Electron paramagnetic resonance study of ion implantation induced defects in amorphous hydrogenated carbon

Electron paramagnetic resonance (EPR) measurements have been made of defects in amorphous hydrogenated carbon (a-C:H) thin films. The films were grown on silicon substrates on the earthed electrode of an rf-powered plasma enhanced chemical vapour deposition reactor and were subsequently implanted with a range of doses of boron, carbon or nitrogen ions with energies from 20 keV to 32.5 keV. Two paramagnetic centres are observed, the carbon defect in the film with g = 2.0028(1) and a silicon defect in the substrate with g = 2.0058(6). The volume concentration of the carbon defect increases approximately linearly with dose, from approximately 3 × 10¹⁷ cm^-3 for unimplanted samples to 2.7 × 10²⁰ cm^-3 at the highest implantation of 2 × 10¹⁶ B⁺ ions cm^-2. The increase in dose over this range also causes a narrowing of the EPR line (from 0.83 mT to 0.13 mT) and a significant decrease in the spin-lattice relaxation time (from 3 × 10^-5 s to 6 × 10^-8 s) which approaches the spin-spin relaxation time at the highest dose. The narrowing is attributed to motional averaging produced by either exchange or hopping. We also report the effects of annealing samples implanted with a range of boron doses. The prime novelty of this paper is that it is the first EPR study of defects produced by the implantation of a range of ions into polymer-like amorphous hydrogenated carbon.