Strong spin-orbit coupling structure, fabrication method and SOT device

Abstract

This invention discloses a strong spin-orbit coupling structure, its fabrication method, and a System-on-Track (SOT) device. The fabrication method of the strong spin-orbit coupling structure includes the following steps: providing a silicon substrate; using magnetron sputtering, alternately depositing elemental bismuth films and elemental platinum films on the silicon substrate using elemental bismuth and elemental platinum targets respectively to obtain a platinum-bismuth alloy film; and depositing a capping layer on the platinum-bismuth alloy film using magnetron sputtering. This invention, based on magnetron sputtering, uses alternating growth of elemental bismuth and elemental platinum films with elemental bismuth and elemental platinum targets combined with an annealing process to prepare the platinum-bismuth alloy film in the strong spin-orbit coupling structure, which has advantages such as large area, atomically flat surface, and precise controllable composition.

Description

Technical Field

[0001] This invention belongs to the field of spintronics technology, specifically relating to a strong spin-orbit coupling structure, its fabrication method, and a SOT device. Background Technology

[0002] In recent years, the use of current to achieve magnetization reversal or magnetic moment precession in magnetic units based on the spin-orbit torque (SOT) effect in the field of spintronics has attracted widespread attention. The SOT effect refers to the generation of spin-orbit torque through charge-induced spin current based on spin-orbit coupling, thereby modulating the magnetic moment of magnetic materials. The physical processes involved include the generation and transport of spin current and its interaction with local magnetic moments. The spin-orbit coupling layer that generates the spin current in an SOT device is the core factor determining the device's energy consumption. Effective integration with the widely used MgO-CoFeB magnetic tunnel junction is a necessary condition for the spin-orbit coupling layer. The current-to-spin current conversion efficiency can be measured by the spin Hall angle; a larger spin Hall angle means that a smaller current can be used to drive magnetization reversal, thus reducing the power consumption of the SOT device.

[0003] Bismuth-based topological materials, including bismuth-based topological insulators and bismuth-based topological half-metals, are a novel type of spin-orbit coupling layer material, and studies have confirmed their high current-to-spin-current conversion efficiency. For example, the research group of Kanglong Wang at the University of California utilized high-quality topological insulators (Bi... 0.5 Sb 0.5 The spin flow generated by the topological surface states of 2Te3 drives the Cr-doped (Bi) 2-x Sb x 2Te3 magnetic thin film with a critical driving current density as low as 10 4 A / cm 2 The driving critical current is on the order of magnitude higher than that of ordinary heavy metals (approximately 10). 6 A / cm 2 ~10 8 A / cm 2 Several orders of magnitude lower. In 2018, Pham Nam Hai's research group at the Tokyo Institute of Technology in Japan fabricated a BiSb topological insulator with good conductivity and achieved magnetization reversal using current-driven vertically magnetized MnGa thin films. The spin Hall angle of the BiSb thin film reached as high as 52°. Due to the large conductivity of BiSb, which is two orders of magnitude higher than that of Bi₂Se₃ and Bi₂Te₃, the energy consumption of the BiSb-based spin-orbit moment device fabricated in this experiment is several orders of magnitude lower than that of previous devices.

[0004] Bismuth-based topological materials, as spin-orbit coupling layers, still face many challenges in their practical application. Metallic bismuth itself is a high atomic number atom with strong spin-orbit coupling properties, but its low melting point of only 271℃ makes it prone to crystallization and volatilization during magnetron sputtering deposition. This is a major reason for the large surface roughness and significant chemical ratio deviation between bismuth-based topological materials and the target material. This is mainly reflected in:

[0005] (1) Bismuth-based topological material thin films are usually prepared by molecular beam epitaxy (MBE) or by molten salt method to prepare high-quality single crystal topological materials. These preparation methods are slow to grow and cannot grow on a large area, which is incompatible with the semiconductor manufacturing industry that is currently widely used.

[0006] (2) Topological insulators have a large resistance and the spin current channel is confined to a very narrow surface, which makes the driving voltage of SOT devices based on topological insulators greater than 5V, and cannot be driven by transistors under advanced semiconductor processes.

