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

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 I_D/I_G 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 I_D/I_G 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.