RU2786616C1 - Heterostructure based on the josephson tunnel junction superconductor-insulator-superconductor with integral shutting - Google Patents

Abstract

FIELD: superconductors.

SUBSTANCE: heterostructure based on a superconductor-insulator-superconductor (SIS) Josephson tunnel junction includes a silicon substrate, the first, second, and third niobium layers separated by a barrier layer, and an integral shunting element of the SIS junction. The integral shunting element is made in the form of a superconductor - insulator - normal metal (SIN) junction located coaxially with the said SIS junction, and the radius r of the SNJ transition is selected from the condition of a given degree of shunting. The first layer of niobium is placed on the substrate, the first layer of aluminum with the formed barrier layer of aluminum dioxide and the second layer of aluminum are placed on top of it, in the central part of which the second layer of niobium with an outer radius r1 is placed, jointly forming a SIN junction. The third layer of niobium contacts only the second layer of niobium and is separated from said structure by a layer of silicon dioxide.

EFFECT: providing greater compactness while expanding the frequency range of operation of Josephson tunnel junctions.

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Description

The invention relates to the field of superconducting microelectronics and can be used to create generators and receivers of THz and sub-THz radiation, superconducting quantum interferometers (SQUIDs), single-quantum digital devices.

Various devices are known from the prior art using heterostructures based on Josephson tunnel junctions.

Known devices based on the Josephson tunnel junction superconductor-insulator-superconductor (SIS) (RU2504049 C2, "Research Institute for Physical Problems named after F.V. Lukin", 01/10/2014. Superconducting Josephson device based on a multilayer thin-film heterostructure containing two superconductor layer forming electrodes and a layer with metal conductivity between them made of a metal-doped semiconductor, the layer has a locally inhomogeneous structure and is configured to simultaneously form two independent current transport channels in its volume.

The disadvantage of this solution is that the junction inductance, as well as the normal junction resistance, have a nonmonotonic dependence on the thickness of the intermediate semiconductor layer (Gudkov, A.L., Kupriyanov, M.Y., & Likharev, K.K. (1988). Properties of Josephson junctions with amorphous -silicon interlayers Zh Eksp Teor Fiz 94(3), 19-332). This can lead to the fact that with small deviations of the thickness of the intermediate layer due to the peculiarities of the technological process from the set values, the inductance and normal resistance will change nonmonotonically and unpredictably, and hence the characteristic voltage and the McCumber hysteresis parameter.

Other superconducting devices are also known using SNS type transitions (for example, US 6734454 (B2), Van Duzer, et al., 05/11/2004; EP 1365456 (A2), YAMAMORI HIROTAKE et al., 11/26/2003), in which transport current was carried out by the direct flow of current through the layer with a purely metallic conductivity and there is a significant suppression of superconductivity in the electrodes (RU 2599904 C1). In such transitions, as a rule, the value of the characteristic voltage does not exceed tens of microvolts, which corresponds to operating frequencies up to 20 GHz. Most practical applications require Josephson junctions with characteristic voltages of hundreds of microvolts and operating frequencies of 200 GHz and higher.

It is known to manufacture junctions with a high current density, which provides a high characteristic voltage and hysteresis-free current-voltage characteristic without the use of external shunting (Tolpygo SK et al Properties of unshunted and resistively shunted Nb/AlOx-Al/Nb Josephson junctions with critical current densities from 0.1 to 1 mA /μm2 //IEEE Transactions on Applied Superconductivity., V. 27, No. 4. - pp. 1-15 (2017)). The disadvantage is a complex technological process - it is necessary to manufacture structures with a very thin insulator layer (of the order of several atomic layers). This process is difficult to control and unpredictable. Also, an increase in current density leads to the fact that in order to manufacture junctions with resistances of the order of several ohms, which is required for most practical applications, it is necessary to significantly reduce their size (the junction area is less than 0.1 μm ² ). This also greatly complicates the manufacturing process.

It is known that shunting the tunnel junction ensures that the volt-ampere characteristic is hysteresis-free. There is also a way to shunt with an external resistor (see the above article by Tolpygo S.K. et al., p. 3). According to this method, the shunt is a thin-film resistor made of normal metal, the thickness of which is chosen taking into account the current density, and the geometric dimensions - taking into account the parameters of the tunnel junction and the required degree of shunting. The disadvantage is the high parasitic inductance of the thin-film shunt, as well as the large total size of the structure compared to the area of one shunted SIS junction. Even in the most modern designs, the shunt area is many times larger than the transition area.

The closest to patentable is the heterostructure according to US patent 6734454 (B2), UNIV ARIZONA, UNIV CALIFORNIA - 11.05.2004 - prototype. The Josephson junction has its own resistance, which effectively shunts the junction and thus eliminates the need for a separate shunt resistor and thus reduces the surface area in an integrated circuit including a plurality of Josephson junctions. The Josephson junction consists of a stack of _Nb layers and a superconductor with T _C >9°K having a greater penetration depth than _Nb , e.g. and 1 Ohm.cm, for example, TaxN. The Josephson junction can be formed on a carrier substrate such as silicon with a ground plane such as Nb on the substrate and an insulating layer such as SiO ₂ separating the ground plane from the array.