[0007] (3) Current bismuth-based topological material thin films have relatively large surface roughness. Even high-quality Bi2Se3 and Bi2Te3 films prepared by MBE still cannot achieve atomic-level flatness. Growing the currently widely used MgO-CoFeB magnetic tunnel junction thin films on these rough surfaces will cause a sharp deterioration in magnetic properties, making them impractical.

[0008] Bismuth itself is a high atomic number atom in platinum-bismuth alloys, giving the alloy stronger spin-orbit coupling properties than the conventional heavy metal platinum. Furthermore, when the stoichiometric ratio of platinum to bismuth is 1:2, it forms a topological half-metal, exhibiting low resistance and strong spin-orbit coupling. Therefore, the fabrication of platinum-bismuth alloys is highly advantageous for realizing low-power spintronic devices. However, currently, only small-sized, high-quality single crystals can be fabricated; large-area, smooth-surface, high-quality thin films are still not feasible.

[0009] Therefore, to address the aforementioned technical problems, it is necessary to provide a strong spin-orbit coupling structure, its fabrication method, and an SOT device. Summary of the Invention

[0010] In view of this, the purpose of this invention is to provide a strong spin-orbit coupling structure, a preparation method and a SOT device. The prepared platinum-bismuth alloy thin film has the advantages of being compatible with semiconductor manufacturing processes, having a large area, atomically flat surface and precise controllable composition.

[0011] To achieve the above objectives, an embodiment of the present invention provides the following technical solution:

[0012] A method for fabricating a strongly spin-orbit coupled structure, the method comprising the following steps:

[0013] Provide a silicon substrate;

[0014] Based on the magnetron sputtering process, elemental bismuth thin films and elemental platinum thin films are alternately deposited on the silicon substrate using elemental bismuth targets and elemental platinum targets, respectively, to obtain platinum-bismuth alloy thin films;

[0015] A capping layer was deposited on a platinum-bismuth alloy thin film using magnetron sputtering.

[0016] In one embodiment, the preparation method further includes:

[0017] In-situ annealing is performed during the platinum-bismuth alloy thin film deposition process at temperatures ranging from 50°C to 250°C; and / or,

[0018] After platinum-bismuth alloy thin film deposition, vacuum annealing is performed at a temperature of 100℃ to 500℃, with a vacuum level of less than or equal to 10. -3 Pa.

[0019] In one embodiment, the alternating deposition period of the elemental bismuth film and the elemental platinum film is 2 to 100, and the sum of the thicknesses of the elemental bismuth film and the elemental platinum film in each period is 0.1 nm to 5 nm.

[0020] In one embodiment, the content of the elemental bismuth target and the elemental platinum target is greater than or equal to 99.9%; and / or, the atomic percentage of bismuth in the platinum-bismuth alloy film is 20% to 80%.

[0021] In one embodiment, the preparation method further includes:

[0022] Liquid nitrogen is used for cooling during the deposition of platinum-bismuth alloy thin films to suppress crystallization of elemental bismuth films during growth; and / or,

[0023] A metal thin layer is deposited between a bismuth thin film and a platinum thin film. The metal thin layer is made of an alloy of one or more of manganese, chromium, cobalt, iron, nickel, neodymium, europium, gadolinium, terbium, dysprosium, and holmium. The thickness of the metal thin layer is 0.01 nm to 1 nm.

[0024] In one embodiment, the material of the capping layer is one or more of silicon oxide, silicon nitride, aluminum oxide, tantalum, ruthenium, platinum, chromium, titanium, molybdenum, and tungsten; and / or, the thickness of the capping layer is 0.1 nm to 20 nm.

[0025] In one embodiment, the preparation method further includes:

[0026] A buffer layer is deposited on a silicon substrate using a magnetron sputtering process.

[0027] The buffer layer is made of one or more of silicon, tantalum, tantalum nitride, ruthenium, and platinum; the thickness of the buffer layer is 0.1 nm to 10 nm, and the surface roughness is less than or equal to 0.5 nm.

[0028] In one embodiment, the vacuum level in the magnetron sputtering process is less than or equal to 10. -5 Pa, the working gas is an inert gas, and the working gas pressure is 0.13 Pa to 1.3 Pa.

[0029] Another embodiment of the present invention provides the following technical solution:

[0030] A strong spin-orbit coupling structure is prepared by the above-described preparation method.

