In this study, the role of the pulse duty ratio was investigated during the deposition of diamond films in a hot filament chemical vapour deposition reactor with a pulsed-tic biased substrate positively relative to the hot filaments. The voltage-current characteristics showed that the discharge current rose with the increase of biasing voltage, which was modified by the duty ratio. Before deposition, two approaches were adopted for the pre-treatment of the silicon substrates, respectively, and the substrates were scratched by diamond paste or seeded by diamond powders using the so-called ＇soft dry polished＇ technique. Diamond films were deposited under a fixed discharge power by changing the duty ratios. In the first group with scratched substrates, it was found that under a high duty ratio the diamond grew slowly with quite poor nucleation, while in the second case a high duty ratio induced a high deposition rate and good diamond qual- ity. Reactive hydrocarbon species with high energy are essential for the initial nucleation process, which is more effectively achieved at a high biasing voltage in the condition of a low duty ratio. In the film growth process, the large discharge current at a high duty ratio represents an increased concentration of electrons and reactive species as well, promoting the growth of diamond films.
