Characterization of the silicon network disorder in hydrogenated amorphous silicon carbide alloys with low carbon concentrations

The network disorder of amorphous Si1-xCx:H films, containing carbon concentrations below 25 at.%, has been studied by means of Raman spectroscopy. Two different radiations were employed to excite the Raman scattering of these materials, one that is strongly (458 nm) absorbed and another that is weakly (581 nm) absorbed. The variation in probed depth attained with these excitation radiations together with the significant differences observed in the Raman spectra excited with them indicate the existence of silicon network disorder inhomogeneities at the short-range (bond angle) and intermediate-range (dihedral angle) levels along the axis perpendicular to the films which are considerably larger than those existing in device-quality a-Si:H. Such inhomogeneities are observed to develop abruptly at the smallest carbon concentration. The findings explain the various previous conflicting reports in the literature regarding the behavior of the Si-network disorder in these alloys. When excited with strongly absorbed radiations (e.g., 514 and 488 nm), the Raman spectra do not show a definite trend as a function of carbon concentration, although it can still be concluded that there is less order than in a-Si:H. On the other hand, the Raman spectra excited with weakly absorbed radiation show that the order actually improves in the bulk of Si-rich a-SiC:H films. The above findings indicate that silicon is forming compact clusters in the interior of the films on this compositional regime. Besides, our results show that hydrogen dilution of the gas mixture during film growth helps improve the short-range order in the bulk of the films with the smallest carbon concentrations, but has no definite effect over the intermediate-range order or over the short-range order at the near surface. It is also found that the spectral intensity enhancement observed in these materials with increasing carbon content is due to a scattering volume effect.