Grain size control of superconducting thin film materials for josephson junctions

Abstract

The disclosed Josephson junction device (110) includes a lower superconducting material layer formed on a substrate 2 (220), a junction layer (240) formed over the lower superconducting material layer, and an upper superconducting material layer (260) formed over the junction layer. Alternatively, the lower superconducting material layer is an alloy, preferably an aluminum alloy, having an average grain size of less than 20 nm and less than the width and length of the junction.

Description

Technical Field

[0001] The presently claimed embodiments of the invention relate to a method for controlling the grain size of a superconducting material for a Josephson junction and a Josephson junction circuit having the grain size. Background Technology

[0002] Quantum circuits incorporating Josephson junctions have numerous applications. Quantum computing circuits employ qubits, which can include Josephson junctions. These junctions are often formed as a stack of a lower superconducting material layer (e.g., an aluminum superconducting layer), a thin oxide layer, and an upper superconducting material layer (e.g., an aluminum layer), wherein the stack is formed on a substrate such as silicon. The grain size in the thin film of the superconducting material layer (e.g., the aluminum layer) is a significant part of the size of the Josephson junction. Therefore, the junction can contain variable amounts of aluminum grains. Consequently, the ratio of grain boundary (GB) area to superconducting aluminum (Al) grain area can vary as it moves between junctions, leading to material-based inhomogeneities in the Josephson junction characteristics.

[0003] Furthermore, during the fabrication of the Josephson junction, oxidation of the aluminum surface occurs at different rates on different surfaces or at grain boundaries, with different penetration rates at different GB orientations. If the superconducting (aluminum) grain size in these superconducting material layers is similar in size to the size of the Josephson junction, the number of grains and the grain boundary area (where oxidation can be different) can vary between junctions, resulting in material-based variations in the Josephson junction properties.

[0004] Therefore, it is desirable to solve the above and other problems of the prior art by, for example, controlling the grain size of one or more superconducting material layers. Summary of the Invention

[0005] One aspect of the present invention is to provide a superconducting circuit. The superconducting circuit includes a Josephson junction device, the Josephson junction device comprising: a lower superconducting material layer formed on a substrate; a junction layer formed on the lower superconducting material layer; and an upper superconducting material layer formed above the junction layer. At least the lower superconducting material layer includes grains, the size of which is larger than the size of the Josephson junction of the Josephson junction device.

[0006] In one embodiment, the lower superconducting material layer is formed from single-crystal grains. In another embodiment, the lower superconducting material layer is epitaxial with the aid of a substrate. In another embodiment, the epitaxial lower superconducting material is formed of aluminum. In another embodiment, the aluminum is oriented such that the (110) plane is parallel to the substrate. In another embodiment, the grains in the lower superconducting material layer are formed by increasing the size of the grains through annealing. In another embodiment, the substrate has at least one of silicon (Si) and sapphire.

[0007] Another aspect of the present invention is to provide a superconducting circuit comprising a Josephson junction device. The Josephson junction device comprises: a superconducting alloy layer; and a junction layer formed on the superconducting alloy layer. The average grain size of the superconducting alloy layer is smaller than the width and length of the Josephson junction of the Josephson junction device and is less than about 20 nm.

[0008] In one embodiment, the superconducting alloy layer is an aluminum alloy layer. In another embodiment, the average grain size of the aluminum alloy layer is less than 10 nm. In yet another embodiment, the superconducting alloy layer is amorphous. In one embodiment, the aluminum alloy comprises aluminum and at least one element selected from Si, Mg, V, Nb, Ta, La, Zr, Hf, Ti, Zn, In, and Sn. In yet another embodiment, the aluminum content in the aluminum alloy is in the range of about 66% to about 97%.

[0009] Another aspect of the present invention is to provide a method for forming a Josephson junction device. The method includes: forming a superconducting material layer on a substrate; and annealing the superconducting material layer such that a single grain of the superconducting material layer exceeds the width and length of the Josephson junction of the Josephson junction device.

[0010] In one embodiment, the superconducting material layer is an aluminum layer. In one embodiment, the individual grains are epitaxially aligned with the substrate. In one embodiment, the substrate has at least one of Si and sapphire. In one embodiment, annealing is performed in a temperature range from about 200°C to about 570°C, and preferably from about 400°C to about 570°C.

[0011] Another aspect of the present invention is to provide a method for forming a Josephson junction device. The method includes forming a superconducting alloy layer of the Josephson junction device on a substrate. The average grain size of the superconducting alloy layer is smaller than the width and length of the Josephson junction and is less than about 20 nm.

