INFLUENCE OF THE SELF-BIAS VOLTAGE ON THE MICROSTRUCTURE OF HYDROGENATED AMORPHOUS CARBON FILMS ELABORATED BY RF-PECVD
Keywords:
PECVD, a-C, H, FTIR, Raman spectroscopy, graphitizationAbstract
Hydrogenated amorphous carbon films (a-C:H) were deposited at room temperature in an rf-PECVD reactor using different DC self-biases (150 – 240 V). The microstructure of the films was then analysed by infrared transmission spectroscopy and Raman spectroscopy. The IR results show that hydrogen is bonded to both sp3C and sp2C atoms in all samples: more than 60% of bonded H is in sp3CHx groups (x = 2, 3) while only 13 to 17% is in sp2CH species in graphitic clusters (probably aromatic rings). The influence of self-bias voltage Vb on the microstructure of the a-C:Hfilms is also demonstrated by Raman scattering analysis. Indeed, a linear relationship between the ratio of the integrated intensity of the D peak over that of the G one (ID/IG) and the G peak FWHM (G) was found for films prepared at relatively high Vb values (Vb higher than -150 V). As the DC bias Vb increases, the ratio ID/IG increases while G becomes narrower. Also G is strongly affected by the graphitization of the material and hence the presence of graphitic clusters. The observed values of G (166 – 178 cm-1) and ID/IG (≤ 1) for our films suggest that the graphitic clusters sizes are less than 10 Å. Moreover, the relative low hydrogen content of these films, as shown by infrared transmission spectra, in addition to the former characteristics and to the values of band gap width, indicate that
our a-C:H films seem to exhibit properties which lie between those of polymeric and DLC films.
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