Characterization of Diamond Thin Films by Core-Level Photoabsorption and UV Excitation Raman Spectroscopy

Abstract

AbstractNear-edge x-ray absorption fine structure (NEXAFS) and Raman spectroscopies have been used to characterize both nanocrystalline and faceted, microcrystalline diamond thin films grown by microwave plasma chemical vapor deposition under a variety of conditions. Raman spectroscopy is perhaps the most commonly used method of characterizing diamond film quality, whereas photoabsorption requires a synchrotron radiation source and is much less commonly used. Both methods characterize the sp2 (graphitic) /sp3 (diamond) electronic bonding character of the films, but Raman spectroscopy is considerably more sensitive to sp2 than sp3 bonded carbon. For nanocrystalline diamond, the use of a visible excitation source for Raman spectroscopy also gives rise to an intense fluorescence band which may completely mask the sp3 Raman line, even in films with negligible (<1%) sp2 bonding character. Near-edge x-ray absorption fine structure (NEXAFS) is a more localized measurement of the electronic bonding character, and does not have as large a variation in sensitivity between sp2 and sp3 phases.Raman spectra measured using 632.8 nm light exhibit a sharp diamond phonon peak at 1332 cm−1 for faceted, large grain diamond films, and no obvious diamond phonon peak for the nanocrystalline diamond films. However, Raman spectra excited by 228.9 nm light exhibit a sharp diamond phonon peak for both nanocrystalline and large grain, faceted diamond films. The UV Raman results are consistent with the C(ls) NEXAFS measurements which show clear bulk diamond excitonic and sp3 features, with little evidence of sp2 character for all of the tested films. We conclude that the use of visible excitation Raman spectroscopy as the sole criterion for the assessment of quality in nanocrystalline diamond films is misleading, but that UV Raman spectroscopy and NEXAFS provide a consistent and reliable characterization. Nanocrystalline films grown using a fullerene precursor in an Ar microwave plasma with very low levels of hydrogen (2%) are found by both of the latter methods to consist almost entirely of sp3-bonded carbon with ˜2% non-tetrahedral bonding.