[0012] In one embodiment, the superconducting alloy layer is an aluminum alloy layer. In one embodiment, the average grain size is less than 10 nm. In one embodiment, the superconducting alloy layer is amorphous. In one embodiment, the aluminum alloy layer is amorphous. In one embodiment, the aluminum alloy layer comprises aluminum and at least one element selected from the group consisting of Si, Mg, V, Nb, Ta, La, Zr, Hf, Ti, Zn, In, and Sn. In one embodiment, the aluminum content in the aluminum alloy layer is in the range of about 66% to about 97%. Attached Figure Description

[0013] The operation and function of the relevant elements of this disclosure and structure, as well as the economy of combination and manufacture of the components, will become more apparent when the following description and appended claims are considered in conjunction with the accompanying drawings, all of which form part of this specification, wherein similar reference numerals denote corresponding components in the various drawings. However, it should be clearly understood that the drawings are for illustrative and descriptive purposes only and are not intended to be limiting of the invention.

[0014] Figure 1 It is a Josephson junction circuit 100 according to some embodiments of the present invention;

[0015] Figure 2A This is a side cross-sectional view of a Josephson knot according to some embodiments of the present invention;

[0016] Figure 2B According to some embodiments of the present invention Figure 2A Top cross-sectional view of the Josephson junction device;

[0017] Figure 3A A portion of the bottom aluminum layer prior to annealing is shown according to some embodiments of the present invention;

[0018] Figure 3B A portion of the bottom aluminum layer after annealing is shown according to some embodiments of the present invention;

[0019] Figure 3C This illustration shows a portion of the bottom aluminum layer overlapping the Josephson junction after alloying, according to some embodiments of the invention.

[0020] Figure 4 Temperature-dependent X-ray diffraction (XRD) intensity maps of aluminum according to some embodiments of the present invention are shown;

[0021] Figure 5A and Figure 5B The presence of the XRD Al(111) peak as a function of temperature and time is shown in some embodiments of the present invention.

[0022] Figure 6A and Figure 6B Plots of XRD measurements before and after annealed deposited aluminum are shown according to some embodiments of the present invention;

[0023] Figure 7 These are transmission electron microscope images illustrating the effects of annealing Al(111) on Si(100) according to some embodiments of the present invention; and

[0024] Figure 8 The images are transmission electron microscope images showing the effect of annealing Al(111) on Si(100) according to some embodiments of the present invention over a large area of ​​the device. Specific Implementation

[0025] To address the problems arising from superconducting grains having dimensions similar to those of a Josephson junction, according to certain embodiments, the microstructure of the Josephson junction is controlled such that the superconducting grain size is either much larger or much smaller than the size of the Josephson junction.

[0026] In one embodiment, increasing the grain size of the superconducting material (e.g., aluminum) can be achieved by high-temperature annealing followed by cooling. By significantly increasing the grain size, it is possible to provide a Josephson junction using a single-grain superconducting material, which is equivalent to constructing it with a single-crystal superconducting material.

[0027] As an alternative to increasing the superconducting material grain size to be larger than the Josephson junction size, the superconducting material grain size can be controlled to be much smaller than the Josephson junction size. For example, the superconducting material grain size can be reduced by alloying the superconducting material. To reduce the superconducting material grain size, a Josephson junction constructed using a superconducting material with smaller grains can include hundreds of grains and be similar between Josephson junctions, thus reducing the variation in the Josephson junction structure due to the superconducting material grain size. Finally, the amorphous superconducting material alloy layer, similar to a single superconducting material grain junction, can also have a material that is uniform between Josephson junctions. According to some embodiments, the alloyed superconducting material can be formed via evaporation or sputtering.

[0028] Figure 1 This is a Josephson junction circuit 100 according to some embodiments. The Josephson junction circuit 100 includes a Josephson junction device 110 and circuit elements 120 coupled to the Josephson junction (JJ) device 110.

[0029] Figure 2AThis is a side cross-sectional view of a Josephson junction (JJ) device according to some embodiments of the present invention. The Josephson junction device includes a bottom superconducting material layer 210 formed on a substrate 220. A top superconducting material layer 230 is formed above the bottom superconducting material layer 210, and a thin oxide layer 240 serving as a tunneling layer is provided between the bottom superconducting material layer 210 and the top superconducting material layer 230.