[0031] Another embodiment of the present invention provides the following technical solution:

[0032] A SOT device, the SOT device comprising a strong spin-orbit coupling structure and a magnetic tunnel junction structure.

[0033] The present invention has the following beneficial effects:

[0034] This invention is based on magnetron sputtering and uses alternating growth of single bismuth and single platinum thin films on single bismuth and single platinum targets combined with an annealing process to prepare platinum-bismuth alloy thin films in a strong spin-orbit coupling structure. It has the advantages of large area, atomically flat surface, and precise controllable composition. Attached Figure Description

[0035] To more clearly illustrate the technical solutions in the embodiments of this application or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments recorded in this application. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0036] Figure 1 This is a schematic diagram of the strong spin-orbit coupling structure in this invention;

[0037] Figure 2 This is a schematic diagram of the internal structure of the platinum-bismuth alloy thin film in this invention;

[0038] Figure 3 This is a schematic flowchart of the method for preparing the strong spin-orbit coupling structure in this invention;

[0039] Figure 4 This is a schematic diagram of the structure of the SOT device in this invention;

[0040] Figure 5This is the atomic force surface topography of the platinum-bismuth alloy film surface in a specific embodiment of the present invention;

[0041] Figure 6 This is the X-ray diffraction pattern of the platinum-bismuth alloy film in a specific embodiment of the present invention. Specific embodiments

[0042] In order to enable those skilled in the art to better understand the technical solutions in the present invention, the following will clearly and completely describe the technical solutions in the embodiments of the present invention with reference to the accompanying drawings in the embodiments of the present invention. Obviously, the described embodiments are only a part of the embodiments of the present invention, rather than all the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without making creative efforts shall fall within the protection scope of the present invention.

[0043] Refer Figure 1 As shown, the present invention discloses a strong spin-orbit coupling structure, which sequentially includes a silicon substrate 10, a platinum-bismuth alloy film 30, and a covering layer 40 from bottom to top. Preferably, the substrate uses the silicon substrate 10 commonly used in the semiconductor manufacturing field, and a buffer layer 20 is further included between the silicon substrate 10 and the platinum-bismuth alloy film 30.

[0044] Combined Figure 2 As shown, the atomic percentage of bismuth in the platinum-bismuth alloy film 30 is 20% - 80%. The platinum-bismuth alloy film 30 includes alternately distributed elemental bismuth films 31 and elemental platinum films 32. Preferably, the number of periods is 2 - 100, and the sum of the thicknesses of the elemental bismuth film 31 and the elemental platinum film 32 in each period is 0.1 nm - 5 nm.

[0045] Among them, the material of the buffer layer 20 is one or more of silicon, tantalum, tantalum nitride, ruthenium, platinum, with a thickness of 0.1 nm - 10 nm and a surface roughness less than or equal to 0.5 nm; the material of the covering layer 40 is one or more of silicon oxide, silicon nitride, aluminum oxide, tantalum, ruthenium, platinum, chromium, titanium, molybdenum, tungsten, with a thickness of 0.1 nm - 20 nm.

[0046] Refer Figure 3 As shown, the preparation method of the strong spin-orbit coupling structure in the present invention includes the following steps:

[0047] S1. Provide a silicon substrate.

[0048] Exemplarily, the silicon substrate selects a wafer or fragment with a flat surface, and the surface roughness is not greater than 0.5 nm.

[0049] Preferably, a buffer layer is first deposited on a silicon substrate using a magnetron sputtering process. The buffer layer is made of one or more of silicon, tantalum, tantalum nitride, ruthenium, platinum, etc., with a thickness of 0.1 nm to 10 nm and a surface roughness of less than or equal to 0.5 nm.

[0050] The magnetron sputtering process in each step of this invention is performed using a magnetron sputtering machine with a back-ground vacuum level of less than or equal to 10. -5 Pa, the working gas for sputtering coating is an inert gas such as argon, krypton, or xenon, and the working gas pressure is 0.13 Pa to 1.3 Pa (i.e., 1 mTorr to 10 mTorr).

