Electron delocalization in amorphous carbon by ion implantation

The electrical properties of amorphous carbon are governed by the high localization of the sp π states, and conventional methods of altering the sp content result in macroscopic graphitization. By using ion beams we have achieved a delocalization of the π states by introducing nanoclustering and hence improving the connectivity between existing clusters, as demonstrated by the increase in the conductivity by two orders of magnitude without modification of the band gap. At higher doses, paramagnetic relaxation-time measurements indicate that exchange effects are present. This unveils the possibility of amorphous carbon-based electronics by tailoring the ion-beam conditions, which we demonstrate in the form of a rectifying device

An EPR study at X- and W-band of defects in a-C:H films in the temperature range 5-300K

Electron paramagnetic resonance (EPR) measurements have been made at X-band (approximate to 9.5 GHz) and W-band (approximate to 95 GHz) of a-C:H films on (1 0 0) silicon substrates; the sample temperature was varied in the range 5-300 K. Two types of film were examined. The first type are amorphous hydrogenated carbon (a-C:H) films grown by plasma enhanced chemical vapour deposition (PECVD) with negative self bias voltages in the approximate range 100-500 V. The second type were initially highly polymeric-like a-C:H films grown on Si placed on the earthed electrode of a PECVD system but were subsequently implanted with either 6 X 10(15) cm(-2) B+ or 2 X 10(16) cm(-2) B+ ions. At both X- and W-band and throughout the temperature range 5-300 K the EPR signal of the carbon unpaired electrons consists of a single symmetric line with g = 2.0026 +/- 0.0002. As the temperature is lowered, several samples develop a dependence on sample orientation of the external field required for resonance. This anisotropy is explained in terms of the demagnetising fields more usually encountered in ferromagnetic resonance. (C) 2003 Elsevier Science B.V. All rights reserved.</p