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(100) substrates. The 1/f noise parameter eta for the NbN/AlN/NbN junctions greatly increased with decreasing the critical current density J(c), 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 eta values did not depend on the J(c). The log eta-A (E) curve was well fitted by the following equation: log eta proportional to 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. (C) 2004 Elsevier B.V. All rights reserved.