Spatially resolved electron energy loss spectroscopy is used to characterize the cross-sectional structure of highly tetrahedral amorphous carbon films, particularly concentrating on the ${\mathrm{sp}}^2$ bonded surface layer. The surface layer is shown to be due to subsurface conversion from ${\mathrm{sp}}^2$ to ${\mathrm{sp}}^3$ bonding at the depth of carbon ion implantation during film growth. The thickness of the surface layer is used as a measure of the ion penetration depth, varying from $0.4\pm 0.2\mathrm{nm}$ for 35 eV ions to $1.3\pm 0.3\mathrm{nm}$ for 320 eV ions. The influence of growth temperature is investigated, and it is found that the temperature above which ${\mathrm{sp}}^3$ bonding is not stable is greatly reduced in the region affected by ion bombardment.

• Received 6 March 1997

DOI:https://doi.org/10.1103/PhysRevLett.80.3280