All-NbN Single Flux Quantum Circuits Based on NbN/AlN/NbN Tunnel Junctions

Hirotaka TERAI  Zhen WANG  

IEICE TRANSACTIONS on Electronics   Vol.E83-C   No.1   pp.69-74
Publication Date: 2000/01/25
Online ISSN: 
Print ISSN: 0916-8516
Type of Manuscript: Special Section PAPER (Special Issue on Superconductive Devices and Systems)
Category: Digital Applications
NbN integrated circuit,  NbN/AlN/NbN tunnel junction,  overdumped junction,  sheet inductance,  SFQ circuit,  operating margin,  

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We report on the fabrication and operation of all-NbN single flux quantum (SFQ) circuits with resistively shunted NbN/AlN/NbN tunnel junctions fabricated on silicon substrates. The critical current varied by about 5% in 400 NbN/AlN/NbN junction arrays, where the junction area was 88 µm2. Critical current densities of the NbN/AlN/NbN tunnel junctions showed exponential dependence on the deposition time of the AlN barrier. By using the 12-nm-thick Cu film as shunted resistors, non-hysteretic current-voltage characteristics were achieved. From dc-SQUID measurements, the sheet inductance of our NbN stripline was estimated to be around 1.2 pH at 4.2 K. We designed and fabricated circuits consisting of dc/SFQ converters, Josephson transmission lines, and T flip-flop-based SFQ/dc converters. The circuits demonstrated correct operation with a bias margin of more than 15% at 4.2 K.