Optical properties of disordered carbon-based materials

This paper gives an overall picture of the optical properties of various non-hydrogenated, hydrogenated and/or nitrogenated amorphous-carbon thin films. It shows how the density of electronic states (DOS) of such materials is related to the structural arrangement of the ions and to their hybridisation, a crucial role being played by the pi electrons. As a consequence of the peculiarities of the pi-DOS, the existence of features and trends of the dielectric constants, both in their real and imaginary parts, are demonstrated. The importance of the effects of overlapping and localisation on the wavefunction of pi states is addressed. As far as nitrogen effects are concerned, particular attention is devoted to discussing the role of electrons belonging to lone pairs in nitrogenated amorphous carbons. Finally, the role of the inhomogeneous microstructure of amorphous carbon-based materials is considered. Such a characteristic is reflected in the threshold from localised to percolating pi states, which is not featured by a clear mobility edge and a corresponding well-defined change in the DOS shape, such as in (homogeneous) amorphous silicon-based materials. Hence, estimation of the percolation threshold of carbon-based materials turns out to be not an easy problem. An optical model able to estimate the percolation threshold in amorphous carbon-based films from the analysis of optical properties is presented. (C) 2002 Elsevier Science B.V. All rights reserved.