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.
Thin-film transistors incorporating a hot-wire chemical-vapor-deposited silicon layer have been shown to exhibit superior electronic stability as compared to glow-discharge-deposited amorphous silicon devices. Hot-wire-deposited silicon films of various thicknesses (37–370 nm) on silicon dioxide were investigated. The films are structurally inhomogeneous. Raman measurements and transmission electron microscopy show that isolated cone-shaped crystals grow within a primarily amorphous layer. The amorphous interface region has a low hydrogen content of 2.0±0.2 at. %, while the films exhibit an enhanced hydrogen concentration in the surface region. The bond-angle distribution in the amorphous phase is comparable to that of device-quality glow-discharge-deposited amorphous silicon.
We used Fourier Transform Infra-Red (FTIR) analysis of bi-layers of plasma-grown hy-drogenated amorphous silicon-carbide films to investigate the role of the material structure in the hydrogen diffusion process. In the bi-layers one layer was deposited using CH4/SiH4 and in the other layer CD4/SiD4 was applied. The carbon concentration was 20 at.%. In previous work we showed, using Elastic Recoil Detection (ERD) and Thermal Desorption Spectrometry (TDS), that the hydrogen moves molecular through these films in the temperature range 325 < T < 450 °C [1]. Using FTIR we obtained information about the number of Si-H and Si-D bonds and their change upon annealing. The FTIR data indicate a structural change during annealing. A comparison with the TDS spectra led us to the conclusion that at higher temperatures the out-diffusion of hydrogen stops because of the hindrance of the molecular transport.
In low temperature processed amorphous silicon and its alloys like silicon nitride and carbide the presence and the behavior of hydrogen is of utmost importance for the applicability of these materiall!: Besides to beneficially passivate Si dangling bond defects, hydrogen increases the reactivity of these materials and is a cause for instability. In this review we discuss examples of the study of the migration of single and molecular hydrogen inside the material as distinct steps in the process of effusion of H from the plasma deposited films. Also examples of the reverse process i.e. the uptake of hydrogen during a post deposition hydrogen treatment or during the deposition itself are presented.In these studies fruitfully use is made of (bi)layer structures in which hydrogenated and deuterated materials are spatially separated. We apply -MeV ion beam t. echniques to measure concentration depth profiles of Hand D. The high energy ion beams are also used to provoke molecule formation in the material, enabling in a novel manner the study of the permeability of thin films for H2 and D2 at relatively low temperature.
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