A method for improving spin injection efficiency by using a buffer layer

Abstract

The application relates to a method for improving spin injection efficiency by using a buffer layer, relating to spintronics. A long strip-shaped single-layer graphene is prepared on a silicon substrate, a double-layer h-BN is prepared and transferred onto the single-layer graphene as a tunneling layer, PMMA is spin-coated, a ferromagnetic electrode shape is exposed by using an EBL system, and development is carried out after the exposure is completed; the sample is placed into an electron beam / thermal evaporation composite plating film system, a low-melting-point metal indium is pre-evaporated as a buffer layer by using a thermal evaporation method, cobalt is electron beam evaporated as a ferromagnetic electrode, an In / Co miscible alloy interface is formed by annealing at 200 DEG C under an argon atmosphere for 1 h, and a spin valve device with a buffer layer is obtained. The method forms a lossless interface of a ferromagnetic electrode layer-buffer layer-tunneling layer, and spin injection efficiency is improved. The method is easy to realize, low in cost, low in operation difficulty and technical requirement, and beneficial to large-scale industrial application; the method is very good in universality, and can be popularized to other spin electronic devices with a tunneling layer to improve the spin injection efficiency of the devices.

Description

TECHNICAL FIELD

[0001] The present application relates to spintronics, and in particular to a method for improving spin injection efficiency by using a buffer layer. BACKGROUND

[0002] Spintronics mainly studies the injection, transport and detection processes related to electron spin, and aims to develop spin memory devices and spin logic devices. The spin memory device currently commercially available is a non-volatile magnetic random access memory (MRAM) based on a magnetic tunnel junction (MTJ), but the spin logic device that can achieve spin manipulation at room temperature is still a difficult problem to be solved by the scientific community and the industry. One of the key links to solve this problem is how to achieve efficient injection and lossless transport of spin electrons at room temperature. The basic device for studying the spin injection and transport process is a lateral spin valve, so it is extremely important to prepare a high-performance spin valve.

[0003] Since the discovery of two-dimensional materials, graphene has been considered as the most ideal spin transport channel due to its ultra-low spin-orbit coupling and ultra-long spin relaxation time. At the same time, it is found that hexagonal boron nitride (h-BN) with a layered structure meets all the characteristics of ideal tunneling layer materials. At present, the mainstream spin injection scheme is a ferromagnetic electrode tunneling injection, which is easy to implement, and the Curie temperature of the ferromagnetic metal is much higher than room temperature, but the spin injection efficiency needs to be improved. The reason is that the contact interface quality between the ferromagnetic electrode and the tunneling layer is too low. Because the main methods for preparing ferromagnetic metal electrodes are electron beam evaporation and thermal evaporation, in this process, the high kinetic energy and thermal energy of ferromagnetic metal atoms often break down the h-BN tunneling layer of less than 1 nm, causing element interdiffusion, and even forming a leakage channel, which will greatly reduce the spin injection efficiency.

[0004] Therefore, the spin injection efficiency will be directly affected by the type (such as ohmic contact, Schottky contact, tunneling contact) and quality (such as cleanliness, flatness, presence or absence of pinholes, etc.) of the contact interface. At present, there is an urgent need for a method that can prepare a high-quality non-destructive tunneling layer contact interface to achieve high-efficiency spin injection at room temperature. SUMMARY

[0005] The present application aims to solve the problem of low spin injection efficiency of ferromagnetic metal electrodes at room temperature, and provides a method for improving spin injection efficiency by using a buffer layer, and a spintronic device prepared by the method, which has good tunneling contact, stable and controllable device quality, and significantly improves the spin injection efficiency of the device.

