Parameters of the Tunnel Barrier of Superconducting Niobium-Based Structures
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Parameters of the Tunnel Barrier of Superconducting Niobium-Based Structures M. E. Paramonova, *, L. V. Filippenkoa, P. N. Dmitrieva, M. Yu. Fominskya, A. B. Ermakova, and V. P. Kosheletsa a Kotel’nikov
Institute of Radio Engineering and Electronics, Russian Academy of Sciences, Moscow, 125009 Russia *e-mail: [email protected] Received March 26, 2020; revised March 26, 2020; accepted April 2, 2020
Abstract—The main parameters of the tunnel barrier of Josephson Nb/AlOx/Nb and Nb/AlN/Nb junctions are estimated in a wide range of the current density using the Simmons method. The dependences of the tunnel barrier height and width of the resistivity are determined experimentally for each type of the junctions. A decrease in the tunnel barrier height of the junction with the AlN interlayer by 0.3 eV as compared to an oxide junction enables us to obtain the junctions with a current density higher than 15 kA/cm2 at an insulating layer thickness of 10 Å allowable technologically, which gives the possibility to obtain quality parameter Rj/Rn no lower than 25. Keywords: superconductivity, superconducting tunnel junction, tunnel barrier parameters, Simmons method, terahertz-range receivers DOI: 10.1134/S1063783420090231
1. INTRODUCTION
2. METHODS OF FABRICATION OF THE TUNNEL STRUCTURES The SIS junctions based on Nb/AlOx/Nb and Nb/AlN/Nb structures were fabricated using the selective niobium etching and anodization process
1534
RnS, : Pm2 20
200
2
35 30 25 Rj/Rn
Mixers based on the superconductor–insulator– superconductor (SIS) tunnel junctions are most sensitive input elements for coherent receivers at frequencies from 0.1 to 1.2 THz: their noise temperature is only restricted by the quantum limit. Now SIS receivers are used as staff devices in most of ground-based and space radio telescopes worldwide [1–6]. To increase the operating frequency of superconducting circuits based on SIS junctions and to extend their band, high current-density tunnel junctions are necessary. However, there is the limit of enhancing the barrier transparency for alumina-based SIS junctions. This limit is 10–15 kA/cm2; a further increase in the current density leads to sharp degradation of the junction quality [7–10] (Fig. 1). To overcome this limit, the technology of fabricating tunnel SIS junctions with an AlN barrier has been developed; this barrier is formed by the nitridation of an Al surface in a plasma RF discharge in a pure N2 atmosphere [4–6, 10–12]. This new type of the junctions enables one to obtain the current densities higher than 15 kA/cm2 for the realization of wide-band receivers and generators of the THz range. An important feature of the junctions with an AlN interlayer is the possibility to use it in combination with the top NbN electrode, which makes it possible to increase the junction gap voltage from 2.8 to 3.7 mV [10] and to increase the upper frequency boundary of operating RF devices.
20 15 10
NbAlOxNb NbAlNNb NbAlNNbN
5 0.1
1
10 J, kA/cm2
100
Fig. 1. Quality parameter Rj/Rn as a function o
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