NbN Josephson Junctions for Single-Flux-Quantum Circuits

Hiroyuki AKAIKE  Naoto NAITO  Yuki NAGAI  Akira FUJIMAKI  

IEICE TRANSACTIONS on Electronics   Vol.E94-C   No.3   pp.301-306
Publication Date: 2011/03/01
Online ISSN: 1745-1353
DOI: 10.1587/transele.E94.C.301
Print ISSN: 0916-8516
Type of Manuscript: Special Section PAPER (Special Section on Superconducting Signal Processing Technologies)
NbN,  Josephson junctions,  self-shunted,  AlNx,  radical-nitridation,  

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We describe the fabrication processes and electrical characteristics of two types of NbN junctions. One is a self-shunted NbN/NbNx/AlN/NbN Josephson junction, which is expected to improve the density of integrated circuits; the other is an underdamped NbN/AlNx/NbN tunnel junction with radical-nitride AlNx barriers, which has highly controllable junction characteristics. In the former, the junction characteristics were changed from underdamped to overdamped by varying the thickness of the NbNx layer. Overdamped junctions with a 6-nm-thick NbNx film exhibited a characteristic voltage of Vc = 0.8 mV and a critical current density of Jc = 22 A/cm2 at 4.2 K. In the junctions with radical-nitride AlNx barriers, Jc could be controlled in the range 0.01-3 kA/cm2 by varying the process conditions, and good uniformity of the junction characteristics was obtained.