[0006] The method for improving spin injection efficiency by using a buffer layer according to the present application comprises the following steps:

[0007] 1) A mechanical exfoliation method is used to prepare a long strip-shaped single-layer graphene on a pretreated silicon substrate;

[0008] 2) Preparation of double-layer h-BN and transfer to single-layer graphene as a tunneling layer;

[0009] 3) Spin-coating PMMA, exposing the designed ferromagnetic electrode shape by using an EBL system, developing after the exposure, and then fixing in isopropyl alcohol;

[0010] 4) Placing the sample into an electron beam / thermal evaporation composite coating system, pre-evaporating low-melting-point metal indium as a buffer layer by using thermal evaporation, and then evaporating cobalt as a ferromagnetic electrode layer by using electron beam evaporation

[0011] 5) Annealing at 200°C for 1h in an Ar atmosphere to form an In / Co miscible alloy interface, which is a lossless interface of the ferromagnetic electrode layer-buffer layer-tunneling layer and is conducive to improving the spin injection efficiency.

[0012] In step 1), the specific steps of the pretreatment include: sequentially placing a silicon substrate into acetone, isopropyl alcohol and deionized water for ultrasonic cleaning, and then blowing dry the silicon substrate, wherein the silicon substrate comprises pure silicon and a 300nm top oxide layer.

[0013] In step 2), the specific steps of the preparation of double-layer h-BN and transfer to single-layer graphene as a tunneling layer include:

[0014] (1) Obtaining double-layer h-BN crystals on a pure silicon substrate coated with a PMMA / PMGI double-gel layer by using a mechanical peeling method;

[0015] (2) Centering the double-layer h-BN, drawing a circle along the edge of the field of view under the objective lens, and dropping MF319 solvent along the scratch to dissolve the PMGI photoresist on the bottom layer;

[0016] (3) After the PMGI is completely dissolved, floating the circular PMMA film carrying the double-layer h-BN crystals on the surface of deionized water;

[0017] (4) Using a hollow metal ring to lift the PMMA film and dry it, turning it upside down, and then transferring the h-BN / PMMA film to a long strip-shaped graphene sample by using a transfer system;

[0018] (5) Placing the transferred PMMA / h-BN / graphene sample in high-purity acetone to sufficiently dissolve the PMMA, then washing in isopropyl alcohol, taking out and blowing dry, to obtain an h-BN / graphene heterojunction sample with a clean interface.

[0019] In step 3), the specific steps of the spin-coating PMMA, exposing the designed ferromagnetic electrode shape by using an EBL system, developing after the exposure, and then fixing in isopropyl alcohol include:

[0020] (1) h-BN / graphene heterojunction sample is adsorbed on a spin coater, and polymethyl methacrylate (PMMA) is uniformly spin-coated and dried;

[0021] (2) The PMMA spin-coated sample is placed in the electron beam cavity of an electron beam exposure machine (EBL), and the cavity is vacuumed to 10 -6 Pa or less, and an electrode shape is designed in the EBL system, and according to the design drawing, the PMMA spin-coated sample is bombarded by an electron beam to perform exposure operation, and a pattern after exposure is obtained;

[0022] (3) The obtained sample is placed in a developing solution to develop and remove PMMA in the exposed area to expose the surface of h-BN, and then the sample is moved to isopropyl alcohol to stop developing, and after fixing, it is blown dry.

[0023] In step 4), the specific method of the pre-deposited low-melting-point metal indium as a buffer layer can be:

[0024] (1) The sample obtained in step 3) is placed in an electron beam / thermal evaporation composite plating system and fixed on a sample stage;

[0025] (2) Close the door and start vacuuming, and the limit vacuum degree in the chamber needs to be better than 1x10 -7 Torr;

[0026] (3) Select thermal evaporation mode, select metal indium (In) as the target material, first cool the sample stage to 20℃, set the evaporation rate to 0.2A / s-0.3A / s, the evaporation time is 100s, and the buffer layer thickness is 2-3nm.

[0027] In step 4), the specific method of using electron beam evaporation of cobalt as a ferromagnetic electrode layer can be: after the low-melting-point metal is evaporated, the chamber door is not opened to realize continuous multi-layer plating without exposure to air, the sample stage is cooled to 20℃, electron beam evaporation mode is selected to evaporate the ferromagnetic metal cobalt (Co), the vacuum degree needs to be better than 5x10 -6 Torr, the electron gun power is set to 2kW, the beam current is 200mA, the evaporation rate is controlled at 0.5A / s-0.7A / s, and the plating time is 700s, and the thickness of the ferromagnetic electrode layer is 35-49nm.

