Raman scattering from extremely thin hard amorphous carbon films

Hard amorphous hydrogenated carbon (a-C:H) films with thicknesses varying from 10 to 2000 A were plasma deposited on Si or Ge and studied by Raman spectroscopy. Using optical multichannel detection it was possible to record light scattering spectra from even the thinnest films. The main peak in the Raman spectrum, which is at about 1520 wavenumbers for thick (a-C:H) films, shows a frequency down shift for layers thinner than 100 A. This mode softening, which amounts to 70 wavenumbers for 10-A a-C:H on Si, is substrate dependent. It reflects the free surface of the film as well as the carbon-substrate bonding at the interface. The Raman scattering intensity shows a pronounced enhancement for films with an optical thickness of about 1/4 of the exciting laser wavelength. This behavior is explained in terms of interference-enhanced Raman scattering. (IAF)