Deep submicron Josephson tunnel junction and preparation method thereof

Abstract

The present invention provides a deep submicron Josephson tunnel junction and a preparation method thereof, the structure comprising: a substrate; a Josephson tunnel junction extending in a lateral direction and formed on the upper surface of the substrate, the Josephson tunnel junction including a first electrode, a barrier layer and a second electrode from left to right; a first electrode lead-out structure formed integrally with the first electrode; and a second electrode lead-out structure formed integrally with the second electrode. By preparing a three-layer film of the Josephson tunnel junction in a lateral direction (i.e., in a horizontal direction) on the substrate, a Josephson tunnel junction extending in a lateral direction is formed, the size of the junction is not limited by the limitation of the photolithography limit, and the film thickness can be used as a side length of the junction, which will greatly reduce the junction area; in addition, since the three-layer film structure is prepared separately, and its electrode lead-out structure is formed integrally with the corresponding electrode, the insulating layer and wiring layer required in the traditional process are reduced, the process flow is simplified, the process cycle is shortened, and the process controllability is improved.

Description

Technical Field

[0001] The invention belongs to the field of superconducting electronics, and in particular relates to a deep submicron Josephson tunnel junction and a preparation method thereof. Background Art

[0002] The Josephson tunnel junction is a quantum element based on the Josephson effect and is the core component of most superconducting quantum devices. Structurally, the Josephson tunnel junction is a "sandwich" structure of superconductor-insulator-superconductor (SIS), in which the superconducting films on both sides serve as the top electrode and the bottom electrode respectively. Many superconducting quantum devices such as superconducting quantum interference devices (SQUIDs) and single flux quantum circuits (SFQs) are based on Josephson tunnel junctions to achieve specific device functions. For SQUID, one or two Josephson junctions are included, and the Josephson junction parameters directly determine the SQUID performance. For example, the junction resistance and junction capacitance directly determine the SQUID noise and energy resolution. From the perspective of SQUID design, the junction capacitance is required to be as small as possible. For SFQ, the number of Josephson junctions can reach tens of thousands or even hundreds of thousands. In order to improve the integration and meet the needs of high-speed digital circuits, the size of the Josephson junction is also required to be reduced.

[0003] With the development of semiconductor technology, the preparation level of superconducting devices has also been greatly improved. The introduction of high-precision lithography technologies such as electron beam lithography (EBL), laser direct writing and focused ion beam (FIB) has made it possible to reduce the size of the Josephson junction to the deep submicron level. However, considering the preparation cycle and mass production requirements, the stepper is still the most commonly used device in the preparation of Josephson tunnel junctions. Figure 1 As shown, the Josephson tunnel junction is usually realized by longitudinally stacking three layers of film, that is, along the longitudinal direction of the substrate, it includes two layers of superconducting material 1 and a thin film insulating layer 2 located between the two layers of superconducting material 1. The size of the layer depends largely on the photolithography accuracy of the exposure machine and the control of the etching depth. Limited by the limit resolution of the stepper, the junction size is difficult to reach the deep submicron level. In addition, this structure needs to consider the electrode lead 3 of the junction top electrode. It is necessary to etch an electrode lead hole 5 smaller than the junction area size on the insulating layer 4 used to isolate the junction top electrode 1 and the lead, which further limits the reduction of the size of the Josephson tunnel junction. Summary of the invention

[0004] In view of the above-mentioned shortcomings of the prior art, an object of the present invention is to provide a deep submicron Josephson tunnel junction and a preparation method thereof, so as to solve the above-mentioned problems existing in the prior art.

[0005] To achieve the above-mentioned object and other related objects, the present invention provides a method for preparing a deep submicron Josephson tunnel junction, the method comprising:

[0006] 1) providing a substrate, and forming a first superconducting material layer on the upper surface of the substrate;

[0007] 2) etching and removing a portion of the first superconducting material layer, and the remaining first superconducting material layer serves as the first electrode and the first electrode lead-out structure of the Josephson tunnel junction;

[0008] 3) forming an insulating layer on the surface of the structure obtained in step 2), and etching away the insulating layer covering the first superconducting material layer and a portion of the substrate, and retaining the insulating layer on the side of the first superconducting material layer to form a barrier layer of the Josephson tunnel junction;

[0009] 4) forming a second superconducting material layer on the surface of the structure obtained in step 3), and etching away the second superconducting material layer covering the first superconducting material layer, the insulating layer and a portion of the substrate, retaining the second superconducting material layer on the side of the insulating layer, and making the retained second superconducting material layer serve as the second electrode and second electrode lead-out structure of the Josephson tunnel junction, thereby forming the Josephson tunnel junction extending in the lateral direction.