[0028] Compared with the prior art, the beneficial effects of the present application are:

[0029] (1) The present application is easy to implement, low in cost, low in operation difficulty and technical requirement, and very beneficial to large-scale industrial application of spintronic devices;

[0030] (2) The spin valve device prepared by the application has perfect contact interface between the tunneling layer and the buffer layer, because they are good tunneling contact, so the contact resistance of the prepared device will not have too much difference, and the device quality is stable and controllable;

[0031] (3) The method of directly evaporating Co will damage hBN, compared with the traditional lateral spin valve device, the device prepared by the method of protecting the contact interface by the buffer layer has very significant improvement in spin injection efficiency. It is beneficial to the practical application of spintronic devices;

[0032] (4) The method of protecting the contact interface by the buffer layer has very good universality, and can be popularized to other spintronic devices with tunneling layer to improve the spin injection efficiency of the device, such as spin light emitting diode, spin field effect transistor and MRAM based on magnetic tunnel junction. BRIEF DESCRIPTION OF DRAWINGS

[0033] Figure 1 The structure schematic diagram of the lateral spin valve device with a buffer layer prepared by the embodiment of the application;

[0034] Figure 2 The structure schematic diagram of the hollow metal ring described in the embodiment of the application;

[0035] Figure 3 The schematic diagram of the metal electrode design used in the embodiment of the application;

[0036] Figure 4 The schematic diagram of the cross-sectional structure of the lateral spin valve device with a buffer layer prepared by the embodiment of the application.

[0037] Figure 5 TEM diagram of directly evaporating ferromagnetic metal.

[0038] Figure 6 Composition analysis xds spectrum of directly evaporating ferromagnetic metal. DETAILED DESCRIPTION

[0039] In order to make the purpose, technical scheme and advantages of the application clearer and more apparent, the following embodiments will further illustrate the application with reference to the drawings. It should be understood that the specific embodiments described herein are only used to explain the application, and are not used to limit the application. On the contrary, the application covers any substitution, modification, equivalent method and scheme defined by the claims within the essence and scope of the application. Further, in order to make the public have a better understanding of the application, some specific details are described in detail in the following detailed description of the application. The application can also be completely understood without the description of these details by those skilled in the art.

[0040] The embodiment of the present application provides a preparation method of a transverse spin valve device, which pre-evaporates a low-melting-point metal as a buffer layer between a metal ferromagnetic electrode and a tunneling layer, and comprises the following steps:

[0041] Step S1, preparing a strip-shaped single-layer graphene 2 on a Si / SiO2 substrate 1.

[0042] Step S1.1, cutting a silicon wafer into a square substrate with a side length of 2 cm, first putting it into acetone for ultrasonic cleaning for 5 min, then putting it into isopropyl alcohol for ultrasonic cleaning for 6 min, then putting it into deionized water for ultrasonic cleaning for 4 min, and finally blowing dry with nitrogen. The silicon wafer is composed of pure silicon and a 300-nm top oxide layer, and acetone and isopropyl alcohol are used to remove contaminants such as debris on the silicon wafer, and deionized water is used to remove residual acetone and isopropyl alcohol.

[0043] Step S1.2, preparing graphene on the Si / SiO2 substrate 1 by using a mechanical exfoliation method. Find and calibrate strip-shaped single-layer graphene 2 under a high-resolution metallographic microscope, and require that the length of the graphene is greater than 20 um and the width is less than 4 um.

[0044] Step S2, transferring double-layer h-BN 3 as a tunneling layer.

[0045] Step S2.1, the device structure of the transverse spin valve is as shown in Figure 1 First, cut a pure silicon wafer (without an oxide layer) into a square substrate with a side length of 2 cm, and then perform ultrasonic cleaning. The cleaning process is the same as that in step S1.1.

[0046] Step S2.2, put the cleaned pure silicon wafer into an oxygen plasma cleaning instrument, set the oxygen flow to 20 sccm, the radio frequency power to 20 W, and clean for 3 min. In this way, the pure silicon wafer becomes hydrophilic, and the adhesion to the glue is enhanced, which is beneficial to the subsequent glue coating.

