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An investigation of superconductor/semiconductor/superconductor Josephson junctions with separations between the Nb superconducting electrodes down to 20 nm is presented. To achieve such short distances, a preparation technique employing an anodic Nb oxide film as a spacer has been developed. The Nb electrodes are coupled through the quasi two-dimensional electron gas in the native inversion layer at the surface of p-type InAs. High characteristic voltages up to 1.35 mV at T=2 K are observed. A sensitive dependence of the magnitude of the characteristic voltage on interface quality is demonstrated. Good agreement is found between the experimental temperature dependence of the critical current and a theoretical model valid for short weak links.

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