Microstructure and optical band gap control of DLC film deposited by pulsed discharge plasma CVD

DLC films were deposited on silicon and quartz glass substrates by pulsed discharge plasma chemical vapor deposition (CVD), where the plasma was generated by pulsed DC discharge in H2–CH4 gas mixture at about 90 Torr in pressure, and the substrates were located near the plasma. The repetition frequency and duty ratio of the pulse were 800 Hz and 20%, respectively. When CH4 / (CH4 + H2) ratio, i.e. methane concentration (Cm), increased from 3 to 40%, C2 species in the plasma was increased, and corresponding to the increase of C2, deposition rate of the film was increased from about 0.2 to 2.4 μm/h. The absorption peaks of sp3C–H and sp2C–H structures were observed in the FT-IR spectra, and the peak of sp2C–H structure was increased with increasing Cm, showing that sp2 to sp3 bonding ratio was increased when Cm was increased. Corresponding to these structural changes due to the increase of Cm, optical band gap (Eg) was decreased from 3 to 0.5 eV continuously when Cm was increased from 3 to 40%.