Sandwich atomic structure in tetrahedral amorphous carbon: Evidence of subplantation model for film growth from hyperthermal species - art. no. 115318

High-resolution transmission electron microscopy (HRTEM) was used to charac terize 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 existin g 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 (s imilar to 40 nm), and consisted of sp(2)-bonded carbon domains and quasicon tinuous two-dimensional layers. The initial pure carbon layer on silicon su bstrate exhibits relatively ordered atomic configuration, which can be attr ibuted to the presence of graphitelike structure. The surface layer (A) was investigated in detail by using both cross-sectional and planar HRTEM obse rvations. Results indicated a large number of ordered structure existed in the surface, in the manner of entangled ribbons that were identified to be sp(2)-bonded glassy carbon. The ordered sp(2)-bonded surface layer is propo sed to form immediately while stopping deposition, i.e., the final stage of film growth, due to thermal spike-induced stress relaxation on surface. Th e interior film (B) is predicted to possess higher sp(3)-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 DeltaR(p) of carbon species implan tation into silicon substrate, were conducted for further investigating and interpreting the observed atomic structures in surface, interior film, and interface. The fact that sp(2)-bonded surface and interface are present in a primarily sp(3)-bonded film gives a direct corroboration of subplantatio n model and compressive stress mechanism for sp(3)-bonded film growth from hyperthermal species.