[0010] Optionally, in step 2), the angle between the etched side surface of the first electrode and the substrate is an acute angle.

[0011] Optionally, in step 2), the angle between the etched side surface of the first electrode and the substrate is between 45° and 60°.

[0012] Optionally, before performing step 3), the method further includes performing ion beam cleaning on the surface of the structure obtained in step 2).

[0013] The present invention also provides another method for preparing a deep submicron Josephson tunnel junction, the method comprising:

[0014] 1) providing a substrate, and forming a first superconducting material layer on the upper surface of the substrate;

[0015] 2) etching and removing a portion of the first superconducting material layer, and the remaining first superconducting material layer serves as the first electrode and the first electrode lead-out structure of the Josephson tunnel junction;

[0016] 3) continuously forming an insulating layer and a second superconducting material layer on the surface of the structure obtained in step 2);

[0017] 4) continuously etching the second superconducting material layer and the insulating layer, retaining the insulating layer and the second superconducting material layer on the side of the first superconducting material layer, and making the retained second superconducting material layer serve as the second electrode and second electrode lead-out structure of the Josephson tunnel junction, and retaining the insulating layer that is in contact with the first superconducting material layer and the second superconducting material layer at the same time as the barrier layer of the Josephson tunnel junction, thereby forming the Josephson tunnel junction extending in the lateral direction.

[0018] Optionally, in step 2), the angle between the etched side surface of the first electrode and the substrate is an acute angle.

[0019] Optionally, in step 2), the angle between the etched side surface of the first electrode and the substrate is between 45° and 60°.

[0020] Optionally, before performing step 3), the method further includes performing ion beam cleaning on the surface of the structure obtained in step 2).

[0021] The present invention also provides a deep submicron Josephson tunnel junction, the Josephson tunnel junction comprising:

[0022] substrate;

[0023] A Josephson tunnel junction is formed on the upper surface of the substrate and extends in a lateral direction. The Josephson tunnel junction includes a first electrode, a barrier layer and a second electrode from left to right.

[0024] A first electrode lead-out structure, formed integrally with the first electrode;

[0025] The second electrode lead-out structure is formed integrally with the second electrode.

[0026] Optionally, a height of the first electrode of the Josephson tunnel junction is less than 0.5 μm.

[0027] Optionally, a height of the first electrode of the Josephson tunnel junction is between 100 nm and 150 nm.

[0028] Optionally, the length of the barrier layer is less than 10 nm.

[0029] Optionally, the second electrode is formed on an upper surface of the barrier layer.

[0030] As described above, the deep submicron Josephson tunnel junction and the preparation method thereof of the present invention form a Josephson tunnel junction extending in the lateral direction by preparing three layers of film of the Josephson tunnel junction on a substrate in the lateral direction (i.e., in the horizontal direction). The size of the junction is not limited by the limitation of the photolithography limit, and the film thickness can be used as the length of one side of the junction, which will greatly reduce the junction area. In addition, since the three-layer film structure is prepared separately and its electrode lead-out structure is integrally formed with the corresponding electrode, the insulating layer and wiring layer required in the traditional process are reduced, the process flow is simplified, the process cycle is shortened, and the process controllability is improved. BRIEF DESCRIPTION OF THE DRAWINGS

[0031] Figure 1 Shown is a schematic diagram of the cross-sectional structure of an existing Josephson tunnel junction.

[0032] Figure 2 Shown is a process flow chart of the method for preparing a deep submicron Josephson tunnel junction of the present invention.

[0033] Figure 3 It is a schematic structural diagram presented in step S1 of the method for preparing a deep submicron Josephson tunnel junction according to the first embodiment of the present invention.

[0034] Figure 4 It is a schematic structural diagram of step S2 in the method for preparing a deep submicron Josephson tunnel junction according to the first embodiment of the present invention.

[0035] Figure 5 It is a schematic diagram of a top view of the structure presented in step S2 of the method for preparing a deep submicron Josephson tunnel junction according to the first embodiment of the present invention.