[0030] In one embodiment, the bottom or lower superconducting material layer 210 is formed from a single crystal grain. In another embodiment, the lower superconducting material layer 210 is epitaxially grown using a substrate 220. In another embodiment, the lower superconducting material layer 210 and / or the upper superconducting material layer 230 are formed of aluminum. In another embodiment, the substrate 220 comprises silicon (Si) and / or sapphire. In another embodiment, the thin oxide layer (tunneling junction layer) 240 comprises a thin oxide layer of the lower superconducting material layer 210.

[0031] Figure 2B yes Figure 2A The top cross-sectional view of the Josephson junction device shows the Josephson junction (JJ) 260, in which a thin oxide layer 240 contacts and separates the bottom superconducting layer 210 from the top superconducting layer 230. The dimensions of the Josephson junction 260 are determined by... Figure 2B The pattern in the image summarizes the area where the bottom superconducting material layer 210 and the top aluminum layer 230 contact the thin oxide layer 240, which acts as a tunneling barrier.

[0032] Although Figure 2A and Figure 2B The Josephson junction device shows a single Josephson junction, but the number of Josephson junctions in the Josephson junction device can be multiple (i.e., two or more).

[0033] Figure 3A A portion of the bottom superconducting material layer 210 prior to annealing region A is shown, wherein the grains of the bottom superconducting material layer 210 are shown within the region of the Josephson junction 260.

[0034] Annealing to increase grain size: Figure 3B A portion of the bottom superconducting material layer 210 after annealing is shown, overlapping with a Josephson junction 260 having a region A, wherein grains of the bottom superconducting material layer 210 are shown within the region of the Josephson junction 260. In an embodiment, the grains in the bottom or lower superconducting material layer 210 are formed by increasing the grain size through annealing. Comparison Figure 3A and Figure 3BIt can be seen that the annealing of the bottom superconducting material layer 210 increases the grain size so that individual grains of the bottom superconducting material layer 210 overlap with the region of the Josephson junction 260. Annealing the bottom superconducting material layer 210 causes the single-crystal grains of the bottom superconducting material layer 210 to exceed the width and length of the Josephson junction of the device. For example, annealing can be performed in a temperature range from, for example, about 200°C to about 570°C, and preferably in a temperature range from about 400°C to about 570°C.

[0035] Although Figure 3B Annealing of the bottom superconducting material layer 210 increases the grain size so that a single grain of the bottom superconducting material layer 210 overlaps with a region of the Josephson junction 260. Alternatively, the top superconducting material layer 230 can be annealed to increase the grain size so that a single grain of the top superconducting material layer 230 overlaps with a region of the Josephson junction 260.

[0036] Reference Figure 2A The substrate 220 may be formed of silicon, such as Si(001). Due to the annealing process, at least the bottom superconducting material (e.g., aluminum) layer 210 may be formed from single-crystal grains and may be epitaxially formed by means of the substrate 220, wherein the single-crystal grains are formed by increasing the grain size of the superconducting material (e.g., aluminum) through annealing.

[0037] Figure 4 Temperature-dependent X-ray diffraction (XRD) intensity maps of superconducting materials (e.g., aluminum) are presented. In-situ XRD was performed using a linear detector on a Si(001) substrate during annealing of a 35 nm thick superconducting material (e.g., aluminum) at a heating rate of 4.5 °C / s in purified He gas. The lower temperatures revealed that the deposited Al had a (111) plane oriented parallel to the surface of the silicon substrate. The Al(111) peak and X-ray scattering background disappeared at approximately 50 °C below the eutectic melting temperature. Melting of the superconducting material was detected by the increase in background X-rays at the eutectic melting temperature of approximately 577 °C.

[0038] Figure 5A and Figure 5B The presence of the Al(111) peak in XRD is shown as a function of temperature and time, where the Al(111) peak disappears at higher temperatures and the sample is cooled before reaching its melting point. Cooling aluminum after annealing does not cause the Al(111) peak to reappear. Quenching in the featureless temperature region within the detected K-space region does not show an Al peak in the detector geometry. After quenching, the detector geometry does not detect the Al(111) peak. Therefore, quenched (111) oriented aluminum produces aluminum without the Al(111) peak.

[0039] Figure 6AThe θ2θXRD measurements performed on the freshly deposited aluminum film are shown, showing only the Al(111) peak with a swing FWHM (full width at half maximum) of approximately 8 degrees (Ω) (fiber width). Figure 6B The θ2θXRD measurements performed on annealed aluminum are shown, where no Al(111) peak was detected after quenching.

