Sandwich atomic structure in tetrahedral amorphous carbon: Evidence of subplantation model for film growth from hyperthermal species

High-resolution transmission electron microscopy (HRTEM) was used to characterize the cross-sectional and planar atomic structures and bonding states of highly tetrahedrally bonded amorphous carbon (ta-C) films, particularly concentrating on the surface layer and interface between substrate and pure ta-C film. A ``sandwich'' cross-sectional structure was found to be existing in ta-C grown from hyperthermal carbon species, and can be expressed as A/B/A layer-by-layer stacks. The interface (A) was shown to be very thick (~40 nm), and consisted of sp²-bonded carbon domains and quasicontinuous two-dimensional layers. The initial pure carbon layer on silicon substrate exhibits relatively ordered atomic configuration, which can be attributed to the presence of graphitelike structure. The surface layer (A) was investigated in detail by using both cross-sectional and planar HRTEM observations. Results indicated a large number of ordered structure existed in the surface, in the manner of entangled ribbons that were identified to be sp²-bonded glassy carbon. The ordered sp²-bonded surface layer is proposed to form immediately while stopping deposition, i.e., the final stage of film growth, due to thermal spike-induced stress relaxation on surface. The interior film (B) is predicted to possess higher sp³-bond content than that measured by electron-energy-loss spectrum. In addition, slow positron annihilation, as well as a classical-trajectory calculation concerning the projected range R_p and range straggling ΔR_p of carbon species implantation into silicon substrate, were conducted for further investigating and interpreting the observed atomic structures in surface, interior film, and interface. The fact that sp²-bonded surface and interface are present in a primarily sp³-bonded film gives a direct corroboration of subplantation model and compressive stress mechanism for sp³-bonded film growth from hyperthermal species.