EFFECTS OF SUBSTRATE PRETREATMENT AND METHANE FRACTION ON THE OPTICALTRANSPARENCY OF NANOCRYSTALLINE DIAMOND THIN-FILMS

Optical transmittance of the nanocrystalline diamond films has been st udied as a function of grain size of the diamond powder used for subst rate pretreatment and the methane fraction in the source gas. It has b een observed that for CH4 fractions below 13%, the films grown on subs trates pretreated with finer diamond powder are more transparent, whil e this trend reverses for CH4 fractions above 13%. These variations in the transparency of the films correlate vel-v well with their corresp onding surface roughness. Nanocrystalline/amorphous diamond films with transmittance of greater than 80% beyond 700 nm and with average surf ace roughness as low as 61 Angstrom have been obtained for CH4 fractio ns 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 sp(3)-bonded phase also seems to be insensit ive to the CH4 content of the source gas.