Critical temperature of thin niobium films on heavily doped silicon.

Abstract

Measurements of the critical temperature are presented for 20--40 nm niobium films on heavily doped n- and p-type silicon, for a wide range of silicon doping concentrations and sample preparation methods. Contrary to a normal-metal--superconductor bilayer, a depression of the critical temperature does not occur in the semiconductor-superconductor system. We show the absence of a depression to be consistent with conventional theories for the proximity effect between a superconductor and a normal metal. It is due to the large mismatch in material parameters between the two layers.