Graphene-based magnetic tunnel junction device with high magnetoresistance change rate and preparation method thereof

Abstract

The invention discloses a graphene-based magnetic tunnel junction device with a high magnetic resistance change rate and a preparation method thereof. The graphene-based magnetic tunnel junction device comprises a ferromagnetic substrate, a graphene layer and an insulating film layer which are sequentially arranged from top to bottom, the insulating film layer comprises a nanopore array, an upper ferromagnetic layer and a conductive metal film, the upper ferromagnetic layer and the conductive metal film are located in the nanopore array, the upper ferromagnetic layer is located between the graphene layer and the conductive metal film, the pore size of the nanopore array is 10-200 nm, the thickness of the insulating film layer is 50-300 nm, the magnetic resistance change rate is high, the preparation process is simple, and the cost is low. According to the preparation method, the problem of short circuit of the nano junction region is avoided, and the magnetic resistance change rate of the device is remarkably improved.

Description

Technical Field

[0001] The present invention relates to the technical field of tunnel junction devices, and in particular to a graphene-based magnetic tunnel junction device with a high magnetoresistance change rate and a preparation method thereof. Background Art

[0002] With the rapid development of new sensitive materials such as nanomaterials and two-dimensional materials, the performance and functions of sensors have been significantly improved. For example, the strong spin filtering effect between the interface of graphene and single-crystal ferromagnetic metals can theoretically greatly improve the magnetoresistance change rate of the magnetic tunnel junction. However, grain boundaries and defects in materials such as graphene and ferromagnetic metals are inevitable. In order to avoid these areas, the size of the magnetic tunnel junction is usually designed to be at the nanometer level. Although existing micro-nano processes such as high-resolution electron beam lithography, ion beam etching, and evaporation electrodes can realize the preparation of sensor nanoarray sensitive elements, when processing extremely thin new nano and graphene magnetic tunnel junctions, it is easy to cause short circuits in the junction area, which in turn affects the magnetoresistance change rate of the actual device and limits the performance of the device.

[0003] In the prior art, ion beam etching is usually used for tunnel junction preparation, such as the IBE etcher and etching method of the dual ion source disclosed in Chinese patent application number CN201910875909.4. When the thickness of the insulating layer is thick, the metal particles attached to the sidewall can be effectively removed to avoid conduction. However, the thickness of the graphene insulating layer of the tunnel junction device is less than 1nm. Even after using inclined etching, it is still easy to cause the short circuit problem in the nanojunction area, because during the etching process, due to the impact of high-energy ions or neutral particles on the surface of the material, atoms or molecules on the surface of the material are knocked out, forming sputtering. When the ions in the plasma hit the surface of the silicon wafer at high speed, they can not only remove the material on the surface, but also make some atoms on the surface obtain enough kinetic energy, thereby detaching from the surface of the material and splashing into the nanojunction area, causing a short circuit. Therefore, if this technical solution is used to prepare an extremely thin new nanometer and graphene magnetic tunnel junction sensor, it is still difficult to completely solve the problem of short circuit in the nanojunction area, resulting in a low TMR (tunnel magnetoresistance effect, magnetoresistance change rate) of the device and a high scrap rate. Summary of the invention

[0004] The technical problem to be solved by the present invention is to overcome the deficiencies of the prior art and provide a graphene-based magnetic tunnel junction device with a high magnetoresistance change rate and a preparation method thereof.

[0005] In order to solve the above technical problems, the present invention adopts the following technical solutions:

[0006] A graphene nano-magnetic tunnel junction device with a high magnetoresistance change rate comprises a ferromagnetic substrate, a graphene layer and an insulating film layer which are arranged in sequence from top to bottom, wherein the insulating film layer comprises a nano-hole array and an upper ferromagnetic layer and a conductive metal film located in the nano-hole array, wherein the upper ferromagnetic layer is located between the graphene layer and the conductive metal film, the aperture size of the nano-hole array is 10 to 200 nm, and the thickness of the insulating film layer is 50 to 300 nm.

