Optical and electronic properties of plasma-deposited hydrogenated amorphous carbon nitride and carbon oxide films

A comparative investigation of the nanostructural vs. electronic properties of DECR plasma deposited carbon nitride and carbon oxide alloys provides new insights in the changes of the sp2-clusters and sp3 matrix resulting from the decrease in the average coordination number, as N and O atoms are increasingly incorporated. This review emphasizes the similar incorporation efficiency of N and O atoms in the carbon matrix as a function of the gas phase mixture (C2H2, N2) or (C2H2, O2), the decrease in the alloy density deduced from Nuclear Reaction Analysis and optical measurements, and the better ordering of the sp2-hybridized carbon phase evidenced by Raman studies. Opposite variations in the ohmic conductivity σ(T), apparent activation energy EACT and hopping transport parameters (related to the localization parameter N(EF)γ−3) are found as a function of the (N/N+C) and (O/O+C) stoichiometries. This surprising result gives evidence of some competition between the improved ordering of the sp2 phase shown by Raman spectra in both alloys (stronger in a-C1−xNx:H with a large influence of aromatic clusters) and the increasing polymer-like character of the sp3 matrix, dominant in a-C1−xOx:H films where the weaker connectivity of the sp3 matrix is attributed to two-fold bonded oxygen atoms C-O-C and terminating CO groups. The role of π  and π* states localization is discussed in the context of bandtail hopping conductivity.