Structural studies of argon-sputtered amorphous carbon films by means of extended x-ray-absorption fine structure

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Abstract
The temperature-dependent microscopic structure of plasma-deposited a-C:H and magnetron-sputtered a-C films, in situ sputter cleaned by argon bombardment, has been investigated by near-edge (NEXAFS) and extended (EXAFS) x-ray-absorption fine-structure studies. We find that the microscopic structure of the two films becomes indistinguishable after sputtering with a loss of hydrogen for the a-C:H sample. The structure of the sputtered films at 30 °C is characterized by a first-neighbor C-C bond length of 1.445(10) Å. Upon annealing the bond length approaches that of graphite (1.421 Å) with a value of 1.427(10) Å at 1050 °C, the highest annealing temperature used. Analysis of the EXAFS amplitude of the first-neighbor shell leads to a two-phase structural model consisting of a ‘‘graphitelike’’ network and a statically and dynamically disordered ‘‘random matrix.’’ The fraction of carbon atoms in the ‘‘graphitelike’’ network increases from 60(6)% at 30°C to 92(9)% at 1050°C. Analysis of the higher-neighbor-shell EXAFS signals leads to a model for the ‘‘graphitelike’’ regions, consisting of a network of conjugated odd- and even-membered rings, without long-range order. In contrast, the ‘‘random matrix’’ is suggested to be a mostly chainlike network of double and single bonds. Our results suggest that the ‘‘graphitelike’’ matrix is a precursor state for crystallization.