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 fractio n in the source gas. Specifically? the substrate pretreatment involved usin g two different grain sizes, 4 mm and 0.1 mu m, of the diamond powder for u ltrasonic agitation. Transmission electron microscopy, optical transmission spectra, and X-ray absorption spectra have been employed to characterize t he resultant films. For each him, the grain size distribution was obtained by directly analyzing the dark-field images of the film. For films deposite d with 0.1 mu m diamond powder pretreatment, the grain size of the film dec reased as the methane fraction increased. The opposite trend was observed f or 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 e vidence of sp(2) bonding even for films with an average grain size as low a s 3.6 nm. Furthermore, a blue shift of the exciton state and conduction ban d edge was observed with the decrease of the grain size of the film, indica ting a quantum confinement effect. On the other hand, no direct relationshi p between the grain size of the film and the optical transmission was obser ved. The key parameter that dictated the optical transparency of the film w as the surface roughness provided that the film maintained high sp(3) conte nt. (C) 2000 Elsevier Science S.A. All rights reserved.