Irradiation of hydrogenated amorphous silicon and its alloys with MeV ions causes the formation of hydrogen molecules inside the material in the region where they were originally bonded. In this paper this effect is experimentally detected by making use of double-layer structures consisting of plasma-deposited hydrogenated and deuterated amorphous silicon-carbon (a-Si 1Ϫx C x :H) alloys, with various carbon contents. At the same time we use this effect to study the low-temperature transport of hydrogen molecules through this class of materials. We deduce that at temperatures below the temperature of the onset of thermal desorption, hydrogen molecules can migrate through material having xϭ0.2, and escape into the ambient. For smaller values of x the formation of molecules inside the material eventually results in morphological damage of the films. The results indicate that in the process of hydrogen loss from the material during annealing at lower temperatures, the rate-limiting step is the formation of molecules inside the material. The observation of an isotope effect in the extent of the ion-beam-induced hydrogen desorption leads to a modification of an existing model for this process.
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