Relationship between 1/f noise parameter and Andreev reflection probability of epitaxial NbN/AlN or MgO/NbN tunnel junctions

We have investigated 1/f noise properties of epitaxial NbN/AlN or MgO/NbN tunnel junctions grown on single-crystal MgO(1 0 0) substrates. The 1/f noise parameter η for the NbN/AlN/NbN junctions greatly increased with decreasing the critical current density Jc, i.e. with increasing the barrier thickness. The result may be associated to the imperfection of epitaxial growth of the thick AlN barrier. On the other hand, for the NbN/MgO/NbN junctions the η values did not depend on the Jc. The logη-A(E) curve was well fitted by the following equation: logη∝aA(E)-5/{exp(bA(E)-1)-1}, where a and b are fitting parameters, and A(E) is Andreev reflection probability. This behavior is very interesting for the development of low noise NbN-based devices