Comparison of Bulk and Surface Structure in Hydrogenated Amorphous Carbon Films

Hydrogenated amorphous carbon (a-C:H) films having different characteristics (diamond like, graphite like and polymer like) are analysed. By comparing the results obtained with a surface-sensitive technique (high resolution electron energy loss spectroscopy) and a bulk-sensitive technique (IR spectrometry) knowledge of the hydrogen and sp3-sp2 depth distributions is achieved. For instance, it is shown that diamond-like films have a hydrogen enrichment near the surface, while the sp3 to sp2 ratio for protonated sites does not change with depth. By means of spectral ellipsometry, the real and imaginary parts of the dielectric constant are measured. Their shapes for the three types of a-C:H are discussed on the basis of the sp2 cluster model. For instance, it is shown that, in the bulk of graphite-like a-C:H, sp2 sites are grouped in a limited range of almost undistorted cluster sizes.