ELECTRONIC-PROPERTIES OF AMORPHOUS-CARBON (A-C-H)

Amorphous carbon (a-C:H) films were deposited from C2H2 in an r.f.-discharge reactor. By changing the area ratio of the two electrodes both anode-type (polymer-like) and cathode-type (diamond-like) films were obtained. They were characterized by their optical absorption constant alpha using photothermal deflection spectroscopy (PDS), photoluminescence (PL) and electron spin resonance (ESR). We define the optical gap as E03, the energy, where alpha = 10(3) cm-1. The broad absorption edge is exponential with an Urbach energy E0 almost-equal-to 250 meV independent of E03 and the deposition conditions. The ESR spectra of all films reveal a single line with g almost-equal-to 2.0027 and lorentzian lineshape whose linewidth DELTAH(pp) varies strongly with E03. The variation of both DELTAH(pp) and the spin density N(s) with E03 is different in anode- and cathode-type films. In spite of the high values of N(s) (4 x 10(18)-7 x 10(20) cm-3), all films exhibit photoluminescence, whose intensity I(PL) increases by three orders of magnitude with increasing E03. Unlike in a-Si:H, there is no well defined correlation between I(PL) and N(s), indicating that the paramagnetic states are not the dominant centres for non-radiative recombination in a-C:H. These results are discussed in terms of the graphitic-cluster model in which the variation of the film properties can be related to the variation of cluster sizes.