Characterization of the Structure of Carbon Material Through the sp 3 /sp 2 Bonding Ratio Measurements
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CHARACTERIZATION OF THE STRUCTURE OF CARBON MATERIAL THROUGH THE sp 3/sp 2 BONDING RATIO MEASUREMENTS F. Demichelis, C. De Martino, C.F. Pirri and A. Tagliaferro Dipartimento di Fisica Politecnico, Torino (Italy)
ABSTRACT Hydrogenated amorphous carbon (a-C:H) films have been deposited by sputter assisted plasma chemical vapor deposition (CVD). The relative concentration of sp3 and sp2 hybridized carbon in samples is determined by infrared (IR) and nuclear magnetic resonance (NMR) spectroscopies and by a new method through the complex dielectric constant deduced from optical transmittance and reflectance. The results are compared and discussed.
INTRODUCTION The films of hydrogenated amorphous carbon (a-C:H) and diamond-like carbon (DLC) have recently received considerable attention because of their mechanical, optical and electrical properties of technological interest [1]. They are indeed mechanically hard, optical transparent over a wide spectral range, highly electrically resistive and chemically inert to both acids and alkalis. There is, however, a great variety of diamond, graphite and hydrocarbon polymers films according to the varying portion of hydrogen, sp3 carbon and sp2 carbon. The hydrogen content and the fraction of sp3 and sp2 carbon sites are the most important parameters which individualize the structure of the films. The hydrogen content can be estimated from nuclear measurements and from3 the2 intensities of C-H infrared bond stretching modes. The evaluation of the sp /sp ratio is more complicated and many different techniques have been applied to get quantitative data on carbon binding. The relative concentration of sp3 and sp2 hybridized carbon can be obtained by nuclear magnetic resonance (NMR) [1-4], electron energy loss spectroscopy (EELS) [5,6], IR spectroscopy [7] and by optical measurements [8,9]. Some of those techniques are felt to be reliable, while others impose limitations to the measurements, so further consideration is needed. In this paper we report the results on the determination of sp3/sp2 ratio for a-C:H films measured with different techniques. We discuss and compare the results and we analyze the approximations and the validity of different methods. Until now a comparison between the values of sp3/sp 2 as obtained by different techniques has not been reported in literature. The reason could be that it is difficult to compare the results obtained by the different techniques because the characterizations are performed on different films.
Mat. Res. Soc. Symp. Proc. Vol. 270. @1992 Materials Research Society
488
EXPERIMENTAL Films of a-C:H were deposited by means of a sputter assisted plasma chemical vapor deposition (CVD) system. A 99.999% purity graphite target was sputtered in an Ar + H, atmosphere. The RF power was in the range 250 - 300 W, the pressure 2 -4 Pa, the Ar flow rates were in the range 80 - 100 sccm, the H, flow rates 0.9 4 sccm and the temperature of the substrate was around 80 °C due to the self-heating during deposition. Optical measurements were recorded
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