Quantitative study of C—H bonding in polymerlike amorphous carbon films using<i>in situ</i>infrared ellipsometry

Abstract

Polymerlike hydrogenated amorphous carbon $(a\ensuremath{-}\mathrm{C}:\mathrm{H})$ films have been deposited by plasma CVD at low temperature and low pressure. Vibrational properties have been investigated by in situ infrared ellipsometry as a function of film thickness. Hydrogen distribution within the films has been changed by varying the ion energy impinging on the film surface. Vibrational properties of $\mathrm{C}\mathrm{---}\mathrm{H}$ stretching and bending modes have been analyzed as function of self-bias ${(V}_{\mathrm{bias}})$ in terms of frequency, bandwidth, and intensity. Absorption strengths are associated with effective charges that have been calculated for the different ${\mathrm{sp}}^{3}$ ${\mathrm{CH}}_{n}$ groups. In order to make a comparison with values of organic chemistry, a general review of infrared spectra including alkanes, alkenes, and aromatic hydrocarbons is presented. A dipole description taking into account the local environment of CH bonds is developed showing that methyl and methylene group effective charges are similar for polymeric $a\ensuremath{-}\mathrm{C}:\mathrm{H}$ and ${\mathrm{C}}_{x}{\mathrm{H}}_{y}$ organic compounds. Line broadening and frequency shift effects are found to depend on the type of CH groups and are explained through a model including strain and dipole-dipole interactions. The sensitivity of effective charges to the local environment and the determination of ${\mathrm{CH}}_{n}$ group densities are used to propose a description of the hydrogenated network structure.