Superconducting characteristics and the thermal stability of tungsten-based amorphous thin films

Superconducting characteristics and the thermal stability of sputtered, tungsten-based, amorphous thin films are investigated. Electronic properties and crystalline structures are analyzed as a function of the metalloid content in the films. It is well known that the superconducting T_c of a bulk crystalline tungsten is 0.01 K, which is one of the lowest transition temperatures among the superconducting metals. We have found that the W film containing 5 to 70 at. % metalloids exhibits a great enhancement in T_c. In the region of 15 to 35 at. % metalloids, the T_c shows the maximum of 5.0 K, and the transition from normal to superconducting state occurs very sharply. SEM observation together with x-ray diffraction analysis indicates that these films are amorphous in structure. The electrical resistivity is about 150 μΩ-cm, and shows little temperature dependence from T_c to 300 K. In addition, the W–Si amorphous superconductor is thermally very stable after annealing at 700 °C, but the W–Ge amorphous alloy crystallizes at 600 °C.