Influence of short coherence length on the superconducting proximity effect of silicon-coupled junctions

Superconducting critical currents of NbN-Si-NbN and Nb-Si-Nb junctions with a coplanar structure are measured as a function of temperature and of the spacing between superconducting electrodes. The current decreases exponentially with increasing temperature above 4.2 K and with increasing spacing. It is found that the coherence length in the semiconductor is determined only by the physical properties of the semiconductor, even though the superconducting electrode with a short coherence length such as NbN is used. Moreover, the measured coherence length in Si agrees with the value derived from the model of Seto and Van Duzer [J. Low Temp. Phys. LT-13, 323 (1972)].