[0007] As a further improvement of the above technical solution:

[0008] The ferromagnetic substrate is a Ni substrate. Preferably, the upper ferromagnetic layer is a Ni film with a thickness of 10nm to 100nm; and the conductive metal film is an Au film with a thickness of 50nm to 200nm.

[0009] A method for preparing a graphene nano-magnetic tunnel junction device with a high magnetoresistance change rate comprises the following steps:

[0010] A1. growing a graphene layer on a ferromagnetic substrate to obtain a graphene sensitive material substrate;

[0011] A2, transferring the insulating film with the nanopore array to a graphene sensitive material substrate to obtain a graphene sensitive material substrate with an insulating film, wherein the pore size of the nanopore array in the insulating film is 10 to 200 nm, and the thickness of the insulating film is 50 nm to 300 nm;

[0012] A3, preparing an upper ferromagnetic layer and a conductive metal film on the surface of the graphene sensitive material substrate having an insulating film, to form a thin film material having an upper ferromagnetic layer and a conductive metal film;

[0013] A4. Remove the upper ferromagnetic layer and the conductive metal film on the thin film material except inside the nanohole array holes to obtain a graphene nanomagnetic tunnel junction device.

[0014] As a further improvement of the above technical solution:

[0015] In step A1, a graphene layer is grown on a ferromagnetic substrate by chemical vapor deposition.

[0016] In step A2, the insulating film is transferred to the graphene sensitive material substrate by using a mask transfer method.

[0017] In step A2, the insulating film is an anodic aluminum oxide or silicon oxide or silicon nitride insulating film.

[0018] In step A3, the upper ferromagnetic layer and the conductive metal film are prepared by molecular beam epitaxy, wherein the upper ferromagnetic layer is a Ni film and the conductive metal film is an Au film.

[0019] In step A3, the molecular beam epitaxy method comprises the following steps:

[0020] B1. First, grow a Ni film at a growth rate of 0.1-1 nm / min until the thickness reaches 10 nm-100 nm;

[0021] B2. Grow an Au film at a growth rate of 4 to 10 nm / min until the thickness reaches 50 nm to 200 nm.

[0022] In step B1, during the growth of the Ni film, the graphene sensitive material substrate is rotated.

[0023] In step A4, the upper ferromagnetic layer and the conductive metal film outside the nanohole array holes are removed by tilted ion beam etching, and the ion beam tilt angle during the tilted ion beam etching is 60° to 85°.

[0024] Compared with the prior art, the advantages of the present invention are:

[0025] The graphene-based magnetic tunnel junction device with a high magnetoresistance change rate of the present invention comprises a ferromagnetic substrate, a graphene layer and an insulating film layer arranged in sequence from top to bottom, wherein the insulating film layer comprises a nanohole array and a conductive metal film located in the nanohole array, the pore size of the nanohole array is 10 to 200 nm, the thickness of the insulating film layer is 50 nm to 300 nm, the magnetoresistance change rate is high, and the preparation process is simple.

[0026] The preparation method of the graphene-based magnetic tunnel junction device with a high magnetoresistance change rate of the present invention comprises the following steps: firstly growing a graphene layer on a ferromagnetic substrate layer to obtain a graphene sensitive material substrate, then transferring an insulating film with a nanohole array to the graphene sensitive material substrate, patterning the graphene sensitive material substrate, then growing a conductive metal film, and finally removing the conductive metal film outside the nanohole array holes to obtain a graphene nanomagnetic tunnel junction device, thereby avoiding the short circuit problem in the nanojunction area and significantly improving the magnetoresistance change rate of the device. BRIEF DESCRIPTION OF THE DRAWINGS

[0027] Figure 1 is a brief schematic diagram of each step of the preparation method in Example 1 of the present invention, Figure 1 (a) shows the AAO membrane with a porous structure transferred onto a Ni / graphene substrate. Figure 1 (b) Ni and Au are deposited on the transferred substrate. Figure 1 (c) polishing by ion beam etching, Figure 1 (d) is the obtained graphene-based magnetic tunnel junction device.

