Optical properties of amorphous carbons and Amorphous carbon nitrides
G. Fanchini, A. Tagliaferro
Dip. Fisica & Unità INFM, Politecnico di Torino, Torino, Italy
In this presentation, we will review the current assessment about the density of electronic states (DOS) in amorphous carbons, either hydrogenated (a-C:H) or not (a-C) and discuss its role in determining the optical constants (refractive indexes n(E), extinction coefficients k(E), real and imaginary parts of the complex dielectric constant, e1(E) + i·e2(E)) in the visible-near UV photon energy range (E=1-5 eV). A-C(:H) films differ from amorphous silicon (a-Si:H) ones as they involve a significant contribution of p-electrons, arising from the sp2 (and, possibly, sp1) hybridised carbon sites.
We will discuss how the density of p-states is affected by the organisation of the sp2 carbon sites in nano-clusters with different electronic structure and gap states, and by the statistical distribution of nano-clusters of different shape and size and the indirect effect of the sp3 (diamond-like) backbone in determining the rigidity and the amount of disorder. In a-C(:H) parameters such as the Tauc optical gap and the Urbach energy bear only a conventional meaning, since they are not only controlled by tail properties (such as in a-Si:H) or by the shape and size of a typical, isolated, sp2 cluster (such as in an hypothetical ‘cluster model ’ of a-C:H). The optical properties of a-C:H can be better understood in terms of mutual interdependency relationships between causally related optical constants (e.g n(E) and k(E), or e1(E) and e2 (E)) of different films, occurring at each given photon energy E .
Nitrogen addition in amorphous carbons not only leads to larger amounts of p-electrons, but it is often accompanied by strongly localised lone-pair (LP) electrons. LP states are stabilised in energy by the local structure of amorphous materials. Thus nitrogen not only affects the density and the nature of the states determining the optical properties but also their spatial extension and it may introduce strong changes in the DOS . We will address the very important role of lone-pairs and LP-p mixing in determining the DOS and the interdependency relationships between the optical constants in amorphous carbon nitrides, to be compared with the corresponding relationships between e1(E) and e2(E)and existing in non-nitrogenated amorphous carbons. A picture of the DOS of amorphous carbon nitrides in the visible-near UV energy range will be given.
Finally we will discuss some applications of the above mentioned concepts to the study of the polarisability of the electronic states and the photoluminescence of amorphous carbons, either nitrogenated or not.
 G. Fanchini et al., Phys. Rev. B 61 (2000) 5002
 G. Fanchini et al., Phys. Rev. B 66 (2002) 195415