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

Electron paramagnetic resonance (EPR) measurements have been made of amorphous hydrogenated carbon (a-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 $V_b<100\mathrm{}\mathrm{V},$ 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 ${\mathrm{H}}^1.$ The former yields a Lorentzian line, the latter a Gaussian, and the resultant spectrum has the Voigt shape. The empirical relation $\Delta B_{\mathrm{pp}}^{\mathrm{G}}\mathrm{}\left(\mathrm{i}\mathrm{n}\mathrm{}\mathrm{G}\mathrm{a}\mathrm{u}\mathrm{s}\mathrm{s}\right)=(0.18\mathrm{}\pm 0.05)\times (\mathrm{a}\mathrm{t}.\mathrm{}\mathrm{}\%\mathrm{}\mathrm{H})$ between the peak-to-peak Gaussian contribution (in Gauss) $\Delta B_{\mathrm{pp}}^{\mathrm{G}}$ and the hydrogen content in atomic percentage is obtained. For $V_b>100\mathrm{}\mathrm{V}$ 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 π-type radicals.