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Hydrogenated amorphous carbon (a-C:H) films have optical and electrical properties that vary widely depending on deposition conditions, however, the electrical conduction mechanism, which is dependent on the film structure, has not yet been fully revealed. To understand the relationship between the film structure and electrical conduction mechanism, three types of a-C:H films were prepared and their film structures and electrical properties were evaluated. The sp2/(sp2 + sp3) ratios were measured by a near-edge X-ray absorption fine structure technique. From the conductivity–temperature relationship, variable-range hopping (VRH) conduction was shown to be the dominant conduction mechanism at low temperatures, and the electrical conduction mechanism changed at a transition temperature from VRH conduction to thermally activated band conduction. On the basis of structural analyses, a model of the microstructure of a-C:H that consists of sp2 and sp3-bonded carbon clusters, hydrogen atoms and dangling bonds was built. Furthermore, it is explained how several electrical conduction parameters are affected by the carrier transportation path among the clusters.

Diamond-like carbon (DLC) films were deposited using the pulsed plasma-enhanced chemical vapor deposition (PECVD) method with different source gases such as methane (CH4), acetylene (C2H2), acetylene with methane (C2H2:CH4), ethylene (C2H4), and ethylene with methane (C2H4:CH4). The chemical characteristics of these DLC films were investigated using suitable spectroscopic techniques. Raman spectroscopy analysis illustrated decrease of the ID/IG ratio with the addition of CH4. Importantly, X-ray photoelectron spectroscopy (XPS) and near-edge X-ray absorption fine structure (NEXAFS) spectroscopy not only enabled evaluation of the relative content of sp2 and sp3, but also clarified the chemical bonding of the films with different source gases. The XPS results showed lower sp2 content than the NEXAFS results. Moreover, glow discharge optical emission spectrometry (GD-OES) illustrated increase in the hydrogen content, which is in good agreement with increase of the sp3 content based on Raman, XPS, and NEXAFS analyses. Consequently, the synthesis and characterization indicate a significant role for differences of chemical composition of the films. The study described herein provides useful information for preparing high-quality DLC films, as well as for evaluating appropriately the distribution of the carbon sp2/(sp2 + sp3) ratio and the hydrogen content in the films with different source gases.

Hydrogenated amorphous carbon (a-C:H) films are only empirically known to be decomposed by heating, and this heat resistance is expected to be dependent on the structure of the film. To understand the decomposition processes and their uniformity, the thermal decomposition characteristics of two types of a-C:H film were investigated after the structural determination of the sp2/(sp2 + sp3) ratios and hydrogen contents. The mass of the gas released by thermal decomposition of the a-C:H films was detected by evaluation of the thermal desorption spectroscopy to determine the starting temperature the film with high hydrogen content started to desorb hydrogens at 400 °C and hydrocarbons at 750 °C. The film with low hydrogens started to desorb decomposition gases at 750 °C. Moreover, uniform decomposition of the films was observed using synchrotron-based X-ray photoemission electron microscopy (X-PEEM) and near edge X-ray absorption fine structure (NEXAFS) spectroscopy. The thermal decomposition of a-C:H films with uniform film structure proceeds relatively uniformly, and the mechanism of thermal decomposition of the films depends on the source materials of the films. The thermal decomposition proceeds uniformly at the micron scale when the initial structure is uniform.