Structural study of ultrathin hydrogenated amorphous carbon films using spectroscopic ellipsometry and ultraviolet Raman spectroscopy

Ultrathin hydrogenated amorphous carbon (a-C:H) films in the initial stage of growth were prepared by the electron cyclotron resonance (ECR) plasma enhanced chemical vapor deposition (PECVD) method, varying the thickness within the range of 20 nm. Structural changes of the films were estimated by spectroscopic ellipsometry (SE) and ultraviolet (UV) Raman analysis. The SE results show that the refractive index increases and the extinction coefficient decreases superlinearly with the increase of the thickness up to 5 nm. The deposition rate decreases and the sp3 fraction increases simultaneously. The increment of the sp3 fraction decreases toward the region which is more than 10 nm in thickness. Direct observation of sp3 C-C by UV Raman analysis completely coincides with the SE analysis, i.e., the sp3 peak intensity becomes stronger and the peak position shifts to the lower wavenumber region with an increase of thickness. These facts imply that an sp2 fraction-rich growth zone exists on the surface and that the sp2 rich contribution decreases with an increase of thickness.