The disadvantage of this shunting is the low reproducibility of the required parameters. Studies are described (Yu, Lei, et al. "Internally shunted Josephson junctions with barriers tuned near the metal-insulator transition for RSFQ logic applications." Superconductor Science and Technology 19.8 (2006): 719) indicating that samples made on the same plate, the critical currents I _C and resistances R _n vary greatly due to violation of stoichiometry. Moreover, if even for one substrate there are noticeable variations in the values of the critical current and normal resistance, then they will be much stronger from one manufacturing cycle to another.

The present invention is aimed at solving the technical problem of reducing parasitic inductance and expanding the frequency range of operation of Josephson tunnel junctions of the superconductor-insulator-superconductor (SIS) type, while ensuring greater compactness, which is the technical result of the invention.

The patented heterostructure based on the Josephson tunnel junction superconductor-insulator-superconductor (SIS) includes a silicon substrate with a first niobium layer on it, a second niobium layer separated from the first niobium layer by a barrier layer, a third niobium layer, and an integral shunting element of the SIS junction.

The difference is as follows. The integral shunting element is made in the form of a cylindrical superconductor - insulator - normal metal (SIN) junction located coaxially with the said SIS junction, of a cylindrical shape, and the value of the radius r of the SIS junction is selected from the condition of a given degree of shunting, while over the first layer of niobium placed on the substrate , the first layer of aluminum is placed with the formed barrier layer of aluminum dioxide and the second layer of aluminum, in the central part of the second layer of aluminum is placed the second layer of niobium with an outer radius r ₁ , which together form the SIN transition, and the third layer of niobium is placed so that it contacts only with a second layer of niobium and separated from said structure by a layer of silicon dioxide.

The heterostructure can be characterized in that the area of the SIS junction is 1.4 µm ² with the outer radius of the SIS junction in the range r =1.5 to 3.5 µm.

The proposed method of shunting consists in the manufacture of tunnel SIS junctions with integral shunting with a superconductor-insulator-normal metal (SIN) junction located around the main junction and manufactured with it in the same technological process. The inner radius of the SIS junction is determined by the radius of the SIS junction, while the outer radius is selected taking into account the required resistance. In this case, it is possible to achieve a reduction in parasitic inductance and an expansion of the frequency range of operation of Josephson tunnel junctions of the superconductor-insulator-superconductor type.

The essence of the invention is illustrated in the figures:

fig. 1 - heterostructure in cross section;

fig. 2 - heterostructure, section along the line A-A;

Fig. 3 - 6 - the sequence of formation of the heterostructure;

fig. 7 - results of numerical simulation: dependence of the characteristic voltage V _c of the junction with SIN shunting from the outer radius r of the shunt;

fig. 8 - results of numerical simulation: dependence of the total subgap voltage of the shunt and transition R _jtotal on the outer radius r of the shunt for various current densities;

fig. 9 - results of numerical simulation: hysteresis parameter β _C depending on the outer radius r of the shunt for different current densities;

fig. 10 - current-voltage characteristics of the fabricated structures with shunting of the SIN transition under the influence of a microwave signal;

fig. 11 - current-voltage characteristics of the obtained structures with different degrees of shunting.

In FIG. 1, 2 shows the topology of the patented heterostructure. On the substrate 1 of silicon is placed the first layer 2 of the Nb film, then the first layer 3 of the Al film. On top of layer 3 a barrier layer 4 of Al ₂ O ₃ is formed, followed by a second layer 5 of Al.

Further, in the central part of layer 5, a small size SIS junction is formed from Nb layer 6. Next, a layer 7 of SiO ₂ is placed, on top of which an upper electrode is formed from a layer of 8 Nb.

An example of the sequence of formation of heterostructure layers is shown schematically in FIG. 3-6. The heterostructure is performed in one technological cycle and includes the following operations:

- deposition of a four-layer structure Nb-AlOx/Al-Nb on a silicon substrate (Fig. 3);

- the formation of the SIN transition 11 by plasma-chemical etching, and through the anodization of the upper layer 5 of aluminum outside the SIN transition 11 (Fig. 4);

formation of the main SIS transition 10, deposition of a layer 7 of an insulator SiO ₂ with a thickness of 250 nm (Fig. 5);

- deposition of the upper (closing) electrode 8 of Nb with a thickness of 300 nm (Fig. 6).

Substrate 1 - a plate of single-crystal unoxidized high-resistance silicon, which is a dielectric at low temperatures. The size of the substrate is 24×24×0.5 mm. A layer of Al ₂ O ₃ 100 nm thick was deposited on the cleaned substrate 1, which is protective when transitions are formed by reactive ion etching.

The geometry of the lower electrode in layer 2 was formed by the method of "explosive" lithography.

The deposition of the four-layer structure Nb/Al-AlOx/Al/Nb is carried out in a single vacuum cycle (Fig. 3). 2.6 Nb layers and 3.5 Al layers were deposited by DC magnetron sputtering.