[0047] Step S2.3, bake the pure silicon wafer treated by the oxygen plasma at 130 DEG C for 5 min using a hot plate, and then put it into a glue coating machine to spin coat PMGI at a speed of 4000 rpm for 70 s.

[0048] Step S2.4, bake the pure silicon wafer again at 130 DEG C for 5 min using a hot plate, and then put it into a glue coating machine to spin coat PMMA at a speed of 3400 rpm for 70 s. After the spin coating is completed, place the wafer on the hot plate to bake at 130 DEG C for 5 min, and obtain a pure silicon substrate coated with PMMA / PMGI double-layer glue.

[0049] Step S2.5, prepare h-BN on the substrate coated with PMMA / PMGI double-layer glue by using a mechanical exfoliation method. Find and calibrate suitable double-layer h-BN under a high-resolution metallographic microscope, and require that the length and width of the double-layer h-BN are both greater than 15 um.

[0050] Step S2.6, with the target double-layer h-BN sample as the center, a circle is drawn along the edge of the field of view with a sharp tweezer under the 5x objective of the microscope (at this time the PMMA / PMGI double-layer film is cut open to expose the section), then a circle of MF319 solvent is dripped along the cut to dissolve the PMGI photoresist of the bottom layer (note: PMMA is not soluble in MF319).

[0051] Step S2.7, after the PMGI photoresist of the bottom layer is completely dissolved, the circular PMMA film loaded with double-layer h-BN crystals is placed on the surface of deionized water (note: the surface of h-BN is always upward and does not contact the deionized water).

[0052] Step S2.8, the circular PMMA film loaded with double-layer h-BN crystals is fished up with a hollow metal ring as shown in Figure 2 , after drying, it is turned upside down, and then the h-BN / PMMA film is transferred to the long strip-shaped single-layer graphene sample prepared in step S1.2 by using a directional transfer system.

[0053] Step S2.9, the PMMA / h-BN / graphene sample obtained by transfer is placed in high-purity acetone to completely dissolve the PMMA, then it is washed in isopropyl alcohol, and finally it is taken out and dried to obtain an "h-BN / graphene" heterojunction sample with a clean interface.

[0054] Step S3, evaporation of low-melting-point metal electrodes and ferromagnetic electrodes;

[0055] Step S3.1, the obtained "h-BN / graphene" heterojunction sample is adsorbed on a spin coater, and polymethyl methacrylate (PMMA) is uniformly spin-coated at a rotation speed of 3400 rpm for 70 s, and then baked at 130°C for 5 min by using a hot plate.

[0056] Step S3.2, the PMMA spin-coated sample is placed in the electron beam cavity of an electron beam exposure machine (EBL), and the cavity is pumped to a vacuum of 10 -6 Pa or less. The electrode shape is designed in the EBL system, and the electrode design is as shown in Figure 3 , and the PMMA spin-coated sample is bombarded with an electron beam for exposure operation.

[0057] Step S3.3, the exposed sample is placed in a developing solution for developing operation, which removes the PMMA in the exposed area to expose the surface of h-BN, so that the shape of the electrode is obtained, then the sample is moved to isopropyl alcohol to stop developing, and after fixing, it is dried with nitrogen.

[0058] Step S3.4, the sample is put into an electron beam / thermal evaporation composite coating system which can realize continuous multi-layer composite coating without exposure to air, the chamber door is opened, and the sample is fixed on the sample table. The electron beam / thermal evaporation composite coating system can adopt a DM500 type electron beam / resistance evaporation composite coating system, Hefei Gusheng Vacuum, Hefei, China.

[0059] Step S3.5, the chamber door is closed, vacuum is pumped, and the vacuum degree in the chamber needs to be better than 1x10 -7 Torr to start evaporation.

[0060] Step S3.6, low melting point metal is first evaporated as a buffer layer 4, because the evaporation temperature is low, a thermal evaporation mode is generally selected, the target material is selected as metal indium (In), the sample table is first water-cooled to 20℃, the evaporation rate is set to 0.2A / s-0.3A / s, the evaporation time is 100s, and the thickness of the low melting point metal buffer layer is 2-3nm after evaporation.

