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

摘要

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 sp~2-clusters and sp~3 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 (C_2H_2, N_2) or (C_2H_2, O_2), the decrease in the alloy density deduced from Nuclear Reaction Analysis and optical measurements, and the better ordering of the sp -hybridized carbon phase evidenced by Raman studies. Opposite variations in the ohmic conductivity σ(T), apparent activation energy E_(ACT) and hopping transport parameters (related to the localization parameter N(E_F)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 sp~2 phase shown by Raman spectra in both alloys (stronger in a-C_(1-x)N_x:H with a large influence of aromatic clusters) and the increasing polymer-like character of the sp~3 matrix, dominant in a-C_(1-x)O_xH films where the weaker connectivity of the sp~3 matrix is attributed to two-fold bonded oxygen atoms C-O-C and terminating C=O groups. The role of π and π~* states localization is discussed in the context of bandtail hopping conductivity.