Structural and optical investigation of plasma deposited silicon carbon alloys: Insights on Si-C bond configuration using spectroscopic ellipsometry

Amorphous (a-Si(1-x)C(x): H) and microcrystalline (mu c-Si(1-x)C(x): H) thin films have been deposited by plasma-enhanced chemical-vapor deposition using SiF(4)-CH(4)-H(2) rf plasmas. Gas flow rates have been varied to deposit films with different carbon content and microstructure. The microstructure and optical properties have been investigated by IR/Raman spectroscopy and by spectroscopic ellipsometry in the energy range of 1.5-5.0 eV. Ellipsometric spectra have been analyzed in terms of the tetrahedron model combined with the Bruggeman effective-medium approximation to determine the film microstructure and silicon-carbon-bond configurations. Correlation between Si-C bond configurations and optical properties of films has been studied as a function of carbon content and microstructure. It is found that the optical properties and the band gap value depend not only on the carbon content, but also on the Si-C bond configuration and microcrystallinity. The films consist of Si-centered Si-Si(4-v)C(v) (v=0-3) Si-centered tetrahedra with segregation of mu c-Si. The amorphous matrix changes from Si-Si(3)C to Si-SiC(3) with the increase of the mu c-Si phase. The optical gap is found to increase with both carbon content and microcrystallinity.