Hydrogen transport and particle-induced hydrogen release in carbonization layers

The transport of hydrogen through amorphous hydrogenated carbon films (a-C:H) has been studied by investigating the molecular and plasma-driven permeation of hydrogen through an iron membrane which had been coated in-situ with a-C:H and a-C:D films of up to 100 nm thickness. A pronounced initial increase of the plasma-driven permeation flux compared with that of a clean iron membrane is observed during the early stage of the film deposition. This is attributed to a progressive decrease of the recombination rate σkr on the exposed surface of the membrane. At larger film thickness, d, the plasma-driven permeation rate decreases and is inversely proportional to d. The contribution of hydrogen isotopes incorporated during film deposition and that of the plasma species to the permeation is separated in measurements with a-C:H layers exposed to deuterium plasmas. The permeation from molecular driving pressures through the membrane is unaffected by the carbon film up to the largest thicknesses investigated (100 nm). The data are analyzed in the framework of a simple model assuming recombination of hydrogen atoms to molecules within their implantation range and rapid transport of H2 molecules through the film. Tentative values for the transport coefficient have been derived. (orig.)