4 Carbon Films

The transformations between one form of carbon and another can be classified as follows:

a) solid-solid transformations, as between diamond and graphite at points 'B' and 'C' in Fig. 5;

b) solid-solute-solid transformations as in the catalysed transformation of graphite to diamond at 'A' in Fig. 5;

c) solid-liquid-solid transformations; these occur when solid carbon phases are flash heated to temperatures above the melting line for the solid phase; some examples are described by Bundy et al [62];

d) solid-gas-solid transformations in which the product form of carbon is produced by condensation of gaseous carbon species produced by evaporation from the reactant form.

The last process is important in the production of carbon films. Gaseous carbon species can be produced from solid carbon phases by electric arc-induced evaporation, by other forms of plasma-assisted and laser-induced evaporation, and also by electron and ion beam sputtering. Metastable carbon films can be produced from the gaseous phase, particularly under conditions of rapid cooling or quenching. Transformation of the metastable forms to the thermodynamically stable form (i.e., graphite under ambient conditions) is kinetically limited by the high activation energy for the transformation. Some carbynoid forms of carbon, e.g., Carbolite [24], may fall into this category, as do amorphous carbon films (see below).

4.1 Amorphous carbon films

As noted above, amorphous carbon films can be produced from carbon-containing gas phases (physical vapour deposition, PVD). They can also be produced from hydrocarbon-containing gases (chemical vapour deposition, CVD). Both PVD and CVD processes can be thermally-activated or can be plasma- and/or electric field-assisted processes (e.g., microwave assisted CVD and ion beam deposition). As a consequence a wide range of processes have been developed to form amorphous carbon films and a correspondingly complex nomenclature has evolved [70, 71].

Amorphous carbon films may be broadly classified as: (i) amorphous carbon films, a-C films, deposited from carbon-containing gases with low or zero hydrogen content [72] and (ii) hydrogenated carbon films, a-C:H films, formed from hydrocarbon-containing gases [73,74]. Both types of film contain different amounts of sp2 and sp3 bonded carbon. The amount of sp2 bonded carbon can be estimated from X-ray absorption near edge spectroscopy,

XANES, and in a-C:H films the sp2:sp3 ratio can be measured directly using nmr spectroscopy [71].

The classification of amorphous carbon films according to carbon bond type and hydrogen content can be represented in a triangular diagram, Fig. 6 [e.g., 70], The corners at the base of the triangle correspond to graphite (100% sp2 carbon) and diamond (100% sp3 carbon). The apex represents 100% H, but the upper limit for formation of solid films is defined by the tie line between the compositions of polyethene, -(CH2)n-, and polyethyne, -(CH) n -.

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