The lower layer 2 of the Nb film 200 nm thick was formed at a power of 600 W and an Ar pressure of 3⋅10 ^-3 mbar. After the necessary cooling of the Nb film, layer 3 of the Al film with a thickness of 5 nm was deposited at a power of 100 W and an Ar pressure of 2⋅10 ^-3 mbar. Then it was thermally oxidized at room temperature for 20 minutes in an atmosphere of pure oxygen at a pressure of 2⋅10 ^-2 mbar.

After residual oxygen was pumped out, the second layer of 5 Al 7 nm thick was deposited at a power of 100 W and an Ar pressure of 2⋅10 ^-3 mbar, and the upper layer of Nb 100 nm thick was deposited at a power of 600 W and an Ar pressure of 3⋅10 ^-3 mbar.

SIN transition 11 was formed by reactive ion etching of the upper layer of 6 Nb in a mixture of gases CF ₄ +3% O ₂ at a power of 50 W. As a result, regions of niobium layer 6 that were not protected by the resist were completely removed. The etching process automatically stopped at layer 5 of the aluminum film. Then, through the same resistive mask, the through anodization of Al was carried out to a voltage of 15–20 V in an electrolyte solution.

The geometry of the main SIS transition 10 was formed by the method of "explosive" lithography. After photolithography, which determines the size (D = 2r ₁ ) of the transition, reactive ion etching of Nb was carried out in a mixture of gases CF ₄ + 3% O ₂ at a power of 50 W, followed by anodization up to 7-8 V. Then layer 7 of SiO ₂ insulation was deposited with a thickness 250 nm; in this case, the same resistive mask was used as in the second etching and anodization. To avoid difficulties with explosive lithography, the deposition of the insulating layer 7 over the remaining mask was carried out in two stages with a time interval sufficient for cooling. The SiO ₂ film was deposited by RF magnetron sputtering at a power of 450 W and an Ar pressure of 2⋅10 ^-3 mbar.

At the last stage, the upper (closing) electrode was formed. An 8Nb layer film with a thickness of 300-400 nm was deposited by DC magnetron sputtering at a power of 600 W and an Ar pressure of 3⋅10 ^-3 mbar. The geometry of the upper electrode of the 8 Nb layer was formed by the method of "explosive" lithography.

In FIG. 7-9 shows the results of numerical simulation. The dependence of the characteristic voltage V _C of a junction with an LSS shunt on the outer radius r of the shunt is shown in FIG. 7. The dependence of the total subgap voltage of the shunt and the transition _Rjtotal on the outer radius of the shunt r for various current densities is shown in FIG. eight.

In FIG. Figure 9 shows the calculation of the hysteresis parameter β _C (β _C ≤ 1 in the case of no hysteresis) depending on the outer radius r of the shunt for various current densities. The area of the SIS transition S = 1.4 μm ² .

The performed numerical calculations show that when creating structures with a low specific tunneling resistance R _n S ≈ 7 Ohm×µm ² , it is possible to obtain junctions with a high characteristic voltage V _C ≈ 800 mV and with an hysteresis-free current-voltage characteristic.

In FIG. 10 shows the current-voltage characteristics of fabricated structures with shunting of the SIN junction. The current-voltage characteristics of the SSI junction with SIN shunting are given without the influence of an external signal and under the influence of signals with frequencies of 330 GHz, 400 GHz and 460 GHz. It can be seen that the SIS junction with SYN shunt provides efficient operation at high frequencies. For each of the frequencies, the corresponding Shapiro steps are observed at voltages corresponding to the frequency according to the Josephson relation.

In FIG. Figure 11 shows the current-voltage characteristics of fabricated structures with different degrees of shunting for the following parameters: transition area S = 1.4 μm ² . The outer radius r of the shunt varies from 1.5 to 3.5 μm. It can be seen that the patented type of shunting ensures the operability of heterostructures, while the degree of shunting can be controlled by setting the radius of the junction SIN.

Thus, the presented data indicate the achievement of the technical result of the invention: providing greater compactness while expanding the frequency range of the Josephson tunnel junctions.

Claims (2)

1. A heterostructure based on a Josephson tunneling superconductor-insulator-superconductor (SIS) junction, including a silicon substrate with a first niobium layer on it, a second niobium layer separated from the first niobium layer by a barrier layer, a third niobium layer, and an integral shunting element of the SIS junction, characterized in that the integral shunting element is made in the form of a cylindrical superconductor - insulator - normal metal (SIN) junction located coaxially with the said SIS junction, of a cylindrical shape, and the value of the radius r of the SIS junction is selected from the condition of a given degree of shunting, while over the first layer of niobium placed on the substrate, the first layer of aluminum is placed with the formed barrier layer of aluminum dioxide and the second layer of aluminum, in the central part of the second layer of aluminum is placed the second layer of niobium with an outer radius r ₁ , which together form the SIN transition, and the third layer of niobium is placed so that that he only interacts with by a second layer of niobium and separated from said structure by a layer of silicon dioxide. 2. Heterostructure according to claim 1, characterized in that the area of the SIS transition is 1.4 μm ² with an outer radius r of the SIS transition in the range from r=1.5 to 3.5 μm.

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