Density of States in Amorphous Carbon and Its Modification by Annealing.

The effect of annealing on the electron density of states in amorphous carbon a-C and amorphous hydrogenated carbon a-C:H has been studied, a-C and a-C:H layers were grown by magnetron sputtering of a graphite target in, respectively, argon and argon-hydrogen plasmas. Optical transmission spectra were studied experimentally in the range 1.5-5.6 eV, and ellipsometric parameters were measured at the He-Ne laser wavelength. The spectral dependence of the imaginary part of the dielectric function was reconstructed. A model describing the optical response of amorphous carbon was developed on the basis of the hypothesis that there are fluctuations of the sp² fragment sizes in the allotropic composition of amorphous carbon. The optical gap E[sub g] in both types of material is accounted for by the presence of fragments of critical size. Experimental data were used to reconstruct, with the use of model parameters, the energy dependence of the density of states in the ground and excited bands, and the plasma frequencies for electrons involved in optical transitions are found. It is shown that the bands of both the ground and excited states are inhomogeneously broadened sets of levels, which are symmetrical about the Fermi level. The behavior of the model parameters E[sub g] and E[sub G] (energy corresponding to the peak of the Gaussian distribution) and the plasma frequency with annealing temperature shows, for materials of both types, a substantial rise in the size and number of critical fluctuations with increasing temperature. [ABSTRACT FROM AUTHOR]