[0028] Figure 2 It is a schematic diagram of the structure of the graphene nanomagnetic tunnel junction device in Example 1 of the present invention.

[0029] Figure 3 It is a schematic diagram of the product structure obtained in each step of the preparation method in Example 1 of the present invention.

[0030] Figure 4 This is a SEM image of the graphene sensitive material substrate with an insulating film obtained after step S2 in Example 1 of the present invention.

[0031] Figure 5 This is a graph showing the magnetoresistance curve measurement results of the graphene tunnel junction micrometer array device in Example 1 of the present invention.

[0032] Figure 6 It is a graph showing the magnetoresistance curve measurement results of the graphene nanomagnetic tunnel junction device prepared in Comparative Example 1 of the present invention.

[0033] Figure 7 It is a SEM picture of the graphene nanomagnetic tunnel junction device prepared in Comparative Example 2 of the present invention.

[0034] Figure 8 This is a graph showing the magnetoresistance curve measurement results of the graphene nanomagnetic tunnel junction device prepared in Comparative Example 2 of the present invention.

[0035] Fig. 9 It is a brief schematic diagram of each step of the preparation process in Comparative Example 3 of the present invention.

[0036] Fig.10 This is a graph showing the magnetoresistance curve measurement results of the graphene nanomagnetic tunnel junction device of comparative example 3 of the present invention.

[0037] The numbers in the figure represent: 1, ferromagnetic substrate; 2, graphene layer; 3, insulating film layer; 31, nanohole array; 32, upper ferromagnetic layer; 33, conductive metal film. DETAILED DESCRIPTION

[0038] The present invention will be described in further detail below. Unless otherwise specified, the instruments or materials used in the present invention are commercially available.

[0039] like Figure 2 As shown, the graphene nanomagnetic tunnel junction device with high magnetoresistance change rate of the present invention includes a ferromagnetic substrate 1, a graphene layer 2 and an insulating film layer 3 arranged in sequence from top to bottom, the insulating film layer 3 includes a nanohole array 31 and an upper ferromagnetic layer 32 and a conductive metal film 33 located in the nanohole array 31, the upper ferromagnetic layer 32 is located between the graphene layer 2 and the conductive metal film 33, the pore size of the nanohole array 31 is 10 to 200 nm, and the thickness of the insulating film layer 3 is 50 nm to 300 nm.

[0040] like Figure 1 As shown, the method for preparing a graphene nano-magnetic tunnel junction device with a high magnetoresistance change rate of the present invention comprises the following steps:

[0041] A1, growing a graphene layer 2 on a ferromagnetic substrate 1 to obtain a graphene sensitive material substrate;

[0042] A2, transferring the insulating film with the nanohole array 31 to the graphene sensitive material substrate to obtain a graphene sensitive material substrate with an insulating film layer, wherein the pore size of the nanohole array 31 in the insulating film is 10 to 200 nm, and the thickness of the insulating film is 50 nm to 300 nm;

[0043] A3, preparing an upper ferromagnetic layer 32 and a conductive metal film 33 on the surface of the graphene sensitive material substrate having an insulating film, to form a thin film material having an upper ferromagnetic layer 32 and a conductive metal film 33;

[0044] A4. Remove the upper ferromagnetic layer 32 and the conductive metal film 33 on the thin film material except inside the holes of the nanohole array 31 to obtain a graphene nanomagnetic tunnel junction device.

[0045] Example 1

[0046] The graphene nano-magnetic tunnel junction device with high magnetoresistance change rate of this embodiment comprises a ferromagnetic substrate 1, a graphene layer 2 and an insulating film layer 3 arranged in sequence from top to bottom, the insulating film layer 3 comprises a nanohole array 31 and an upper ferromagnetic layer 32 and a conductive metal film 33 located in the nanohole array 31, the aperture size of the nanohole array 31 is 80nm, the thickness of the insulating film layer 3 is 130nm, and the sum of the thickness of the upper ferromagnetic layer 32 and the conductive metal film 33 is 150nm. In this embodiment, the ferromagnetic substrate 1 is a Ni substrate.

