Effects of the C2H2/(C2H2 + NH3) ratio on the properties of thermal chemical vapor deposition (CVD) carbon films are investigated. Experimental results indicate that the deposition rate of carbon films increases with increase of the C2H2/(C2H2 + NH3) ratio. Ten NH3 molecules will suppress about one C2H2 molecule to form carbon films, and the deposition rate of carbon films is proportional to the partial pressure of acetylene with a power of fourth order. A model description of the kinetics of this thermal CVD process is proposed. Moreover, the ordering degree, nano-crystallite size, and the number of sp2 carbon sites decrease with increase of the C2H2/(C2H2 + NH3) ratio, while the electrical resistivity increases. Few nitrogen atoms and no hydrogen are incorporated in the carbon films. When the carbon films are very thin, the carbon atoms would be uniformly deposited on a silica glass plate which results in surface smoothening. Nevertheless, when the carbon films are getting thicker, the surface roughness increases. As the C2H2/(C2H2 + NH3) ratio increases, the decrease of the water contact angle on a carbon film surface is mainly resulting from the decrease of the number of sp2 carbon sites in the carbon films. The deposition temperature and working pressure of thermal CVD carbon films using the C2H2/N2 or C2H2/NH3 mixtures are smaller in comparison to those using a CH4/NH3 mixture. Usually, when the deposition temperature is above 1073 K, NH3 is decomposed into NH2, which would suppress the growth of carbon films. However, as the deposition temperature is below 1073 K, NH3 serves as the dilute gas in the described processes
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