Effects of substrate pretreatment and methane fraction on the optical transparency of nanocrystalline diamond thin films

Optical transmittance of the nanocrystalline diamond films has been studied as a function of grain size of the diamond powder used for substrate pretreatment and the methane fraction in the source gas. It has been observed that for CH₄ fractions below 13%, the films grown on substrates pretreated with finer diamond powder are more transparent, while this trend reverses for CH₄ fractions above 13%. These variations in the transparency of the films correlate very well with their corresponding surface roughness. Nanocrystalline/amorphous diamond films with transmittance of greater than 80% beyond 700 nm and with average surface roughness as low as 61 Å have been obtained for CH₄ fractions as high as 42% in the source gas. Interestingly, these films do not show an obvious presence of any graphitic carbon, and the structural ordering of the amorphous sp³-bonded phase also seems to be insensitive to the CH₄ content of the source gas.