Microstructure, optical and electrical properties of nitrogen doped sp2-rich a-C thin films grown by facing-target magnetron sputtering

N atoms were incorporated into sp2-rich a-C networks using DC facing-target reactive sputtering at various N2 fraction (PN2) and their structure and opto-electrical properties were investigated systematically. As PN2 increases, the fraction of CN bonded carbons (or the N content) increases primarily at the expense of the CC bonded carbons and then reaches its saturated value at PN2 > 40%. The incorporated N preferentially forms different kinds of non-aromatic CN phase, leading to more localization of π electrons and the loss of the connectivity of nanographite fragments in the films, which is different from the case in N-doped sp3-rich a-CNx films. Hence, with increasing PN2, the a-C(:N) film converts from a semiconductor with a narrower optical band gap to an insulator-like material with a wider gap. Additionally, the variation of optical constants (n and k) and spin defects are related to the enhancement of the non-aromatic CN phase.