[0036] Figure 6 It is a schematic structural diagram presented in step S3 of the method for preparing a deep submicron Josephson tunnel junction according to the first embodiment of the present invention.

[0037] Figure 7 It is a schematic diagram of a top view of the structure presented in step S3 of the method for preparing a deep submicron Josephson tunnel junction according to the first embodiment of the present invention.

[0038] Figure 8 It is a schematic structural diagram presented in step S4 of the method for preparing a deep submicron Josephson tunnel junction according to the first embodiment of the present invention.

[0039] Fig. 9 It is a schematic diagram of a top view of the structure presented in step S4 in the method for preparing a deep submicron Josephson tunnel junction according to the first embodiment of the present invention.

[0040] Fig.10 It is a schematic structural diagram of step S4 in the method for preparing a deep submicron Josephson tunnel junction according to the second embodiment of the present invention.

[0041] Fig.11 It is a schematic diagram of a top view of the structure presented in step S4 in the method for preparing a deep submicron Josephson tunnel junction according to the second embodiment of the present invention.

[0042] Component number description

[0043] 1 Superconducting material layer

[0044] 2 Thin film insulation layer

[0045] 3. Electrode extraction

[0046] 4 Insulation layer

[0047] 5 Electrode lead hole

[0048] 10 Substrate

[0049] 11. First superconducting material layer

[0050] 12. First electrode

[0051] 13. First electrode lead structure

[0052] 14 Insulation layer

[0053] 15 Barrier layer

[0054] 16 Second superconducting material layer

[0055] 17. Second electrode

[0056] 18. Second electrode lead structure

[0057] θ Angle

[0058] Steps S1 to S4 DETAILED DESCRIPTION

[0059] The following describes the embodiments of the present invention through specific examples, and those skilled in the art can easily understand other advantages and effects of the present invention from the contents disclosed in this specification. The present invention can also be implemented or applied through other different specific embodiments, and the details in this specification can also be modified or changed in various ways based on different viewpoints and applications without departing from the spirit of the present invention.

[0060] See also Figures 1 to 11 It should be noted that the illustrations provided in this embodiment are only used to illustrate the basic concept of the present invention in a schematic manner, and the illustrations only show components related to the present invention rather than being drawn according to the number, shape and size of components in actual implementation. In actual implementation, the type, quantity and proportion of each component may be changed arbitrarily, and the component layout may also be more complicated.

[0061] Embodiment 1

[0062] The present embodiment provides a method for preparing a deep submicron Josephson tunnel junction, wherein a three-layer film of a Josephson tunnel junction is prepared on a substrate along a lateral direction (i.e., along a horizontal direction), thereby forming a Josephson tunnel junction extending along a lateral direction. The size of the junction is not limited by the limitation of the photolithography limit, and the film thickness can be used as a side length of the junction, which will greatly reduce the junction area; in addition, since the three-layer film structure is prepared separately, and its electrode lead-out structure is integrally formed with the corresponding electrode, the insulating layer and wiring layer required in the traditional process are reduced, the process flow is simplified, the process cycle is shortened, and the process controllability is improved.

[0063] like Figures 2 to 9 As shown, the preparation method comprises the following steps:

[0064] like Figure 2 and Figure 3 As shown, step S1 is first performed to provide a substrate 10 and form a first superconducting material layer 11 on the upper surface of the substrate 10 .

[0065] As an example, the substrate 10 may be, but not limited to, a silicon substrate, a magnesium oxide (MgO) substrate, etc.; the material of the first superconducting material layer 11 may include, but not limited to, Nb (niobium) or NbN (niobium nitride). Preferably, a suitable material of the substrate 10 is selected based on the material of the first superconducting material layer 11. For example, when the first superconducting material layer 11 is Nb, the material of the substrate 10 is suitably a silicon substrate; when the first superconducting material layer 11 is NbN, the material of the substrate 10 is suitably a single crystal MgO substrate with a (100) crystal orientation.