[0047] like Figure 1 As shown, the method for preparing a graphene-based magnetic tunnel junction device with a high magnetoresistance change rate of this embodiment includes the following steps:

[0048] S1, using a Ni (111) crystal ferromagnetic substrate as a ferromagnetic substrate 1, and growing a fully covered graphene layer 2 on the ferromagnetic substrate 1 by a conventional CVD method to obtain a graphene layer as shown in FIG. Figure 3 (a) The graphene sensitive material substrate shown;

[0049] S2, using an anodic aluminum oxide insulating film with a nanopore array as an insulating film (AAO film), and using a mask transfer technique to transfer the insulating film to a graphene sensitive material substrate, to obtain Figure 3 (b) and Figure 4 The graphene sensitive material substrate with insulating film is shown.

[0050] In this embodiment, the dimensions of the AAO membrane are: pore center distance 125nm, pore diameter 80nm and membrane thickness 130nm. The AAO membrane is soaked in acetone several times before use to remove organic matter on the AAO membrane. In other embodiments, the pore size of the nanopore array in the insulating membrane is 10 to 200nm (preferably 70 to 100nm), and the thickness of the insulating membrane is 50nm to 300nm, which can also achieve the same or similar technical effects.

[0051] S3. The upper ferromagnetic layer 32 and the conductive metal film 33 are grown by molecular beam epitaxy. The sum of the thicknesses of the upper ferromagnetic layer 32 and the conductive metal film 33 in this embodiment is 150 nm.

[0052] A1. First, grow a single crystal Ni film as the upper ferromagnetic layer 32, such as Figure 3 (c) shows that the thickness of Ni is 50nm, the growth rate is 0.15nm / min, and the growth temperature is 450℃. When the growth temperature exceeds 500℃, the AAO film will break.

[0053] The rate of Ni should not be too fast, because it will easily lead to the blockage of the nanopores and prevent Ni from being deposited at the bottom of the pores. When growing Ni, the graphene sensitive material substrate needs to be rotated at a rotation rate of 2 to 60RPM so that Ni can better fill the nanopores. In this embodiment, it is 60RPM.

[0054] A2, then use conventional methods to evaporate Au metal film as conductive metal film 33 for signal extraction, such as Figure 3 (d) The Au thickness is 100 nm and the rate is 6 nm / min.

[0055] S4, using an inclined ion beam etching method to remove the upper ferromagnetic layer 32 and the conductive metal film 33 on the surface, so that the Ni film and the Au metal film outside the nanopore array holes are removed, and the conductive metal film 33 inside the nanopore array holes becomes a mutually independent nanojunction array sensitive element structure, such as Figure 3 (e)

[0056] In this embodiment, during tilted ion beam etching, the tilt angle of the ion beam relative to the conductive metal film 33 is set to 60°, the rotation rate is 60RPM, and the etching rate is 6nm / min. In other embodiments, the tilt angle is 60-85°, the rotation rate is 2-30RPM, and the etching rate is 3-10nm / min, and the same or similar technical effects can be obtained. If the tilt angle is too small, it will be easy to etch the magnetic electrode material (Ni, Au) evaporated in the hole. If the tilt angle is too large, the isolated graphene magnetic sensitive unit cannot be reached. Although the tilt angle of this application is appropriate, Au will be partially etched, it will not be completely etched. Different from the purpose of using ion beam etching to etch the junction pattern in the traditional MTJ preparation method, the purpose of using tilted ion beam etching in this application is to electrically isolate the junctions.

[0057] S5. Use a conventional PVD method to evaporate the insulating layer SiO2, and use conventional micron-resolution ultraviolet lithography and IBE etching methods to prepare the top electrode and bottom electrode of the micron-sensitive unit to obtain a graphene nanomagnetic tunnel junction device.

