The origin of preferred orientation during carbon film growth

Carbon films were prepared using a filtered cathodic vacuum arc deposition system operated with a substrate bias varying linearly with time during growth. Ion energies were in the range between 95 and 620eV. Alternating dark, high density (sp3 rich) bands and light, low density (sp2 rich) bands were observed using cross-sectional transmission electron microscopy, corresponding to abrupt transitions between materials with densities of approximately 3.1 and 2.6gcm-3. No intermediate densities were observed in the samples. The low density bands show strong preferred orientation with graphitic sheets aligned normal to the film. After annealing, the low density bands became more oriented and the thinner high density layers were converted to low density material. In molecular dynamics modelling of film growth, temperature activated structural rearrangements occurring over long timescales (ps) caused the transition from sp3 rich to oriented sp2 rich structure. Once this oriented growth was initiated, the sputtering yield decreased and channelling was observed. However, we conclude that sputtering and channelling events, while they occur, are not the cause of the transition to the oriented structure.