhancement of the Raman scattering intensity is observed, which is explained by interference enhanced Raman scattering. (IAF)