Surface Chemical Effects on the Optical Properties of Thin Nanocrystalline Diamond Films
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and oxidation experiments were performed at 450 - 800 C (for 0.5 or 1.0 hour for each temperature) in air, nitrogen or hydrogen atmosphere respectively. The same samples were examined by Raman and optical spectroscopy and by electron microscopic study as well. RESULTS Optical properties. For all the as-grown films, the transmission in the middle and far IR was consistent with the value expected for a film with a smooth surface (with root mean square surface roughness ranging from 20 to 150 nm) and refractive index of 2.35-2.37. The spectra of nanocrystalline diamond films were close to that of the type lba diamonds with the extra absorption near the indirect band edge in the UV-VIS and in the defect-induced one-phonon diamond band in 1R. The concentration of carbon-hydrogen groups in our films was of the order of 1-2 %, as estimated from the integral intensity of the corresponding stretch mode in IR absorption spectra, where three sharp components are seen at 2920 cm- 1 , 2850 cm- 1 and at 2836 cm-1. The peak frequencies and shapes of two higher components are quite close to the well-known spectrum of CH 2 -groups (antisymmetric and symmetric stretches) involving sp3 -bonded carbon in a-C:H and 3 CVD diamond films. Deconvolution of this band shows also the presence of CH 3 and CH sp groups. The interpretation of the nature of the absorption band with the peak at near 2836 cm- 1 (which dominates in the CH, spectra of 10-20 .tim thick nanocrystalline films) is less obvious. This band was never observed in the IR spectra of diamond-like carbon films, so it should originate from hydrogen at diamond grain surfaces or associated with crystal structure defects. Some authors assigned this band to hydrogen located at (111) diamond grain surfaces, while others assigned this strong C-H stretch band to nitrogen incorporation in CVD diamond films. We analyzed UV-vis spectra and IR one-phonon defect-induced band in films under study and calculated the maximum possible nitrogen content, like in [4], which occured to be several times lower, than the concentration of the centers, responsible for the 2836 cm-1 band. Therefore the incorporation of nitrogen alone cannot be responsible for the whole 2836 cm- 1 band in our films. The oxidation in air of diamond films at 590-630 C resulted in gradually decreasing of the diamond film's transmission in the whole spectral range except for the far IR [5]. At these and higher temperatures, oxygen etching results in selective, non-uniform removal of the film material at near surface regions, which leads to formation of surface porosity and hence to increased surface and bulk light scattering. During this stage of oxidation, hydrogen begins to leave the diamond films. As a result, the dangling carbon bonds are occupied by oxygen forming C=O and C-O-C groups, which were registrated in the IR spectra. At the temperatures higher than 630 C the most of the non-diamond carbon is removed from samples investigated and the remaining diamond crystallites in the film structure are attacked by the oxygen.
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