Early stages of carbon film growth: carbide interface formation on molybdenum

In this study we present an investigation of the carbide formation and early stages of carbon film growth using a low energy carbon beam to supply the growth species. Carbon is supplied through electron beam evaporation of graphite and between 0.1 and 40 ML are deposited on a molybdenum substrate (substrate temperature 400 degrees C). Photoelectron spectroscopy in the ultraviolet and X-ray regime was employed to characterize the surface and observe the carbide and carbon film formation. Two regimes, with respect to the surface composition, can be identified: firstly, the carbide formation, and, secondly the growth of a pure carbon overlayer. A carbide interface with Mo2C stoichiometry is created, and the formation of a pure amorphous carbon layer is observed for carbon coverages exceeding about 3.4 ML. But even after the onset of carbon film growth, the carbide interface growth is not terminated, and the extension of the carbide region into the bulk continues to increase. Diffusion through the carbide interface is still present and a dynamic rather than static interfacial layer exists. The diffusion of carbon through the metal carbide dominates the interface formation, which is also evidenced by a delayed onset of carbon film formation at 600 degrees C. Apart from the observation of interface formation, this experiment also enabled us to observe the valence band spectrum of molybdenum carbide (Mo2C) for the first time. The sequential deposition of carbon was shown to be a suitable method to produce clean carbide surfaces. (C) 1999 Elsevier Science S.A. All rights reserved.