First-principles study of structural and opto-electronic characteristics of ultra-thin amorphous carbon films

Most amorphous carbon (a-C) applications require films with ultra-thin thicknesses; however, the electronic structure and opto-electronic characteristics of such films remain unclear so far. To address this issue, we developed a theoretical model based on the density functional theory and molecular dynamic simulations, in order to calculate the electronic structure and opto-electronic characteristics of the ultra-thin a-C films at different densities and temperatures. Temperature was found to have a weak influence over the resulting electronic structure and opto-electronic characteristics, whereas density had a significant influence on these aspects. The volume fraction of sp3 bonding increased with density, whereas that of sp2 bonding initially increased, reached a peak value of 2.52 g/cm3, and then decreased rapidly. Moreover, the extinction coefficients of the ultra-thin a-C films were found to be density-sensitive in the long-wavelength regime. This implies that switching the volume ratio of sp2 to sp3 bonding can effectively alter the transmittances of ultra-thin a-C films, and this can serve as a novel approach toward photonic memory applications. Nevertheless, the electrical resistivity of the ultra-thin a-C films appeared independent of temperature. This implicitly indicates that the electrical switching behavior of a-C films previously utilized for non-volatile storage applications is likely due to an electrically induced effect and not a purely thermal consequence.