Ultrathin undoped tetrahedral amorphous carbon films: The role of the underlying titanium layer on the electronic structure

Abstract

This article continued our research on physical characterisation (Raman spectroscopy, scanning tunnelling spectroscopy (STS), conductive-atomic force microscopy (AFM)) of electrode materials based on bilayers consisting of an underlying titanium layer and an ultrathin undoped tetrahedral amorphous carbon (ta-C) layer. The influence of the quality of the underlying Ti layer and the Ti/ta-C interface on the ta-C bonding structure, electronic and electrical properties of the whole bilayer was considered. It was found that the smoother the substrate surface was, the better quality ta-C was deposited with higher sp3 fraction according to Raman spectroscopy results and, as sequence, with the wider mobility band gap and the higher specific resistance. The presence of either the air-formed native titanium oxide or the titanium nitride layers at the Ti/ta-C interface did not lead to the change of the ta-C bonding structure according to Raman spectroscopy results. However it caused the widening of the “apparent” mobility band gap measured by STS, the sharp increase in the specific resistance and the change of the conduction mechanism from space-charge-limited current (SCLC) defined by monoenergetic traps to SCLC with exponentially distributed shallower traps and lower mobility of free charge carriers (conductive-AFM) in comparison to the sample without mentioned interlayers. The fact that Raman spectroscopy did not show the change of the ta-C bonding structure connected with the band gap value, whereas STS results did show the band gap widening, allowed to conclude that the DOS of the interlayer could have the influence on the resulting STS spectrum. The same effect, one could see with the specific resistance alteration and the change of the conduction mechanism, which both caused by characteristics of the interlayer.