The molybdenum–carbon interface: formation and electronic structure of the carbide layer

The formation of the carbide interface between polycrystalline molybdenum and hydrogen-free carbon films is investigated at different substrate temperatures (ambient, 400 and 600°C) with photoelectron spectroscopy in the ultraviolet (UPS) and X-ray regime (XPS). A carbon beam is created by electron beam evaporation of graphite and the stepwise in-situ deposition of carbon allows to follow the evolution of the interface. In a first reaction step, a molybdenum carbide layer is formed, which is expressed in characteristic changes of the valence band (VB) spectra and the position of the C 1s (283.0 eV), and the Mo 3d5/2 (227.8 eV) core levels. The onset of carbon overlayer growth is delayed at elevated substrate temperatures and observed after the deposition of 3.4 ML carbon at 400°C, while at 600°C, already, more than 11 ML carbon are required. A carbon overlayer with a thickness of 13 ML deposited at ambient temperature can be completely dissolved by a short period of annealing at 800°C. The surface composition just prior to the onset of overlayer growth corresponds to Mo2C. The molybdenum carbide is a poor diffusion barrier at elevated temperatures and continues to grow even after a carbon overlayer has already formed. The carbon layer growth is therefore slowed down considerably, a fact that is of importance in the application of carbon thin films. The sequential deposition of carbon also allows the electronic structure of the carbide to be studied. The peaks in the VB spectra of Mo2C can be assigned to metal, and metalcarbon hybrid bands, following the general description of transition metal carbide valence bands. From this experiment, it is possible to investigate the interface formation itself and to open a simple route to the investigation of the electronic structure of metal carbides.