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 Vb in the approximate range 10--540 V. For Vb<100V, 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 H¹. The former yields a Lorentzian line, the latter a Gaussian, and the resultant spectrum has the Voigt shape. The empirical relation ${Δ}{B}ₚₚG(inGauss)=(0.18±{}0.05)×{}(at.%H)$ between the peak-to-peak Gaussian contribution (in Gauss) ${Δ}{B}ₚₚG$ and the hydrogen content in atomic percentage is obtained. For ${V}b>100V$ 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.