Correlation between gradients of composition and dielectric properties in oxynitride or diamond-like films on Si by means of spectroscopic ellipsometry and ion-beam analysis

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Abstract
The optical absorption spectra in the energy range 0·2–5 eV of anodic oxide films and of diamond-like films (the latter being deposited either by low-energy ion implantation or by sputtering) were investigated by spectroscopic ellipsometry. The paper emphasizes first the difficulty of interpreting the data obtained on complex amorphous systems, consisting of mixtures at the nanometre level of silicon oxide + hydroxide + nitride groups or of C sp3 + C sp2 + C: H, without the full determination of their composition and concentration gradients. On the basis of the most complete analysis which could be performed by using ion-beam techniques (back-and recoil scattering, nuclear reactions, secondary-ion emission), spectroscopic ellipsometry gave valuable information about the effect of dopants on the diffusion processes in silica and also of radiation damage on the formation of amorphous diamond. It was shown for instance that the oxidation of silicon nitride proceeds essentially from the outward diffusion of Si and N, the amounts of N and vacancies of the order of 1% in the oxide being almost the only causes of the increase of electric field and growth kinetics with respect to those of pure silica. Contrary to previous assertions, the preferential sputtering of sp2-bonded C at implantation energies in the 100 eV range is not the best way to increase the percentage of sp3-bonded (diamond-like) material in amorphous films. Films obtained at an energy of 2 keV exhibited absorption properties more characteristic of a semiconductor (with a gap of 0·7 eV) than the previous films or sputtered films, without containing amounts of H which would make them susceptible to the modification of their properties. This result confirmed the previous identification of diamond-like C in 2 keV films, or correspondingly of graphite-like C in others, by soft X-ray emission.