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

Abstract

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 ${V}_{b}<100\mathrm{V},$ 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 $\ensuremath{\Delta}{B}_{\mathrm{pp}}^{\mathrm{G}}\mathrm{}(\mathrm{i}\mathrm{n}\mathrm{}\mathrm{G}\mathrm{a}\mathrm{u}\mathrm{s}\mathrm{s})=(0.18\ifmmode\pm\else\textpm\fi{}0.05)\ifmmode\times\else\texttimes\fi{}(\mathrm{a}\mathrm{t}.\mathrm{}%\mathrm{}\mathrm{H})$ between the peak-to-peak Gaussian contribution (in Gauss) $\ensuremath{\Delta}{B}_{\mathrm{pp}}^{\mathrm{G}}$ and the hydrogen content in atomic percentage is obtained. For ${V}_{b}>100\mathrm{V}$ 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 \ensuremath{\pi}-type radicals.