Observation of the initial stages of growth of hydrogenated amorphous carbon films by scanning tunneling microscopy

Scanning tunneling microscopy was used to investigate the initial stages of growth of hydrogenated amorphous carbon (a-C:H) films. The films were deposited on highly oriented pyrolytic graphite and silicon substrates both on the powered (negatively self-biased) and grounded electrodes from a methane rf plasma (13.56 MHz) at 68–70 mTorr and room temperature. On graphite substrates, films formed at the powered electrode were observed to nucleate in clusters ∼50 Å in diameter. Cluster density increased with deposition time, while cluster size remained relatively constant. The atomic structure of individual clusters was resolved and found to be amorphous, containing areas of both five- and sixfold rings in a disordered matrix. No long-range order was observed. The C–H bonding in these clusters was observed by high-resolution electron-energy-loss spectroscopy. Films formed on graphite substrates placed at the grounded electrode produced very noisy images due to an unstable tunneling gap, indicative of the material’s poor electrical conductivity. On silicon substrates placed at the powered electrode, cluster formation was not observed, and the films formed were measured to be flat to within 3-Å rms. The clusters formed on graphite at the powered electrode may be initiated by nucleation sites created by high-energy ion bombardment. Due to the higher reactivity of silicon, growth onto these substrates proceeds without the requirement of ion bombardment nucleation sites, thus the more homogeneous growth on silicon.