HIGH-QUALITY NB/ALOX-AL/NB JOSEPHSON-JUNCTIONS WITH GAP VOLTAGE OF 2.95-MV

The dependence of the gap voltage on impurities in Nb films is studied. The gap voltage is determined by the number of impurities included in the films. The impurities arise from the residual gas in the sputtering chamber before the deposition of Nb films. By reducing the impurities in the films, we obtain high-quality Nb/AlO(x)-Al/Nb Josephson junctions having the gap voltage of 2.95 mV which is close to that estimated from the energy gap of Nb bulk at 4.2 K. In addition, the use of a thin Al layer is also an important factor in obtaining excellent current-voltage characteristics.