The role of π and π gaussian-like density-of-states bands in the interpretation of the physical properties of a-C and a-C:H films

Effect of substrate bias on the properties of a-C:H films by direct current saddle-field plasma-enhanced chemical-vapor deposition

“…Even though the PL peak shows the same trends of increase with V s , it does not directly correlate to the optical band gap because the PL only reflects the distribution of the sp 2 cluster size by the photons emitted from carrier recombination with an energy similar to the cluster energy gap, and E g depend on the average number of rings in the cluster. 29,20,30 Luminescence above E g comes from the transitions between localized states above E g . 30…”

Metastable states in hydrogenated amorphous carbon

“…10,30 Figure 5 is a schematic band diagram of a-C:H showing the locations of and bands. 11,31 We assumed that the and bands have a Gaussian form. As shown in a previous work, the capture centers in the band are deep states.…”

“…There are dangling-bond defects in a-Si:H, but there may be two kinds of defects in a-C:H. 10 As carbon may form both and bonding, it is possible to have both and defects in a-C:H. The defects are expected to be easily created under an external treatment because the bonding is weak. So, careful attention is needed to apply the models of a-Si:H to a-C:H. In spite of the differences between them, however, many of the basic properties of a-C:H, such as amorphous state, imperfect tetrahedral bonding, and the relaxation of structure by hydrogens, etc., are similar to those of a-Si:H. 11 So, we can borrow some concepts of a-Si:H to interpret the data of a-C:H.…”

The optical absorption edge of diamond-like carbon: A quantum well model

“…8 several observed phenomena, such as Raman spectra 9 and luminescence characteristics. 1 As well, there have been reports of direct microscopic observation of graphitic clusters, consisting of sixfold rings, in amorphous carbon. 10 If the predictions of the QW model are confirmed by experiment, this would provide additional support for the cluster model.…”

“…This absorption spectrum has been found to be unlike that of other amorphous semiconductors: the tail region is unusually broad; there are no clear regions of Tauc and Urbach behavior; and the apparent optical gap varies from practically zero to 5 eV, depending on the deposition parameters. 1 Although experimental spectra have been analyzed using the conventional Tauc and Urbach techniques, the parameters obtained have been inconsistent and have not had clear physical significance. It has been suggested that this problem is related to the unusual structural characteristics of the material, namely, the presence of both tetrahedral sp 3 bonding and trigonal sp 2 bonding.…”