[0061] Step S3.7, without opening the chamber door, the vacuum degree is kept better than 5x10 -6 Torr, the sample table is first water-cooled to 20℃, an electron beam evaporation mode is selected, the ferromagnetic metal electrode layer 5 is evaporated, the target material is selected as metal cobalt (Co), the electron gun power is 2kW, the beam current is 200mA, the evaporation rate is controlled at 0.5A / s-0.7A / s, the coating time is 700s, and finally the thickness of the cobalt electrode is 35nm-49nm, so that continuous multi-layer composite coating in a high vacuum condition is realized.

[0062] Step S3.8, after breaking the vacuum, the power of the composite coating machine is turned off, the chamber door is opened after the sample table is cooled, and the sample is taken out.

[0063] Step S4, device annealing;

[0064] Step S4.1, the device is put into a tube furnace, a temperature rising and falling curve is set, the temperature rising rate is 8℃ / min, the temperature falling rate is 10℃ / min, and the working temperature is set to 200℃;

[0065] Step S4.2, the main valve of the argon cylinder is opened, argon is introduced, after the gas flow is stable, attention is paid to that the pressure gauge of the tube furnace should not be higher than 0.08MPa, the heating power is started, and the device is annealed in an Ar atmosphere at 200℃ for 1h, so that an In / Co miscible alloy interface is formed to improve the spin injection efficiency, and thus the preparation of the transverse spin valve device with a buffer layer is completed, and the cross-sectional view of the transverse spin valve is shown in Figure 4 .

[0066] The working principle of the transverse spin valve prepared in the application is as follows:

[0067] When a current is applied between the ground electrode 51 and the injection electrode 52, electrons become spin-polarized electrons through the ferromagnetic electrode. These spin-polarized electrons tunnel into the graphene, where they accumulate spin on the graphene below the injection electrode 52. Then, they begin spin diffusion through the graphene channels. Thus, a voltage signal related to spin diffusion can be measured at the probe electrode 53 and the reference electrode 54. The injection, transport, and detection processes of spin can be studied using this voltage signal and some device parameters.

[0068] Figure 5 and 6 TEM images and xds spectra of elemental composition of ferromagnetic metals prepared by direct Co evaporation are presented. As can be seen from the figures, the traditional method of direct Co evaporation damages the hBN layer. Compared to traditional lateral spin valve devices, the device prepared by this invention, utilizing a buffer layer to protect the contact interface, exhibits a significantly improved spin injection efficiency. This is beneficial for the practical application of spintronic devices.

[0069] The parameters described above are only preferred embodiments of the present invention. It should be noted that for those skilled in the art, several improvements and modifications can be made without departing from the principle of the present invention, and these improvements and modifications should also be considered within the scope of protection of the present invention.

Claims

1. A method for improving spin injection efficiency using a buffer layer, characterized in that... Includes the following steps: 1) Long strip-shaped monolayer graphene was prepared on a pretreated silicon substrate using a mechanical exfoliation method; 2) The preparation of bilayer h-BN and its transfer onto monolayer graphene as a tunneling layer includes the following steps: (1) A double-layer h-BN crystal was obtained on a pure silicon substrate coated with a PMMA / PMGI double layer by mechanical peeling. (2) Using the double-layer h-BN as the center, draw a circle along the edge of the field of view under the objective lens, and drop MF319 solvent along the scratch to dissolve the PMGI photoresist in the bottom layer. (3) After the PMGI is completely dissolved, the circular PMMA membrane carrying the double-layer h-BN crystals is floated on the surface of deionized water. (4) The PMMA film was lifted out and dried using a hollow metal ring, turned upside down, and then the h-BN / PMMA film was transferred onto the long strip graphene sample using a transfer system. (5) The PMMA / h-BN / graphene sample obtained by transfer was placed in high-purity acetone to fully dissolve PMMA, then washed in isopropanol, taken out and dried to obtain an h-BN / graphene heterojunction sample with a clean interface. 3) Spin-coat PMMA, use an EBL system to expose the designed ferromagnetic electrode shape, develop after exposure, fix and dry; 4) The sample is placed in an electron beam / thermal evaporation composite coating system. First, low-melting-point metal indium is pre-deposited as a buffer layer using thermal evaporation, and then cobalt is deposited as a ferromagnetic electrode layer using electron beam evaporation. 5) Annealing in an Ar atmosphere forms an In / Co miscible alloy interface. This non-destructive interface of ferromagnetic electrode layer-buffer layer-tunneling layer improves spin injection efficiency.

