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Applied Physics Letters

Niobium nitride-niobium Josephson tunnel junctions with sputtered amorphous silicon barriers

D. W. Jillie, Harry Kroger, L. N. Smith, Edward J. Cukauskas, Marty Nisenoff

Published Apr 15, 1982• 40 citations
Publisher

Abstract

Niobium nitride-niobium Josephson tunnel junctions with sputtered amorphous silicon barriers (NbN-αSi-Nb) have been prepared using processing that is fully compatible with integrated circuit fabrication. These junctions are of suitable quality and uniformity for digital circuit and S-I-S detector applications. The junction quality depends critically upon the properties of the NbN surface, and seems to correlate well with the UV/visible reflectivity of this surface.

DOI
10.1063/1.93214

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Niobium nitride-niobium Josephson tunnel junctions with sputtered amorphous silicon barriers (NbN-αSi-Nb) have been prepared using processing that is fully compatible with integrated circuit fabrication. These junctions are of suitable quality and uniformity for digital circuit and S-I-S detector applications. The junction quality depends critically upon the properties of the NbN surface, and seems to correlate well with the UV/visible reflectivity of this surface.
Niobium nitride-niobium nitride Josephson tunnel junctions have been fabricated using amorphous magnesium oxide (a-MgO) films as barriers. These junctions have excellent tunneling characteristics. For example, a large gap voltage (Vg=5.1 mV), a large product of the maximum critical current and the normal tunneling resistance (IcRn=3.25 mV), and a small subgap leakage current (Vm=45 mV, measured at 3 mV) have been obtained for a NbN/a-MgO/NbN junction. The critical current of this junction remains finite up to 14.5 K.
All hard Josephson tunnel junctions, whose base and counter electrodes are composed of double-layered niobium nitride (NbN) and niobium (Nb) films, have been successfully fabricated by isolating a junction sandwich formed on a whole silicon wafer with a reactive ion etching technique. The reactive ion etching technique has been used for patterning both base and counterelectrodes, and self-aligning definition of junction areas has been performed. The fabricated junctions show good quality single-particle tunneling characteristics and excellent uniformity in critical currents.
This program is part of an ongoing effort to fabricate all-refractory S-I-S devices for use as mm-wave detectors and mixers in the quantum regime, and operating in the temperature range of 8-10 K. There are four ongoing approaches to achieve this goal: (1) Development of high-quality NbN-barrier-Nb S-I-S junctions as an intermediate goal towards fabricating all-NbN junctions; (2) Actual development of all-NbN junctions using the results of (1) above as a guide; (3) Development of an in-house NbN deposition capability in order to speed the progress on (2) above; and (4) The use of existing all-NB digital circuit fabrication capability to design and fabricate mm-wave mixers to operate at T less than or approx. 4.2 K, thus gaining valuable experience that can be applied to the eventual design of all-NbN mixers. Sputtered NbN-a:Si-NB Josephson tunnel junctions fabricated using SRC's SNAP process (Selective Niobium Anodization Process) have been developed. All-NbN devices using the plain a:Si barrier have been developed to the point where they exhibit S-I-S tunnelling characteristics. Fourteen types of mm-wave mixer chips have been designed in collaboration with the Goddard Institute for Space Studies using the existing all -NB digital circuit fabrication process. Masks were fabricated, and a trial fabrication proved the integrity of the process. The viability of the proposed mounting scheme has been experimentally ascertained.
Epitaxial NbN/AlN/NbN Josephson junctions are promising alternatives to conventional Al/AlOx_xx​/Al junctions for superconducting qubits due to their crystalline tunnel barriers and the high transition temperature of NbN. However, their integration into quantum circuits typically requires complex fabrication processes involving amorphous interlayer dielectrics, which can degrade qubit coherence times. To address these issues, we developed sidewall spacer passivated epitaxial NbN/AlN/NbN junctions using a fabrication process that reduces both the volume of amorphous dielectric material and fabrication complexity. The fabricated junctions exhibited a large gap voltage of approximately 5.6 mV and low subgap leakage currents. By varying the thickness of the AlN tunnel barrier, high-quality junctions were fabricated over a wide range of critical current densities Jc{\it J}_cJc​, from 0.3 to 104^44 A/cm2^22. In addition, the comparison of the current-voltage characteristics before and after buffered hydrofluoric acid treatment indicated that the wet etching process did not degrade the electrical characteristics of the junctions. These results establish a scalable fabrication process for epitaxial Josephson junctions and provide a promising platform for high-coherence nitride-based superconducting qubits.