Spin density in hydrogenated amorphous carbon films—different originations

Abstract

Hydrogenated amorphous carbon (a-C:H) films were deposited on glass in a plasma-enhanced chemical vapor deposition (PECVD) system using methane as a source gas. The characteristics of the films were studied by electron spin resonance (ESR), Raman, and Fourier transform infrared (FT-IR) spectroscopy. In particular, the relationship between the variation of ESR spin density and the structural change in the a-C:H films has been investigated. It was observed that the ESR spin density of the a-C:H films increased with increasing radio frequency (r.f.) powers in the PECVD deposition. However, for the films deposited on a grounded substrate and a substrate temperature of 320 °C, both the spin density and the Raman I(D)/ I(G) ratio increased with increasing r.f. power; while, for the films deposited at a negative bias voltage (−100 V) and at room temperature (not exceeding ∼80 °C), the I(D)/ I(G) ratio decreased with the increasing r.f. power. The FT-IR measurements were also consistent with the analysis of the Raman spectra. The results were attributed to different originations for the increase of spin densities: predominantly increase in number of sp 2 graphitic clusters of aromatic rings for the non-bias case; while, mainly sp 2 sites in distorted graphitic rings with less ordering, and few if any sp 3 sites for the biased case. This could also explain the origin of the spin density depending on the substrate bias voltage.