Tuning structural, electrical and mechanical properties of diamond-like carbon films by substrate bias voltage

Abstract

Diamond-like carbon (DLC) thin films are deposited by unbalanced RF magnetron sputtering at substrate bias voltages to tune the ratio of the sp3 to sp2 bonds and hydrogen composition in optimized Ar: CH4 ambient. The effect of substrate bias voltage on the sp2 and sp3 bonding ratios and hydrogen concentration is correlated to the physical and elastic properties for the optimized DLC film. Raman spectroscopy qualitatively established sp3/sp2 bonding ratios from the ID/IG ratio at various substrate bias voltages and the trend is in agreement with XPS data. High H content of at least 18.00 ± 2.00 at. % is evaluated from photoluminescence background for all substrate bias voltages. Applying an empirical approach to the ID/IG ratios, we derive the Tauc band gaps in the range between 1.51 and 1.57 eV. The highest band gap correlates to the highest resistivity values (131 × 102 Ω.cm) measured for DLC films deposited at the highest bias voltage of −100 V and with lowest H content. As DLC thin films deposited at −100 V exhibit optimal properties, the elastic constants are subsequently evaluated using the Surface Elastodynamic Greens functions to the discrete phonon dispersion obtained by surface Brillouin scattering. Enhancement in bulk, shear and elastic moduli is ascribed to higher sp3/sp2 ratio and predominantly to H content.