NEXAFS characterization of nanocrystalline diamond thin films synthesized with high methane concentrations

Diamond thin films were deposited on silicon in gas mixtures of methane and hydrogen with different methane concentrations ranging from 1% to 100% using microwave plasma assisted chemical vapor deposition. Both Raman spectroscopy and synchrotron near edge extended X-ray absorption fine structure spectroscopy (NEXAFS) were used to characterize the electronic structure and chemical bonding of the synthesized films. The NEXAFS spectra of the nanocrystalline diamond (NCD) films exhibit clear spectral characteristics of diamond. Close observation reveals that the films (10% CH₄ or above) exhibit a slightly broadened exciton transition with a 0.25 eV blue shift. With the increase in methane concentration, the growth rate, the surface smoothness, and the sp² carbon concentration of the films increase while the grain size decreases. Well-faceted microcrystalline diamond films were synthesized with a methane concentration of 5% or lower, while NCD films were formed with a methane concentration of 10% or higher. Diamond thin films with low surface roughness and fine nanocrystalline structure have been synthesized with high methane concentrations (50% or above). It has been observed that the diamond growth rate increases with methane concentration. The growth rate at 100% methane concentration is approximately 10 times higher than at 1%.