Influence of hydrogen on the structure and properties of tetrahedral amorphous carbon films obtained by the filtered cathodic vacuum arc technique

Hydrogenated tetrahedral amorphous carbon (ta-C: H) films prepared by filtered cathodic vacuum arc at different hydrogen partial pressures (0.0008–0.5mTorr) were studied in terms of their structure and properties. It is shown that a single Breit-Wigner-Fano (BWF) line shape is appropriate to fit the Raman spectra acquired from the ta-C: H films deposited at a hydrogen partial pressure of 0.0008–0.08 mTorr. However, the single BWF fit shows a residual near 1350cm⁻¹ for the film deposited at a higher hydrogen partial pressure. This indicates the presence of graphitic clusters in the corresponding film. The data from infrared spectroscopy show that the hydrogen in the ta-C: H films is associated with triply bonded sp¹, CH_n sp² and CH_n sp³ bonding. The absorption for the CH_n sp³ is maximum for ta-C: H at a hydrogen partial pressure of 0.008 mTorr. It was observed that the film hardness and compressive stress are about 70 GPa and 12 GPa respectively corresponding to the hydrogen partial pressure from 0.0008 to 0.02 mTorr. When the hydrogen partial pressure further increases from 0.02 to 0.5 mTorr, the hardness and compressive stress decrease to about 50 GPa and 8 GPa respectively. The optical bandgap is about 3.0eV for the ta-C: H film deposited at hydrogen partial pressure of 0.008 mTorr compared with 2.7 eV for the hydrogen free ta-C films. A lower intraband absorption coefficient of the ta-C : H sample (hydrogen partial pressure, 0.008 mTorr) indicates that the defect states are lower in this sample.