LO-TO splittings in plasma-deposited siloxane films

The present work presents LO and TO functions in the mid-infrared region for thin films deposited from glow discharge plasmas of tetramethylsilane (TMS) diluted either in Ar or O-2 or in mixtures of these two gases. These functions were calculated through the Kramers-Kronig analysis of transmittance spectra of the films supported on KBr disks. To correlate structural aspects of the films with the observed LO-TO splittings, a group frequency analysis based on the literature was made. Such an analysis indicated that the films deposited from the TMS Ar mixture were formed mainly by a polycarbosilane skeleton, whereas those deposited from TMS-O-2 and TMS-O-2-Ar were formed by a random network of four types of distorted tetrahedra: (CH3)(3)SiO0.5, (CH3)(2)SiO, (CH3SiO1.5), and SiO2. From the LO-TO splitting for the asymmetrical stretching mode of Si-O-Si groups, the density and the presence of defects in samples obtained from TMS-O-2 and TMS-O-2-Ar mixtures were evaluated. The number of defects increased as the Ar-to-O-2 flow rate decreased. We also report for the first time LO-TO splittings for bands related to the bending of CH3 and to the stretching of the Si-C bond in Si(CH3)(x) groups. The knowledge of such splittings is very important for a correct evaluation of the infrared reflection -absorption spectra taken at oblique incidence of thin films containing Si-O bonds and Si(CH3)(x) groups deposited on metals.