Growth of highly transparent nanocrystalline diamond films and a spectroscopic study of the growth

A series of nanocrystalline diamond films with grain size ranged from 4 nm to a few hundreds of nanometers were grown by microwave plasma enhanced chemical vapor deposition. Effects of the substrate pretreatment and the methane fraction in the source gas on the microstructure, surface roughness, and optical transmittance of the resultant films were studied. Specifically, comparison was made between two different sizes, 4 nm and 0.1 μm, of the diamond powder used for substrate pretreatment. Interestingly, the films grown on substrates scratched with coarser powder (0.1 μm) can be smoother and more transparent than those on substrates scratched with finer powder (4 nm), despite of the similarity in the grain size of these two types of films prepared at high methane fractions. It is also demonstrated that the major factor that controls the optical transparency is the surface roughness irrespective of the grain size as long as the sp²-bonded carbon in the film is avoided. In situ optical emission spectroscopy was employed to monitor the plasma chemistry, from which possible growth species for the films were discussed.