In situ photoelectron spectroscopy analysis of tetrahedral amorphous carbon films

In the present study, we describe an in situ investigation of the electronic structure of ta-C (tetrahedral amorphous carbon) films with photoelectron spectroscopy in the X-ray (XPS) and ultraviolet regime (UPS). The carbon films were deposited with a filtered arc source and their structure controlled through adjustment of the ion energy by the application of a dc-bias to the substrate. The different information depths of UPS and XPS are exploited to distinguish between variations in the electronic structure and sp(3)/sp(2) ratio at the surface and in the bulk. It is shown that the sp(3)/sp(2) carbon ratio determined by fitting the Cls peak reflects under certain conditions the true change of this ratio in the bulk, but can yield distorted values if the presence of the sp(2) carbon rich surface layer is neglected. The reliability regime of the core level analysis is established and it is demonstrated how values of sp(3) concentrations in the bulk and at the surface can be distinguished. The analysis of the position of the plasmon loss feature supports the results from the XPS analysis. The UPS-valence band spectra are dominated by the sp(2) surface layer, and reflect the density of states at the surface. The highest surface sp(3) concentration is not equivalent to the optimum ion energy required to achieve a maximum sp(3) content in the bulk, a fact which influences consideration of contact formation. (C) 2004 Elsevier B.V. All rights reserved.