Identification of ternary phases in TiBC/a-C nanocomposite thin films: Influence on the electrical and optical properties

The local structure of TiBC and amorphous carbon (a-C) nanocomposite films (TiBC/a-C) was correlated with their optical and electrical properties. TiBC/a-C films with increasing C content were deposited by magnetron co-sputtering from TiC:TiB₂ (60:40) and graphite targets. Chemical composition is determined by electron energy-loss spectroscopy. Grazing incidence X-ray diffraction reveals that the microstructure of the films is amorphous with small nanocrystallites emerging by increasing the C content that could be attributed to the formation of ternary (TiB_xC_y) or mixed binary (TiB₂ and TiC) phases. Further information was then obtained by studying the chemical bonding by measuring the near-edge fine structure (NES) by electron energy-loss (B K-, C K-, and Ti L-edges) and X-ray absorption (B K- and Ti L-edges) spectroscopies. The NES analysis indicates the formation of a nanocrystalline ternary TiB_xC_y compound concomitant with the segregation of an a-C phase as the carbon content is increased. The optical properties were studied by spectroscopic ellipsometry and the electrical resistivity was measured by the Van der Pauw method between 20 and 300 K. The films continuously lose their metallic character in terms of optical constants and resistivity with increasing carbon content. Theoretical fitting of the electrical properties using the grain-boundary scattering model supported the formation of a nanocomposite structure based on a ternary TiB_xC _y phase embedded in a matrix of a-C. The electron transport properties are mainly limited by the high density of point defects, grain size, and transmission probability. The phase formation of TiBC/a-C nanocomposites prepared by sputtering was assessed by X-ray absorption and energy loss near-edge structure. With the increase of C, a gradual transformation from h-TiB_xC_y to fcc-TiB_xC_y structure is suggested concomitant with the development of a-C matrix. The measurement of the optical and electrical properties and its theoretical simulation supported the formation of ternary TiBC and a-C phases and allowed their quantification.