[0066] The present embodiment does not limit the method for forming the first superconducting material layer 11, and any existing suitable thin film forming method can be used, such as magnetron sputtering, etc. In the present embodiment, it is preferred to use DC magnetron sputtering to form the first superconducting material layer 11. The deposition height of the first superconducting material layer 11 can be determined according to the size of the Josephson tunnel junction to be formed later. This is because the deposition height of the first superconducting material layer 11 will directly affect the length of one side of the Josephson tunnel junction to be formed later (such as Figure 8 As shown in the figure, the deposition height of the first superconducting material layer 11 can be selected to be less than 0.5μm to achieve a deep submicron level, for example, it can be 0.5μm, 0.4μm, 0.3μm, 0.2μm, 0.1μm or other sizes. When considering the good performance of superconducting materials applied to Josephson tunnel junctions, in this embodiment, it is preferred that the deposition height of the first superconducting material layer 11 is between 100nm and 150nm.

[0067] like Figure 2 , Figure 4 and Figure 5 As shown, step S2 is then performed to etch and remove part of the first superconducting material layer 11, and the remaining first superconducting material layer 11 is used as the first electrode 12 of the Josephson tunnel junction and the first electrode lead-out structure 13. Through this step, the first electrode 12 and the first electrode lead-out structure 13 of the Josephson tunnel junction can be formed simultaneously.

[0068] This embodiment does not limit the method for etching the first superconducting material layer 11, and any suitable thin film etching method can be used, such as dry etching or wet etching. This embodiment preferably uses reactive ion etching (RIE) technology for etching. It should be noted that different reaction gases are used for etching different material layers. For example, when the material of the first superconducting material layer 11 is Nb, the etching gas can be CF. 4 (carbon tetrafluoride).

[0069] like Figure 4 As shown, as an example, in order to improve the contact stability between the subsequent deposited material and the first superconducting material layer 11 and improve the coverage yield of the subsequent deposited material, the angle θ between the etched side surface of the first superconducting material layer 11 retained after etching (i.e., the etched side surface in contact with the subsequent deposited material) and the substrate 10 can be set to an acute angle, for example, 85°, 75°, 65°, 55°, 45°, etc. Preferably, considering the coverage yield of the subsequent deposited material and the area of ​​the Josephson tunnel junction, the angle θ is selected to be between 45° and 60°.

[0070] like Figure 2 , Figure 6 and Figure 7 As shown, step S3 is then performed to form an insulating layer 14 on the surface of the structure obtained in step S2, and to etch away the insulating layer 14 covering the first superconducting material layer 11 and a portion of the substrate 10, while retaining the insulating layer 14 on the side of the first superconducting material layer 11 to form a barrier layer 15 of the Josephson tunnel junction.

[0071] As an example, before implementing this step, the surface of the structure obtained in step S2 may be cleaned with an ion beam to remove impurities such as an oxide layer and particles formed on the surface of the structure.

[0072] The length of the insulating layer 14 in the lateral direction is related to its longitudinal deposition thickness. As an example, the length of the insulating layer 14 in the lateral direction remaining after etching is less than 10 nm.

[0073] As an example, the material of the insulating layer 14 may be, but is not limited to, Al (aluminum)-AlOx (aluminum oxide) or AlN (aluminum nitride), or may be other suitable insulating materials. The method of forming the insulating layer 14 is not limited either. In this embodiment, the insulating layer 14 is preferably formed by a magnetron sputtering method, and Al (aluminum)-AlOx (aluminum oxide) is used as an example for explanation. Specifically, a layer of Al thin film is first sputtered to form a layer, and then the Al thin film layer is oxidized in an oxygen atmosphere to obtain an Al-AlOx insulating layer.

[0074] like Figure 2 , Figure 8 and Fig. 9 As shown, step S4 is finally performed to form a second superconducting material layer 16 on the surface of the structure obtained in step S3, and the second superconducting material layer 16 covering the first superconducting material layer 11, the insulating layer 14 and part of the substrate 10 is removed by etching, and the second superconducting material layer 16 on the side of the insulating layer 14 is retained, and the retained second superconducting material layer 16 is used as the second electrode 17 and the second electrode lead-out structure 18 of the Josephson tunnel junction, thereby forming the Josephson tunnel junction extending in the lateral direction. Through this step, the second electrode 17 and the second electrode lead-out structure 18 of the Josephson tunnel junction can be formed at the same time, so that there is no need to additionally form an insulating protection layer, a wiring layer and a photolithography opening process to realize the lead-out of the electrode, which simplifies the process flow and also makes the junction size not limited by the photolithography limit.