[0058] The magnetoresistance performance of the graphene nano-magnetic tunnel junction device prepared in this embodiment was tested by a four-probe method. Figure 5 As shown, the results show that the magnetoresistance change rate of this embodiment is 117%, which is 100 times higher than that of the product prepared by the conventional method (Comparative Example 3).

[0059] Comparative Example 1

[0060] The preparation method of the graphene nanomagnetic tunnel junction device of this comparative example is basically the same as the preparation method of Example 1, except that in step S2, the size of the AAO membrane is different, and a pore size of 3 μm is used.

[0061] The magnetoresistance performance was tested by the same method as in Example 1. The TMR curve of the graphene nanomagnetic tunnel junction device prepared in this comparative example is shown in FIG. Figure 6 As shown in the figure, its TMR effect is very weak, only about 3%, and its resistance is very small, which is not suitable for use in sensors. This shows that the pore size of the AAO membrane is a very important parameter, which determines the size of the junction area. If the pore size is too large, the TMR effect will be greatly affected.

[0062] Comparative Example 2

[0063] The preparation method of the graphene nano-magnetic tunnel junction device of this comparative example is basically the same as the preparation method of Example 1, except that: in step S3, the electrode is not rotated when growing the single crystal Ni film.

[0064] The SEM image of the graphene nanomagnetic tunnel junction device prepared in this comparative example is as follows Figure 7 As shown, only half of a single nanohole has conductive metal deposited.

[0065] The magnetoresistance performance was tested using the same method as in Example 1, and the measured magnetoresistance curve also showed abnormalities, such as Figure 8 As shown, only a single-sided peak appears, while the magnetoresistance curve normally has two peaks.

[0066] Comparative Example 3

[0067] This comparative example is a conventional method for preparing a graphene nano-magnetic tunnel junction device. First, all sensitive materials are grown, and then micro-nano processes are used to process them into nano-devices one by one, including the following steps:

[0068] (1) Preparation of bottom electrode: Fig. 9 (a) to Fig. 9 As shown in (c), it includes two steps: UV lithography and ion beam etching (IBE). First, UV exposure and development are performed to transfer the designed bottom electrode pattern to the photoresist, and then the photoresist is used as a mask to use an ion beam to etch away all the thin films in the surrounding area of ​​the bottom electrode to form independent bottom electrode units.

[0069] (2) Nanojunction region preparation: Fig. 9 (d) to Fig. 9 As shown in (e), it includes two steps: electron beam lithography and ion beam etching. The process idea is basically the same as the top electrode preparation, except that the junction area is very small, only at the nanometer level, and traditional ultraviolet lithography cannot meet the requirements, so a higher resolution electron beam lithography technology is required. During the IBE etching process, the etching time needs to be controlled so that the area around the junction area is just etched to the underlying ferromagnetic film;

[0070] (3) Insulation layer deposition: Fig. 9 (f) to Fig. 9 (g) shows the process, which includes silicon dioxide growth and stripping. First, a SiO2 insulating layer of a certain thickness is deposited by inductively coupled plasma chemical vapor deposition (ICP-CVD) to prevent the upper and lower ferromagnetic layers from making direct electrical contact. Then, the sample is immersed in a chemical solvent to dissolve the photoresist, so that the silicon dioxide covering the junction area is lifted off along with the photoresist, thereby exposing the top ferromagnetic film surface.

[0071] (4) Via preparation: Fig. 9 (h) to Fig. 9 As shown in (i), the SiO2 insulating layer at the electrode Pad is etched away by combining ultraviolet lithography with reactive ion beam etching, thereby exposing the bottom electrode for subsequent electrical connection;

[0072] (5) Preparation of top electrode: Fig. 9 (j) to Fig. 9As shown in (k), it includes UV lithography and capping metal deposition. First, the top electrode pattern is transferred to the photoresist using UV lithography technology, and then a certain thickness of Ti / Au is deposited as a capping layer by physical vapor deposition (generally electron beam evaporation or magnetron sputtering), and the upper and lower ferromagnetic electrodes are respectively led out to facilitate the subsequent transport test using the four-point method.