[0040] Figure 7 This is a transmission electron microscope image showing the effect of annealing Al(111) on Si(100) to a temperature where the (111) peak disappears but before the eutectic melting point, followed by rapid cooling, which increases the aluminum grain size so that individual grains become much larger than the size of the Josephson junction. Furthermore, the orientation of the aluminum changes from (111) to (110). A grain size greater than 5 micrometers is achieved for the aluminum, and aluminum is epitaxially formed on the silicon substrate. The texture of the aluminum film changes from (111) to (110) and is very precisely aligned with the Si(100) substrate. The mismatch between the aluminum and silicon films is provided by some oxide islands and low-density stacking faults and through dislocations at the interface. Therefore, the oxidation of the Josephson junction can be carried out on a single grain of the bottom or lower aluminum layer, thus eliminating the variability originating from grain boundaries.

[0041] Figure 8 The image shows a transmission electron microscope image of the quenched aluminum layer at a lower resolution, in which no grain boundaries are present in the image and only small defects exist in the aluminum layer.

[0042] Alloying reduces grain size: Figure 3C A portion of the bottom superconducting material layer 210 after alloying is shown, overlapping with a Josephson junction 260 having a region A, wherein the grains of the bottom superconducting material layer 210 are shown within the region of the Josephson junction 260. Comparison Figure 3A and Figure 3CAs can be seen, alloying the bottom superconducting material layer 210 reduces the grain size, causing multiple grains of the bottom superconducting material layer 210 to overlap with regions of the Josephson junction 260. Alloying the bottom superconducting material layer 210 results in an average grain size smaller than the width and length of the Josephson junction of the device. In one embodiment, the average grain size of the bottom superconducting material layer 210 may be less than 20 nm, for example. In another embodiment, the average grain size of the bottom superconducting material layer 210 may be less than 10 nm, for example. In one embodiment, the number of grains of the bottom superconducting material layer 210 that contact and overlap with the Josephson junction may be, for example, greater than or equal to 10. In another embodiment, the number of grains of the bottom superconducting material layer 210 that contact and overlap with the Josephson junction may be, for example, greater than or equal to 50. In yet another embodiment, the number of grains of the bottom superconducting material layer 210 that contact and overlap with the Josephson junction may be, for example, greater than or equal to 100. The bottom superconducting material layer 210 may have, for example, a reduced grain size or be amorphous. Although the bottom superconducting material layer 210 may have a reduced grain size or be amorphous, alternatively or additionally, the top superconducting material layer 230 may have a reduced grain size or be amorphous by alloying the superconducting material.

[0043] The small grain size of the superconducting material relative to the junction is advantageous. When the Josephson junction contains hundreds of superconducting grains overlapping the junction, the material variation between the junctions is substantially reduced. Under the constraint of reducing the superconducting grain size, the superconducting material is amorphous, and variability between the junctions is eliminated. Since there are no crystal planes in this amorphous superconducting material, the surface of the superconducting material is identical regardless of its surface angle; that is, the sidewalls and edges of the superconducting film have the same surface.

[0044] Superconducting alloys (e.g., aluminum alloys) may include aluminum and at least one alloying element. The alloying element may include Si, Mg, V, Nb, Ta, La, Zr, Hf, Ti, Zn, In, and / or Sn. The aluminum content in the aluminum alloy ranges from about 66% to about 97%. According to some embodiments, the alloyed aluminum can be formed via evaporation or sputtering.

[0045] In some embodiments, Mg can be used as an alloying element to form a light amorphous aluminum layer. According to some embodiments, the aluminum alloy can be formed having low contents of Si, Mg, V, Nb, Ta, La, Zr, Hf, Ti, Zn, In, and / or Sn. According to some embodiments, alloying elements for capturing oxygen in solution, such as low contents of Zr, Hf, and / or Ti, can be added.

[0046] Various embodiments have been described for illustrative purposes, but are not intended to be exhaustive or limited to the disclosed embodiments. Many modifications and variations will be apparent to those skilled in the art without departing from the scope and spirit of the described embodiments. The terminology used herein has been chosen to best explain the principles of the embodiments, their practical application, or technical improvements to technologies found in the market, or to enable those skilled in the art to understand the embodiments disclosed herein.

