Growth of diamond films: General correlation between film morphology and plasma emission spectra

We have studied the emission spectra from plasmas excited in several compositions of three different gas mixtures useful for the growth of diamond films, namely CH4-H-2 (the classical one), CH4-CO2 and C2H2-CO2 (not previously reported by other authors). In all three cases we find the same correlation between the quality of the obtained diamond films and some spectral features: in particular, the yield of the best diamond films corresponds to the presence of the emission line at wavelength 431 nm ascribed to CH, in the absence of the emission band at 505-517 nm attributed to C2. The appearance and the progressive increase of the latter corresponds to a gradual worsening of the diamond film quality. We propose such spectral features of the plasma as a general and practical gauge of the diamond film growth conditions, for any gas mixture used.