[0075] As an example, the material of the second superconducting material layer 16 may include but is not limited to Nb (niobium) or NbN (niobium nitride), and the materials of the first superconducting material layer 11 and the second superconducting material layer 16 may be the same or different, depending on the specific situation.

[0076] This embodiment does not limit the method for forming the second superconducting material layer 16, and any existing suitable thin film forming method can be used, such as magnetron sputtering, etc. In this embodiment, DC magnetron sputtering is preferably used to form the second superconducting material layer 16. The deposition thickness of the second superconducting material layer 16 can be less than or equal to the deposition thickness of the first superconducting material layer 11, and the contact area between the three thin films of the first superconducting material layer 11, the second superconducting material layer 16 and the barrier layer 15 determines the size of the Josephson tunnel junction.

[0077] Embodiment 2

[0078] This embodiment provides a method for preparing a deep submicron Josephson tunnel junction. The method is basically the same as the method for preparing a deep submicron Josephson tunnel junction in the first embodiment, except for step S3 and step S4. Specifically:

[0079] After forming the first electrode 12 and the first electrode lead structure 13 in step S2, in step S3, an insulating layer 14 and a second superconducting material layer 16 are continuously formed on the surface of the structure obtained in step S2. The method and thickness of forming the insulating layer 14 and the second superconducting material layer 16 can be referred to in the first embodiment, and will not be described again.

[0080] like Fig.10 and Fig.11 As shown, step S4 is finally performed to continuously etch the first superconducting material layer 11 and the second superconducting material layer 16 and the insulating layer 14 on a portion of the substrate 10, retaining the insulating layer 14 and the second superconducting material layer 16 on the side of the first superconducting material layer 11, and making the retained second superconducting material layer 16 serve as the second electrode 17 and the second electrode lead-out structure 18 of the Josephson tunnel junction, and the retained insulating layer 14 that is in contact with the first superconducting material layer 11 and the second superconducting material layer 16 serves as the barrier layer 15 of the Josephson tunnel junction, thereby forming the Josephson tunnel junction extending in the lateral direction.

[0081] The preparation method of this embodiment adopts continuous deposition and then continuous etching of the insulating layer 14 and the second superconducting material layer 16, so that the Josephson tunnel junction can be formed by only two photolithography processes, which further simplifies the process. At the same time, the structure of the formed Josephson tunnel junction is basically the same as that of the first embodiment (because the length of the barrier layer is very thin relative to the superconducting material layer, as described in the first embodiment, the length of the barrier layer is generally less than 10nm), that is, the Josephson tunnel junction formed in this embodiment also extends in the lateral direction.

[0082] Embodiment 3

[0083] This embodiment provides a deep submicron Josephson tunnel junction, which can be prepared by the preparation method of the above-mentioned embodiment 1 or embodiment 2, but is not limited to the preparation method described in embodiment 1 and embodiment 2, as long as the Josephson tunnel junction can be formed. The beneficial effects that can be achieved by the Josephson tunnel junction can also be referred to in embodiment 1 and embodiment 2.

[0084] like Figures 8 to 11 As shown, the Josephson tunnel junction includes:

[0085] Substrate 10;

[0086] A Josephson tunnel junction is formed on the upper surface of the substrate 10 and extends in a lateral direction. The Josephson tunnel junction includes a first electrode 12, a barrier layer 15 and a second electrode 17 from left to right.

[0087] A first electrode lead-out structure 13, formed integrally with the first electrode 12;

[0088] The second electrode lead-out structure 18 is formed integrally with the second electrode 17 .

[0089] By forming the Josephson tunnel junction along the lateral direction on the substrate, the thickness of the film can be used as the length of one side of the junction, and the size of the junction will not be limited by the limitation of lithography, thereby greatly reducing the junction area of ​​the Josephson tunnel junction.

[0090] like Figure 8 and Fig. 9 As shown, as an example, the height of the first electrode 17 of the Josephson tunnel junction is less than 0.5 μm, for example, it can be 0.5 μm, 0.4 μm, 0.3 μm, 0.2 μm, 0.1 μm or other sizes, and when considering the good performance of superconducting materials applied to Josephson tunnel junctions, in this embodiment, it is preferred that the height of the first electrode 17 is between 100 nm and 150 nm. Preferably, in order to increase the effective area of ​​the Josephson tunnel junction, the height of the second electrode 17, the height of the barrier layer 15 and the height of the first electrode 12 can be made substantially the same.

