The effect of the CH4 level on the morphology, microstructure, phase purity and electrochemical properties of carbon films deposited by microwave-assisted CVD from Ar-rich source gas mixtures

Carbon thin films were deposited on Si substrates by microwave-assisted chemical vapor deposition (CVD) using variable CH4 levels in an Ar/H2 (Ar-rich) source gas mixture. The relationship between the CH4 concentration (0.5 to 3 vol.%) in the source gas and the resulting film morphology, microstructure, phase purity and electrochemical behavior was investigated. The H2 level was maintained constant at 5% while the Ar level ranged from 92 to 94.5%. The films used in the electrochemical measurements were boron-doped with 2 ppm B2H6 while those used in the structural studies were undoped. Boron doping at this level had no detectable effect on the film morphology or microstructure. Relatively smooth ultrananocrystalline diamond (UNCD) thin films, with a nominal grain size of ca. 15 nm, were only formed at a CH4 concentration of 1%. At the lower CH4 concentration (0.5%), faceted microcrystalline diamond was the predominant phase formed with a grain size of ca. 0.5 m. At the higher CH4 concentration (2%), a diamond-like carbon film was produced with mixture of sp2-bonded carbon and UNCD. Finally, the film grown with 3% CH4 was essentially nanocrystalline graphite. The characteristic voltammetric features of high quality diamond (low and featureless voltammetric background current, wide potential window, and weak molecular adsorption) were observed for the film grown with 1% CH4, not the films' grown with higher CH4 levels. The C2 dimer level in the source gas was monitored using the Swan band optical emission intensity at 516 nm. The emission intensity and the film growth rate both increased with the CH4 concentration in the source gas, consistent with the dimer being involved in the film growth. Importantly, C2 appears to be involved in the growth of the different carbon microstructures including microcrystalline and ultrananocrystalline diamond, amorphous or diamond-like carbon, and nanocrystalline graphite. In summary, the morphology, microstructure, phase purity and electrochemical properties of the carbon films formed varied significantly over a narrow range of CH4 concentrations in the Ar-rich source gas. The results have important implications for the formation of UNCD from Ar-rich source gas mixtures, and its application in electrochemistry. Characterization data by scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD), visible-Raman spectroscopy and electrochemical methods are presented.