Growth, characterization, optical and X-ray absorption studies of nano-crystalline diamond films

We report here the growth of thin diamond films, with an average grain size ranged from 3.6 nm to 5 mu m, by microwave plasma enhanced chemical vapor deposition via different substrate pretreatment and varying methane fraction in the source gas. Specifically? the substrate pretreatment involved using two different grain sizes, 4 mm and 0.1 mu m, of the diamond powder for ultrasonic agitation. Transmission electron microscopy, optical transmission spectra, and X-ray absorption spectra have been employed to characterize the resultant films. For each him, the grain size distribution was obtained by directly analyzing the dark-field images of the film. For films deposited with 0.1 mu m diamond powder pretreatment, the grain size of the film decreased as the methane fraction increased. The opposite trend was observed for 4 nm diamond powder pretreated films. X-ray absorption spectra at the C K-edge showed clear bulk diamond excitonic and sp(3) features with little evidence of sp(2) bonding even for films with an average grain size as low as 3.6 nm. Furthermore, a blue shift of the exciton state and conduction band edge was observed with the decrease of the grain size of the film, indicating a quantum confinement effect. On the other hand, no direct relationship between the grain size of the film and the optical transmission was observed. The key parameter that dictated the optical transparency of the film was the surface roughness provided that the film maintained high sp(3) content. (C) 2000 Elsevier Science S.A. All rights reserved.