2. The method for improving spin injection efficiency using a buffer layer as described in claim 1, characterized in that... In step 1), the specific steps of the pretreatment include: ultrasonically cleaning the silicon substrate in acetone, isopropanol and deionized water in sequence, and then drying the silicon substrate, wherein the silicon substrate contains pure silicon and a 300nm top oxide layer.

3. The method for improving spin injection efficiency using a buffer layer as described in claim 1, characterized in that... In step 3), the spin-coating of PMMA involves adsorbing the h-BN / graphene heterojunction sample onto a spin coater, uniformly spin-coating PMMA, and then drying it. The exposure of the designed ferromagnetic electrode shape using an EBL system involves placing the spin-coated PMMA sample into the electron beam cavity of an electron beam exposure machine, and evacuating the cavity to 10°C. -6 Below the Pa level, the electrode shape is designed in the EBL system. According to the design, the sample spin-coated with PMMA is bombarded with an electron beam to perform an exposure operation and obtain the exposed pattern.

4. The method for improving spin injection efficiency using a buffer layer as described in claim 1, characterized in that... In step 3), the development involves placing the obtained sample in a developing solution to remove the PMMA from the exposed area, exposing the surface of h-BN. Then, the sample is transferred to isopropanol to stop development, fixed, and dried.

5. The method for improving spin injection efficiency using a buffer layer as described in claim 1, characterized in that... In step 4), the specific method for using pre-deposited low-melting-point indium metal as a buffer layer is as follows: (1) Place the sample obtained in step 3) into the electron beam / thermal evaporation composite coating system and fix it on the sample stage; (2) Close the chamber door and begin evacuation. The ultimate vacuum level inside the chamber needs to be better than 1×10⁻⁶. -7 Torr; (3) Select the thermal evaporation method, use indium metal as the target material, first cool the sample stage to 20 ℃, set the evaporation rate to 0.2A / s-0.3A / s, and the evaporation time to 100s.

6. The method for improving spin injection efficiency using a buffer layer as described in claim 1, characterized in that... In step 4), the specific method for using electron beam evaporation to deposit cobalt as a ferromagnetic electrode layer is as follows: after evaporating the low-melting-point metal, without opening the chamber door, continuous multilayer film deposition is achieved without exposing air. After the sample stage cools to 20 °C, the ferromagnetic metal cobalt is deposited by electron beam evaporation, and the vacuum degree needs to be better than 5 × 10⁻⁶. -6 Torr, electron gun power set to 2kW, beam current 200mA, evaporation rate controlled between 0.5A / s and 0.7A / s, coating time 700s.

7. The method for improving spin injection efficiency using a buffer layer as described in claim 1, characterized in that... In step 4), the thickness of the buffer layer is 2-3 nm, and the thickness of the ferromagnetic electrode layer is 35-49 nm.

8. The method for improving spin injection efficiency using a buffer layer as described in claim 1, characterized in that... In step 5), the specific method for Ar atmosphere annealing is as follows: (1) Place the sample in a tube furnace, set the heating and cooling curves, with a heating rate of 8℃ / min and a cooling rate of 10℃ / min; (2) Open the main valve of the argon cylinder, introduce argon gas, and wait for the gas flow to stabilize. Then anneal at 200℃ in an Ar atmosphere for 1 hour to form an In / Co miscible alloy interface.

9. A spintronic device prepared by a method for improving spin injection efficiency using a buffer layer as described in any one of claims 1 to 8.

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