Comparative study on two all-refractory Josephson tunnel junctions: Nb/AlOx/Nb and Nb/NbN/MgO/NbN/Nb

Two types of all refractory Josephson tunnel junctions have been studied: Nb/AlOx/Nb and NbN/MgO/NbN. Conductance-voltage measurements and subgap current analyses have yielded information about the barrier shape and homogeneity of the oxide layers in both types of junctions. The AlOx in SNINS Nb/AlOx/Nb junctions is intrinsically homogeneous, which results in a subgap current dominated by temperature dependent single-particle tunneling at 4.2 K. Barrier inhomogeneities which are probably introduced during the trilayer anodization cause excess tunneling currents. The MgO barrier in NbN/MgO/NbN junctions is inhomogeneous. Due to this, the subgap current at 4.2 K is dominated by temperature independent tunneling currents possibly in combination with leakage currents. The barrier height of the artificial MgO is significantly lower than in thermally oxidized Mg. During fabrication, reactions of deposited MgO with residual gasses affect the barrier properties considerably. It is suggested that residual H2O reacts with MgO during and after deposition. (orig.)