Superconducting Properties of Amorphous Tantalum Thin Films

We investigate superconducting properties of tantalum thin films. The films are deposited at a rate of 0.1nm/sec on quartz substrates by dc sputtering with an argon pressure of 4 mtorr. The samples are patterned to a Hall bar geometry with a shadow mask, and contacts are made with an indium press via gold pads that are pre-deposited on the substrates before the tantalum deposition. We found that with decreasing film thickness, the superconducting transition temperature ($T_c )$ of these films continuously decreases towards 0K. This is characteristic of the amorphous structure of the superconducting thin films, and in strong contrast to granular superconducting thin films where $T_c $ is nearly independent of film thickness. We also found that an exposure of these films to air at room temperature (at least for several months) does not cause any noticeable change in their superconducting properties. We present our results on superconducting properties measurements and structural studies on these amorphous tantalum films.