Formation of diamondlike nanocrystallites in amorphous carbon films synthesized by radio-frequency sputtering
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Transmission electron microscopy (TEM) and fast Fourier transformation (FFT) analysis were used to examine the microstructures of amorphous carbon (a-C) films deposited on Si(100) by radio-frequency (rf) sputtering without magnetron. TEM analysis revealed that a-C films synthesized under certain deposition conditions contained randomly dispersed nanocrystallites ∼35 Å in size. FFT results indicated that the nanocrystallites possessed diamondlike cubic structures with their close-packed {111} planes parallel to the film surface. The formation of diamondlike nanocrystallites is attributed to metastable carbon atom clusters of trigonal carbon hybridization that were sputtered off from the graphite target under certain process conditions. Cluster distortion upon deposition onto the growing film surface by the bombarding Ar+ ions promoted tetrahedral carbon atom hybridization and, possibly, epitaxial growth of diamondlike nanocrystallites for a short duration.
I. INTRODUCTION
Films of diamondlike carbon and amorphous carbon (a-C) deposited by different techniques have attracted significant research attention because of their unique properties and use in numerous applications.1–3 Hydrogenated a-C films with dispersed diamond nanocrystallites have been synthesized by microwave plasma-enhanced chemical vapor deposition,4–6 ion beam deposition,7 and direct current glow discharge.8 Deposition of metal-doped diamondlike carbon films has also been reported recently.9 Atomic hydrogen produced by thermal decomposition or atomic collisions in a plasma plays an important role in the formation of diamondlike domains.10 Nanocrystallites embedded in tetrahedral amorphous carbon (ta-C) films have been identified in carbon films produced by filtered cathodic vacuum arc deposition.11 These nanocrystallites have been observed only in ta-C films deposited under high carbon ion energies. The majority of the crystals discovered in ta-C films were graphitic and 0.5–2 m in size. The d spacing of crystals in ta-C films has been found to be very close to that of cubic diamond. Conversely to ta-C films, diamondlike nanocrystallites have not been previously observed in a-C films produced by radio-frequency (rf) sputtering. A considerable amount of platelet crystallites with an orientation parallel
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Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2008.0098 700
J. Mater. Res., Vol. 23, No. 3, Mar 2008
to the film surface has been reported for a-C films produced by rf magnetron sputtering under a substrate bias voltage of −200 V.12 These crystallites were of several micrometers in size and exhibited lattice parameters larger than graphite and diamond but smaller than the C60 fullerene crystal structure. The objective of this study was to investigate if carbon crystallites of nanometer size (nanocrystallites) can form in a-C films synthesized by rf sputtering, involving significantly less energetic carbon atom precursors compared to the highly energetic carbon ions in some of the previous film dep
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