[0073] The magnetic resistance performance was tested using the same method as in Example 1. The test results are as follows: Fig.10 As shown, the magnetic resistivity is significantly lower than that of Example 1 of the present application.

[0074] Although the present invention has been disclosed as above with preferred embodiments, it is not intended to limit the present invention. Any technician familiar with the art can make many possible changes and modifications to the technical solution of the present invention by using the technical content disclosed above without departing from the scope of the technical solution of the present invention, or modify it into an equivalent embodiment of equivalent changes. Therefore, any simple modification, equivalent change and modification made to the above embodiments according to the technical essence of the present invention without departing from the content of the technical solution of the present invention should fall within the scope of protection of the technical solution of the present invention.

Claims

1. A graphene nano-magnetic tunnel junction device with a high magnetoresistance change rate, comprising a ferromagnetic substrate (1), a graphene layer (2) and an insulating film layer (3) arranged in sequence from top to bottom, wherein the insulating film layer (3) comprises a nanohole array (31) and an upper ferromagnetic layer (32) and a conductive metal film (33) located in the nanohole array (31), wherein the upper ferromagnetic layer (32) is located between the graphene layer (2) and the conductive metal film (33), and wherein: The pore size of the nanopore array (31) is 10 to 200 nm, and the thickness of the insulating film layer (3) is 50 to 300 nm.

2. The graphene nano-magnetic tunnel junction device with high magnetoresistance change rate according to claim 1, characterized in that: The ferromagnetic substrate (1) is a Ni substrate.

3. A method for preparing a graphene nano-magnetic tunnel junction device with a high magnetoresistance change rate, characterized in that: The following steps are involved: A1, growing a graphene layer (2) on a ferromagnetic substrate (1) to obtain a graphene sensitive material substrate; A2, transferring the insulating film with the nanopore array (31) to a graphene sensitive material substrate to obtain a graphene sensitive material substrate with an insulating film, wherein the pore size of the nanopore array (31) in the insulating film is 10 to 200 nm, and the thickness of the insulating film is 50 nm to 300 nm; A3, preparing an upper ferromagnetic layer (32) and a conductive metal film (33) on the surface of a graphene sensitive material substrate having an insulating film, to form a thin film material having an upper ferromagnetic layer (32) and a conductive metal film (33); A4. Remove the upper ferromagnetic layer (32) and the conductive metal film (33) on the thin film material except in the holes of the nanohole array (31), and obtain a graphene nanomagnetic tunnel junction device.

4. The preparation method according to claim 3, characterized in that: In step A1, a graphene layer (2) is grown on a ferromagnetic substrate (1) by chemical vapor deposition.

5. The preparation method according to claim 3, characterized in that: In step A2, the insulating film is transferred to the graphene sensitive material substrate by using a mask transfer method.

6. The preparation method according to claim 5, characterized in that: In step A2, the insulating film is an anodic aluminum oxide or silicon oxide or silicon nitride insulating film.

7. The preparation method according to claim 3, characterized in that: In step A3, the upper ferromagnetic layer (32) and the conductive metal film (33) are prepared by molecular beam epitaxy, wherein the upper ferromagnetic layer (32) is a Ni film, and the conductive metal film (33) is an Au film.

8. The preparation method according to claim 7, characterized in that: In step A3, the molecular beam epitaxy method comprises the following steps: B1. First, grow a Ni film at a growth rate of 0.1-1 nm / min until the thickness reaches 10 nm-100 nm; B2. Grow an Au film at a growth rate of 4 to 10 nm / min until the thickness reaches 50 nm to 200 nm.

9. The preparation method according to claim 3, characterized in that: In step B1, during the growth of the Ni film, the graphene sensitive material substrate is rotated at a rotation rate of 2 to 60 RPM.

10. The preparation method according to claim 3, characterized in that: In step A4, the upper ferromagnetic layer (32) and the conductive metal film (33) outside the holes of the nanohole array (31) are removed by tilted ion beam etching, and the ion beam tilt angle during the tilted ion beam etching is 60° to 85°.

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