Formation and behavior of unbonded hydrogen in a-C:H of various compositions and densities

It has previously been noted that two types of hydrogen exist in hydrogenated amorphous carbon (a-C:H) prepared by chemical vapor deposition techniques: H bonded to C atoms and unbonded H2 molecules (up to 80% of all H). Little is known on the atomic-scale processes during the formation of the structures containing unbonded hydrogen, and on the effect of unbonded H2 on the structure-based material characteristics. In the present work, we report ab-initio molecular dynamics simulations of low-density H-rich a-C:H materials (H content 25–50%). Using liquid-quench simulations, we investigate the dependence of both (1) H2 formation from H atoms, and (2) survival of H2, present in the gas phase, on the formation conditions of the material. We further focus on the dependence of the characteristics of carbon networks on material composition, density and unbonded H2 content, and on the temperature dependence of H2 formation and dissociation in the networks. The results provide insight into the role of unbonded H2 in a-C:H formation, and explain relationships between experimental conditions and materials characteristics. In particular, the results predict and describe the dependence of (1) unbonded hydrogen fraction on microwave/radiofrequency character of the discharge, ion bombardment characteristics or deposition temperature, and (2) the coordination of carbon atoms on H content, unbonded H2 fraction or material density. For a given composition, the results predict the independence of the H2 content on material density in a wide range, due to a high volume of voids in low-density a-C:H materials.