Characterization by electron spin resonance of defects in a-C:H thin films. Correlation between structural evolutions and optical properties

The structural evolutions of a-C:H thin films prepared by r.f. plasma-enhanced chemical vapor deposition from a CH4/Ar or CH4/He gas mixture have been investigated. Elastic recoil detection analysis, infrared and Raman spectroscopies have been used to quantify, respectively, hydrogen content, the bonding and sp2 proportion carbon of thin films. The variations of electron spin resonance parameters, the peak-to-peak linewidth ΔHpp, g-factor and the spin density, as a function of the film preparation are discussed on the basis of the complete physico-chemical characterization. The type of defects do not change drastically when the conditions of the film preparation vary contrary to the spin–spin interaction: a stronger exchange resulting from the greater delocalization of the π-electron occurs when the bias voltage increases. Moreover, the hyperfine interaction of the spins with the hydrogen atoms decreases. We have found that the optical band gap Eg of these films decreases with the loss of hydrogen (bonded and unbonded) and the increasing of sp2 content while Eu gap increases. These results have been related to the spin densities.