Location of the minimum of the differential tunneling resistance R(V) in a superconductor-degenerate semiconductor Schottky contact

Measurements of differential resistance in a superconductor-degenerate semiconductor junction Nb-n++GaAs at T=1.6K show close similarity to those for a conventional superconductor-insulator-normal metal junction, except for the position of the minimum which is located at 3.6 meV. Using a simple model for the charge screening at the Schottky barrier, we give an argument why this minimum is by far displaced with respect to the superconducting gap energy (delta=1.5meV for bulk Nb). We argue that a rebuilding of the density of states takes place at the barrier, due to the imperfect metal screening in the degenerate semiconductor. Energy states close to the degenerate semiconductor Fermi energy are depleted at the barrier and are not available for tunneling, up to an energy Eg which adds to the superconducting gap.