Hydrogenated amorphous carbons (a-C:H) were prepared from mixtures of propane, methane, argon and hydrogen by plasma chemical vapor deposition (PCVD) processes. In order to study the interrelation between deposition mechanisms and characteristics, the hydrogen bond configurations with deposition conditions were investigated by means of FTIR (Fourier transform infrared) spectroscopy. Thermal analysis by using GC (gas chromatography), DSC (differential scanning calorimetry) and TG-DTA(thermogravimetry-differential thermal analyser) was done to obtain the structural information and to investigate the gas evolution mechanisms. The optical band gap was measured.
Bond configurations of hydrogen, of importance in determining properties, are strongly dependent on the preparation methods and depositions. But, systematic studies on the structural changes with deposition were few to understand the deposition mechanisms. Their structural descriptions, reported by previous researchers, have been restricted to the bond sites of hydrogen. A comparative method about the stretch and deformation vibration modes hydrogen has not yet ben carried. In order to understand hydrogen bond structures, however, it is necessary to know not only bond sites but also vibration modes, and their changes with deposition conditions. In this study the hydrogen bond structures of a-C:H from different gas systems were systematically investigated with the changes of rf power and substrate temperature.
Some reported that hydrogen bonded to both $SP^3$ and olefinic $SP^2$ sites with similar probabilities in a-C:H, but some results showed that hydrogen preferentially bonded to $SP^3$ sites. It is concluded that the preferential bond sites of hydrogen have relation to the hydrocarbon gases used. Deposition mechanisms are discussed to explain the variations of bond sites and vibration modes.
The experimental results showed that at given deposition conditions the deformation vibration was dominantly observed in hydrogen-methane system and the stretch vibration was dominantly observed in argon-propane system. This is considered to be the first report for the C-H bond structure analysis. It is interpreted by the interaction between gases and surfaces. The momentum transfer of depositing species arriving at surface will be high in hydrogen-methane system due to low collisional cross-section and low mass of hydrogen. Large momentum transfer to the surface causes the distortion of a-C:H network and results in the deformed vibration modes of hydrogen.
In this study hydrogens were dominantly bonded to $SP^3$ carbons when using propane and methane. This is consistent with the previous results using saturated hydrocarbon. The reported structure of a-C:H obtained from other hydrocarbons (unsaturated or aromatic) always showed both $SP^{3_-}$ and $SP^{2_-}$ bonded hydrogens. Therefore the hydrocarbon structure is an important factor to determine the bond site of hydrogen in a-C:H network.
The structural changes during heat treatment were studied. Many researchers observed the decrease of hydrogen content upon heating by infrared spectroscopy and proposed hydrogen evolution processes. But a recent investigation by mass spectroscopy showed that hydrocarbon evolution was dominant below about 600°C. Therefore the gas evolution at low temperature should be interpreted in terms of hydrocarbon evolution. But it has not yet been developed. The gas evolution spectra, calorimetric measurement, and some results on the thermal analysis are given here.
The experimental results showed that the hydrocarbon evolution was followed by large weight loss without discernable thickness reduction of a-C:H films. This implies that hydrocarbons are formed in inner region of the deposit and effuse out by molecular transport. The effusion path is considered as the interconnected micropores formed from the surface. Calorimetric measurement showed that the evolution is thermal activation process. The formation of hydrocarbons from C-CnHm pairs in a-C:H network requires a range of thermal activation energies due to interaction distance distribution.
a-C:H films are useful to optical coatings due to their optical transparency. Under given deposition conditions a-C:H from argon-propane system showed high optical band gap up to 3.0 eV. For all gas systems high optical band gap was obtained at low substrate temperature and low rf power conditions. This may be due to carbon bond termination by $CH_3$ components. For the proper definition of optical band gap in amorphous materials, it is suggested that a parameter for the distribution of density of states must be stated. This can be done by choosing correct n value in general relation $(αhv)^{1/n}=B$ (hv-Eo).
