Temperature-dependent properties of niobium nitride Josephson tunnel junctions

Abstract

We report the electrical properties of all niobium nitride(NbN) Josephson tunnel junctions with magnesium oxide(MgO) films as barriers in the temperature range 4.2-15 K. NbN/MgO/NbN Josephson tunnel junctions have large gap voltages(V <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">g</inf> =4.8-5.4 mV), large products of the maximum critical currents and the normal tunneling resistances(I <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</inf> R <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">n</inf> =3.15-3.35 mV), and small subgap leakage currents(V <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">m</inf> =13-138 mV) at 4.2 K. The junction parameters decreased with increase in temperature, but reasonably large junction parameters(for example, V <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">g</inf> =4.9 mV, I <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</inf> R <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">n</inf> =2.67 mV, and V <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">m</inf> =22 mV) have been obtained even at 9 K. The magnetic penetration depth in NbN electrodes has been measured up to 15 K using two-junction interferometers which consist of NbN/MgO/NbN junctions, NbN ground planes, and NbN control lines.