Pressure dependence of hydrogen uptake kinetics in thin metal films

Abstract

The characteristic equilibration rate for hydrogen uptake in a metal film is shown to go through a maximum at a pressure well below that for hydride formation. This is brought about by non-negligible change of bulk hydrogen site occupancy with pressure once the surface chemisorption sites have been saturated. The rate will increase again after going through a minimum if there is significant hydrogen in impurity sites which can saturate or if there is dissociation at weakly bound subsurface or alternate surface sites. In the latter case the degree and form of nonmonotonicity is strongly dependent on the sticking coefficient of the weak sites, and thus can provide information about their nature. Appreciable hydrogen at impurity sites or in grain boundaries can greatly limit the equilibration rate and lower the pressure beyond which it decreases. Results are in qualitative agreement with recent data on Pd-SiO2 films in metal-oxide-semiconductor devices. Quantitative agreement with the long-time behavior is possible if the polycrystalline films had a large amount of hydrogen in grain boundaries.