Low density of defect states in hydrogenated amorphous carbon thin films grown by plasma-enhanced chemical vapor deposition

Abstract

The density of electronic defect states in most forms of amorphous carbon deposited at room temperature is found so far to be very high ${\text{(10}}^{18}{\text{–10}}^{22} {\mathrm{spins cm}}^{\mathrm{-3}}\mathrm{).}$ In this letter, we demonstrate that the radio-frequency plasma-enhanced chemical vapor deposited hydrogenated amorphous carbon $\mathrm{(a-}\mathrm{C:H})$ thin film exhibits the lowest spin density of the order of ${10}^{16} {\mathrm{cm}}^{\mathrm{-3}},$ investigated by using electron spin resonance (ESR) spectroscopy, a very promising reproducible result comparable with high-quality $\mathrm{a-}\mathrm{Si:H}.$ In addition, the optical gap of $\mathrm{a-}\mathrm{C:H}$ has been tailored between a wide range, 1.8–3.1 eV. The ESR spectra of all the films reveal a single Lorentzian line whose linewidth ${\mathrm{\Delta H}}_{\mathrm{pp}}$ varies strongly with the optical gap. Also, there is a strong dependence of spin density on the optical gap, and we show that this dependency is a direct result of structural changes due to ${\mathrm{sp}}^3{\mathrm{/sp}}^2$ carbon bonding network.