EPR linewidth variation, spin relaxation times, and exchange in amorphous hydrogenated carbon

Electron paramagnetic resonance (EPR) measurements have been made of amorphous hydrogenated carbon (alpha-C:H) films grown by plasma enhanced chemical vapor deposition (PECVD) with negative self-bias voltages V-b in the approximate range 10-540 V. For V-b< 100 V, as the film changes from polymerlike to diamondlike, the changes in linewidth and shape are interpreted in terms of changes to two contributions-one due to dipolar interactions between the unpaired spins and one due to unresolved lines arising from hyperfine interactions with H-1. The former yields a Lorentzian line, the latter a Gaussian, and the resultant spectrum has the Voigt shape. The empirical relation Delta B-pp(G) (in Gauss)=(0.18+/-0.05)X(at. % H) between the peak-to-peak Gaussian contribution (in Gauss) Delta B-pp(G) and the hydrogen content in atomic percentage is obtained. For V-b> 100 V the linewidth is shown to be dominated by the dipolar interactions and exchange and it decreases as V-b increases; the change is shown to arise primarily from a change in the exchange interaction. Evidence for this comes from measurements which show that the spin-lattice relaxation time appreciably shortens and the spin-spin relaxation time lengthens as the bias voltage is increased. The magnitude and variation with bias of the linewidth are consistent with the EPR signal originating from the pi-type radicals.