A systematic Raman analysis has been carried out on diamond films prepared by microwave plasma enhanced chemical vapour deposition, using a CH4-CO2 gas mixture at methane concentrations varying between 47 and 52%, at 750 and 850°C substrate temperatures, in order to assess the influence of the growth conditions on the film crystalline quality, as measured by the linewidth of the diamond peak. The trends observed by changing substrate temperature and CH4 concentration are understood on the basis of the results of a complementary film characterisation by means of micro-Raman spectroscopy, scanning electron microscopy and X-ray diffraction, aimed at investigating the local stress distribution, the surface morphology and the preferential orientation, respectively. The existence of a strong correlation is evidenced between the texturing achieved by varying the gas-mixture composition and the lineshape of the diamond peak in the micro-Raman spectra. The competition between different growth sectors within the film promotes the occurrence of large anisotropic stresses that split and inhomogeneously broaden the diamond peak. As a result, quite large macro-Raman diamond peaks are correspondingly detected, suggesting the possible unreliability of linewidth as defect density indicator in the presence of large anisotropic stresses. Finally, a quantitative estimation of the stress level in the investigated films is preliminarily presented.
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