Sputtering pressure dependence of hydrogen-sensing effect of palladium films

The electrical resistivity ρ of palladium (Pd) films prepared by using magnetron sputtering at different pressures φ ranging from 2 to 15 mTorr showed very different hydrogen (H)-induced response. This reaction is because the mean free path of the particles in vacuum changes substantially with φ, such that the structure of the deposits is altered accordingly. A film prepared at a moderate φ value of 6 mTorr has a moderate strength. After a few hydrogenation-dehydrogenation cycles, some cracks are generated because of the great difference in the specific volumes of the metal and hydride phases. Breathing of the cracks in subsequent switching cycles occurred, which led to the response gain of ρ, defined as the resistivity ratio of the dehydrogenated-to-hydrogenated states during a cycle, to increase to 17. This result demonstrates the attractiveness of using the Pd films in H₂ detection application. The H-induced resistive response of the films prepared at other φ values was found to be much smaller.