Microstructure And Properties Of Amorphous Carbon And Carbon Nitride Thin Films

Amorphous carbon (a-C) and carbon nitride (a-CNx) films have been extensively investigated due to their excellent properties, such as high hardness, wear resistance, controllable optical band gap and adjustable electrical conductivity. In literature, the reported results are contradictory regarding the effect of carbon hybridization states, disorder and N incorporation on the optical and electrical properties. In order to further investigate the microstructure and properties of the a-C and a-CNx films, we fabricated the films by DC facing-target reactive magnetron sputtering under different sputtering power (P) and nitrogen partial pressure (PN2), respectively. And the structure, defects, optical and electrical properties of the samples were investigated systematically.Sputtered a-C films are sp2-rich but highly disordered with a rather small sized aromatic sp2C clusters. Compared to the wide-gap insulators, e.g. ta-C and DLC, all the sputtered a-C films possess much narrower optical band gap and relatively higher conductivity. With increasing P, the high energy of incident C atoms causes the conversion of some previously formed C-sp3 bonds to C-sp2 bonds, so that the fraction of sp2C and the number (or the size) of sp2 clusters in the films increase. In general, this structure modification increases the defect density N s determined from the electron paramagnetic resonance (EPR). The optical gap widens from 0.11 to 0.70 eV as P increases, while the room-temperature conductivity decreases by two orders of magnitude. At low P, the structure disorder with more distorted bond-angles is the main reason for narrowed gap. The decreasing localization parameter N ( EF)?γ?3 for the a-C films deposited at high P reflects a smaller hopping conductivity below 300 K.With the increase of PN2, the fraction of CN bonded carbons (or the N content) increases primarily at the expense of the C-C bonded carbons and then reaches its saturated value. The incorporated N preferentially forms different kinds of non-aromatic CN phase, e. g. non-aromatic N–sp3C, C≡N and N–sp2C bonds. As PN2 increases, the changes of optical band gap, optical constants (n, k) and electrical conductivity are related to the enhanced localization of theπelectrons caused by the formation more non-aromatic CN phase. A positive linear correlation between the prefactorσ00 and the slope T0 1/4 was found in our a-CNx films, which indicates that the electrons occupying the localized band tail states contribute to the variable range hopping conductivity. EPR analysis indicates that with increasing PN2, the spin density N s increases and finally decreases at PN2=100%. It was also found that with the increase of PN2, the fraction of Gaussian component in the EPR spectra increases and the linewidth becomes narrower due to the exchange interaction of spin centers.