Correlation between luminescence properties and microstructure of hydrogenated amorphous carbon films investigated by X-ray diffraction and infrared ellipsometry

Plasma deposited polymer-like amorphous carbon (PLC) films show a strong ph otoluminescence (PL) in the visible range at room temperature. However, the origin of the luminescence in terms of microstructure is poorly understood . In this study, by varying the plasma conditions, hydrogenated carbon (a-C :H) films, ranging from soft and transparent to denser and more absorbent m aterials, have been grown. Steady-state PL measurements reveal a steep quen ching of the luminescence intensity when film density is higher than 1.3 g cm(-3). Using in situ infrared ellipsometry, C-H bonding analysis shows tha t this quenching corresponds to a strong decrease of the sp(3) CHn=2,3 grou p concentrations, consistent with a network crosslinking and to an enrichme nt of the methyl group environment in pi bonds. In correlation, the evoluti on of the carbon skeleton has been studied using X-ray diffraction measurem ents. Structure factors and pair correlation functions evidence clear diffe rences in the network structure, in particular a shortening of the carbon-c arbon bond when density increases. (C) 1999 Elsevier Science Ltd. All right s reserved.