Semiconductor-superconductor transitions in Nb/Al structures

We report here the first experimental evidence of semiconductor-superconductor transitions in relatively thick Nb/Al structures. The temperature dependence of resistance R(T) always shows a sharp superconducting transition followed by either a normal metallic R(T) behavior in low-resistivity samples, or a semiconductor-like behavior when the sample resistivity at 10 K is above 100 mu Omega cm. We discuss here the fabrication conditions and the electron localization regime associated with the measured R(T) dependence.