Electronic processes in hydrogenated amorphous carbon

It is shown that disorder limits the degree of clustering of sp(2) sites in a-C:H to relatively small sizes. The optical gap is still controlled by the configuration of sp(2) sites. Experimentally, the gap appears to increase linearly with sp(3) content for all types of a-C:H. The photoluminescence of a-C:H is shown to follow basically the a-Si:H model, with gap states acting as non-radiative centers, the sp(2) cluster states acting like conventional tail states, but with a faster radiative rate, wider band tails, and shorter tail state Bohr radii.