Amorphous carbon luminescence; excitation and emission in a broad energy range

Photoluminescence excitation (PLE) spectra at photon energies of 2.47 to 5.63 eV have been obtained for hydrogenated amorphous carbon samples (a-C:H) deposited by radio frequency glow-discharge decomposition of methane at 0 V self bias. A peak in the PLE spectra was observed at similar to 3.0 eV excitation energy, above which the luminescence intensity slowly decreases with photon energy. Photoluminescence (PL) spectra with a peak at 2.34 eV show similar asymmetric line-shape and peak energy when the excitation photon energy decreases from 5.6 eV to 2.5 eV. The highest photon energy which excites the characteristic PL band is 5.63 eV. Our results demonstrate that light-emitting electron hole pairs remain strongly localized even at excitation energies much higher than the optical gap. A new localization mechanism based on symmetry differences between sigma and pi bonds is proposed. (C) 1998 Elsevier Science B.V. All rights reserved.