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

Abstract The structural evolutions of a-C:H thin films prepared by r.f. plasma-enhanced chemical vapor deposition from a CH 4 /Ar or CH 4 /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 sp 2 proportion carbon of thin films. The variations of electron spin resonance parameters, the peak-to-peak linewidth Δ H pp , 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 E g of these films decreases with the loss of hydrogen (bonded and unbonded) and the increasing of sp 2 content while E u gap increases. These results have been related to the spin densities.