On the structure of thin amorphous carbon films

Abstract Diamond-like carbon films have been prepared by KrF excimer pulsed-laser deposition at a wavelength of 248 nm, with power densities ranging from 0.05 to 1.85 GW cm−2, in vacuum and in a nitrogen atmosphere. Structural analysis was performed first by Fourier transform infrared spectroscopy, which shows that the films are hydrogen free and become transparent above a threshold power density. The band features indicate that the structure of the films is disordered. Combined visible and ultraviolet (UV) Raman spectroscopies allowed us to enhance the scattering cross-section from sp3-coordinated carbon with respect to the sp2 signals. In visible Raman spectra the D and G peaks are found; in UV Raman spectra, besides the blue-shifted G peak and the T peak, a further peak, not reported before, at about 1400 cm−2 is evident in spectra from films deposited in vacuum at low fluences. An analysis of the evolution of Raman fitting parameters with increasing deposition power density shows that above a threshold value of about 0.5 GW cm−2 a structural transition occurs in the films deposited in vacuum from mainly disordered graphitic to predominantly amorphous tetrahedrally bonded carbon. The presence of nitrogen during deposition results in relevant modifications of the Raman spectra, associated with changes in the expansion dynamics of the carbon plasma plume.