Claims

1. A superconducting circuit, comprising: Josephson junction device, including a lower superconducting material layer formed on a substrate; A junction layer is formed on the lower superconducting material layer; as well as An upper superconducting material layer is formed above the junction layer. Wherein, at least the lower superconducting material layer includes more than one grain, each grain having a size larger than the width and length of the Josephson junction of the Josephson junction device, wherein the Josephson junction is defined by the contact region where the lower superconducting material layer overlaps with the upper superconducting material layer and the junction layer.

2. The superconducting circuit according to claim 1, wherein, The lower superconducting material layer is formed from single-crystal grains.

3. The superconducting circuit according to claim 1 or 2, wherein, The lower superconducting material layer is epitaxially grown on a substrate.

4. The superconducting circuit according to claim 1 or 2, wherein, The lower superconducting material of the epitaxial layer is formed of aluminum.

5. The superconducting circuit according to claim 4, wherein, The aluminum is oriented such that the (110) plane of the aluminum is parallel to the substrate.

6. The superconducting circuit according to any one of claims 1, 2, and 5, wherein, The grains in the lower superconducting material layer are formed by increasing the size of the grains through annealing.

7. The superconducting circuit according to any one of claims 1, 2, and 5, wherein, The substrate includes at least one of silicon and sapphire.

8. A superconducting circuit, comprising: Josephson junction devices, including: Lower superconducting alloy layer; A junction layer is formed on the lower superconducting alloy layer; and An upper superconducting material layer is formed above the junction layer. The average grain size of the lower superconducting alloy layer is smaller than the width and length of the Josephson junction of the Josephson junction device and is less than 20 nm. The Josephson junction is defined by the contact area where the lower superconducting alloy layer overlaps with the upper superconducting material layer and the junction layer.

9. The superconducting circuit according to claim 8, wherein, The lower superconducting alloy layer is an aluminum alloy layer.

10. The superconducting circuit according to claim 9, wherein, The average grain size of the aluminum alloy layer is less than 10 nm.

11. The superconducting circuit according to any one of claims 8 to 10, wherein, The lower superconducting alloy layer is amorphous.

12. The superconducting circuit according to claim 9 or 10, wherein, The aluminum alloy comprises aluminum and at least one element selected from the group consisting of Si, Mg, V, Nb, Ta, La, Zr, Hf, Ti, Zn, In and Sn.

13. The superconducting circuit according to claim 9 or 10, wherein, The aluminum content in the aluminum alloy is in the range of 66% to 97%.

14. A method for forming a Josephson junction device, comprising: A lower superconducting material layer comprising more than one grain, a junction layer on the lower superconducting material layer, and an upper superconducting material layer above the junction layer are formed on a substrate. as well as The lower superconducting material layer is annealed such that a single grain of the lower superconducting material layer exceeds the width and length of the Josephson junction of the Josephson junction device, wherein the Josephson junction is defined by the contact region where the lower superconducting material layer overlaps with the upper superconducting material layer and the junction layer.

15. The method according to claim 14, wherein, The lower superconducting material layer is an aluminum layer.

16. The method according to claim 14 or 15, wherein, The individual grains are epitaxially aligned with the substrate.

17. The method according to claim 14 or 15, wherein, The substrate includes at least one of silicon and sapphire.

18. The method according to claim 14 or 15, wherein, The annealing is performed in a temperature range from 200°C to 570°C.

19. The method according to claim 18, wherein, The annealing is performed in a temperature range of 400°C to 570°C.

20. A method for forming a Josephson junction device, comprising: A lower superconducting alloy layer, a junction layer on the lower superconducting alloy layer, and an upper superconducting material layer above the junction layer are formed on a substrate. The average grain size of the lower superconducting alloy layer is smaller than the width and length of the Josephson junction and is less than 20 nm. The Josephson junction is defined by the contact area where the lower superconducting alloy layer overlaps with the upper superconducting material layer and the junction layer.

21. The method according to claim 20, wherein, The lower superconducting alloy layer is an aluminum alloy layer.

22. The method according to claim 20 or 21, wherein, The average grain size is less than 10 nm.

23. The method according to claim 20 or 21, wherein, The lower superconducting alloy layer is amorphous.

24. The method according to claim 21, wherein, The aluminum alloy layer is amorphous.

25. The method according to claim 21, wherein, The aluminum alloy layer comprises aluminum and at least one element selected from the group consisting of Si, Mg, V, Nb, Ta, La, Zr, Hf, Ti, Zn, In and Sn.

26. The method according to any one of claims 21, 24, and 25, wherein, The aluminum content in the aluminum alloy layer is in the range of 66% to 97%.

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