Effects of short-range order on the electronic structure and optical properties of amorphous carbon

We have calculated the joint density of states and absorption coefficient of amorphous carbon, a-C:H, treating it as a ternary alloy of sp(2) carbon, sp(3) carbon and hydrogen. These optical properties have been calculated in terms of the above compositions and a short-range order parameter determining the degree of clustering of the sp(2) (or sp(3)) carbon atoms. Our calculations show that this parameter has a significant effect on the Tauc gap, which seems to be considerably smaller than the mobility gap. By comparing, however, our theoretical values of the Tauc gap with experiments we deduce that the degree of clustering found in most real samples is very small. (C) 2000 Elsevier Science S.A. All rights reserved.