[0091] like Figure 8 As shown, as an example, the length of the barrier layer 15 is less than 10 nm.

[0092] like Fig.10 and Fig.11 As shown, as an example, the second electrode 17 is formed on the upper surface of the barrier layer 15 .

[0093] In summary, the present invention provides a deep submicron Josephson tunnel junction and a preparation method thereof. By preparing a three-layer film of a Josephson tunnel junction on a substrate in a lateral direction (i.e., in a horizontal direction), a Josephson tunnel junction extending in a lateral direction is formed. The size of the junction is not limited by the limitation of the photolithography limit, and the film thickness can be used as a side length of the junction, which will greatly reduce the junction area. In addition, since the three-layer film structure is prepared separately, and its electrode lead-out structure is integrally formed with the corresponding electrode, the insulating layer and wiring layer required in the traditional process are reduced, the process flow is simplified, the process cycle is shortened, and the process controllability is improved. Therefore, the present invention effectively overcomes various shortcomings in the prior art and has a high industrial utilization value.

[0094] The above embodiments are merely illustrative of the principles and effects of the present invention, and are not intended to limit the present invention. Anyone familiar with the art may modify or alter the above embodiments without departing from the spirit and scope of the present invention. Therefore, all equivalent modifications or alterations made by a person of ordinary skill in the art without departing from the spirit and technical concept disclosed by the present invention shall still be covered by the claims of the present invention.

Claims

1. A method for preparing a deep submicron Josephson tunnel junction, characterized in that: The preparation method comprises: 1) providing a substrate, and forming a first superconducting material layer on the upper surface of the substrate; 2) etching and removing a portion of the first superconducting material layer, and retaining the first superconducting material layer as the first electrode and the first electrode lead-out structure of the Josephson tunnel junction, wherein the angle between the etched side surface of the first electrode and the substrate is an acute angle; 3) forming an insulating layer on the surface of the structure obtained in step 2), and etching away the insulating layer covering the first superconducting material layer and a portion of the substrate, and retaining the insulating layer on the side of the first superconducting material layer to form a barrier layer of the Josephson tunnel junction; 4) forming a second superconducting material layer on the surface of the structure obtained in step 3), and etching away the second superconducting material layer covering the first superconducting material layer, the insulating layer and a portion of the substrate, retaining the second superconducting material layer on the side of the insulating layer, and making the retained second superconducting material layer serve as the second electrode and second electrode lead-out structure of the Josephson tunnel junction, thereby forming the Josephson tunnel junction extending in the lateral direction.

2. The method for preparing a deep submicron Josephson tunnel junction according to claim 1, characterized in that: In step 2), the angle between the etched side surface of the first electrode and the substrate is between 45° and 60°.

3. The method for preparing a deep submicron Josephson tunnel junction according to claim 1, characterized in that: Before performing step 3), the method further includes performing ion beam cleaning on the surface of the structure obtained in step 2).

4. A method for preparing a deep submicron Josephson tunnel junction, characterized in that: The preparation method comprises: 1) providing a substrate, and forming a first superconducting material layer on the upper surface of the substrate; 2) etching and removing a portion of the first superconducting material layer, and retaining the first superconducting material layer as the first electrode and the first electrode lead-out structure of the Josephson tunnel junction, wherein the angle between the etched side surface of the first electrode and the substrate is an acute angle; 3) continuously forming an insulating layer and a second superconducting material layer on the surface of the structure obtained in step 2); 4) continuously etching the second superconducting material layer and the insulating layer, retaining the insulating layer and the second superconducting material layer on the side of the first superconducting material layer, and making the retained second superconducting material layer serve as the second electrode and second electrode lead-out structure of the Josephson tunnel junction, and retaining the insulating layer that is in contact with the first superconducting material layer and the second superconducting material layer at the same time as the barrier layer of the Josephson tunnel junction, thereby forming the Josephson tunnel junction extending in the lateral direction.

5. The method for preparing a deep submicron Josephson tunnel junction according to claim 4, characterized in that: In step 2), the angle between the etched side surface of the first electrode and the substrate is between 45° and 60°.

6. The method for preparing a deep submicron Josephson tunnel junction according to claim 4, characterized in that: Before performing step 3), the method further includes performing ion beam cleaning on the surface of the structure obtained in step 2).

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