Micro-Raman spectroscopic analysis of tetrahedral amorphous carbon films deposited under varying conditions

The structure of tetrahedral amorphous carbon (ta-C) films deposited by a filtered cathodic vacuum arc has been studied using micro-Raman spectroscopy in terms of substrate bias, nitrogen gas partial pressure (ta-C:N films) or aluminum content in a mixed aluminum/carbon target (ta-C:Al films) during deposition. The first-order Raman spectra generally show a broad feature overlaid by a disordered (D) peak and a graphitic (G) peak. The contribution of sp³ bonding to the Raman spectrum is not explicit, since the Raman phonon line is more sensitive to the sp² carbon bonding due to its larger Raman scattering cross section. However by comparing the ratios of the intensities, the full widths at half maximum (FWHM), and the peak areas between the D and G peaks, the sp³ contribution may indirectly be reflected by the complex Raman features. The G peak position for the ta-C and ta-C:N films appears to not change significantly with the change of substrate bias voltage or N₂ partial pressure, whereas the shift of the D peak is more appreciable. On the contrary, the G peak position for the ta-C:Al films shows a continuous decrease with increasing Al content. For the undoped ta-C films, the minimum intensity, area, and FWHM ratios between the D peak and the G peak are obtained at a bias around -100 V, which corresponds to the maximum sp³ content in the ta-C films. These ratios for the ta-C:N and ta-C:Al films, however, generally increase with increased N or Al content, which indicates the increase of sp² bonded clusters.