[0051] S2. Based on the magnetron sputtering process, elemental bismuth thin films and elemental platinum thin films are alternately deposited on a silicon substrate (or on the buffer layer if there is one) using elemental bismuth targets and elemental platinum targets respectively, to obtain platinum-bismuth alloy thin films.

[0052] For example, the content of elemental bismuth target and elemental platinum target is greater than or equal to 99.9%, the alternating deposition period of elemental bismuth film and elemental platinum film is 2 to 100, the sum of the thickness of elemental bismuth film and elemental platinum film in each period is 0.1 nm to 5 nm, and the atomic percentage of bismuth in the final platinum-bismuth alloy film is 20% to 80%.

[0053] In a preferred embodiment, liquid nitrogen is used for cooling during the deposition of the platinum-bismuth alloy thin film to suppress the crystallization of the elemental bismuth film during growth and reduce the surface roughness by no more than 0.2 nm.

[0054] Optionally, a metal thin layer is deposited between the elemental bismuth thin film and the elemental platinum thin film for further controllable doping of the platinum-bismuth alloy. The material of the metal thin layer is an alloy composed of one or more of manganese, chromium, cobalt, iron, nickel, neodymium, europium, gadolinium, terbium, dysprosium, and holmium, and the thickness of the metal thin layer is 0.01 nm to 1 nm.

[0055] S3. A capping layer is deposited on a platinum-bismuth alloy thin film using a magnetron sputtering process.

[0056] For example, the material of the capping layer is one or more of silicon oxide, silicon nitride, aluminum oxide, tantalum, ruthenium, platinum, chromium, titanium, molybdenum, tungsten, etc., and the thickness is 0.1 nm to 20 nm.

[0057] Among them, the elemental bismuth thin film and the elemental platinum thin film can deposit the elemental bismuth thin film first and then the elemental platinum thin film, or can deposit the elemental platinum thin film first and then the elemental bismuth thin film. Preferably, the scheme of depositing the elemental bismuth thin film first and then the elemental platinum thin film is adopted, and the top layer is the elemental platinum thin film. At this time, the elemental platinum thin film on the top layer can form an alloy with the covering layer, which can not only effectively isolate the diffusion of bismuth elements into the magnetic tunnel junction structure and cause the reduction of magnetic properties, but also promote the magnetic and electrical properties of the magnetic tunnel junction structure by absorbing boron elements in the magnetic layer.

[0058] It should be emphasized that the platinum-bismuth alloy thin film in the present invention needs to be annealed. The annealing process can be carried out by vacuum annealing alone after the deposition of the platinum-bismuth alloy thin film, or can be carried out by in-situ annealing alone during the deposition of the platinum-bismuth alloy thin film, or can adopt a combination of vacuum annealing and in-situ annealing.

[0059] Among them, when vacuum annealing is carried out after the deposition of the platinum-bismuth alloy thin film, the temperature of the vacuum annealing is 100 °C to 500 °C, and the vacuum degree is less than or equal to 10 -3 Pa; when in-situ annealing is carried out during the deposition of the platinum-bismuth alloy thin film, the temperature of the in-situ annealing is 50 °C to 250 °C

[0060] The principle of the present invention is that metallic bismuth and metallic platinum have good wettability. By alternately growing the elemental bismuth target and the elemental platinum target, a large-area and atomically flat surface thin film can be prepared. Secondly, by controlling the thickness of each film layer, the film composition (the percentage of bismuth atoms is 20% to 80%) can be accurately controlled. Then, through the annealing process, a topological semimetal platinum-bismuth alloy thin film is obtained.

[0061] As Figure 4 shown, the present invention also discloses a SOT device, which includes the above-mentioned strong spin-orbit coupling structure and magnetic tunnel junction structure. Exemplarily, the magnetic tunnel junction structure sequentially includes a free layer 50, a barrier layer 60, a composite reference layer 70, and a top electrode layer 80 from bottom to top. The magnetic tunnel junction structure can be a MgO-CoFeB magnetic tunnel junction structure in the prior art, etc. Each layer in the magnetic tunnel junction structure belongs to the prior art and will not be elaborated here.

[0062] The strong spin-orbit coupling structure in the present invention has the advantages of a large spin Hall angle and a small resistivity. Its effective combination with the currently widely used MgO-CoFeB magnetic tunnel junction is expected to reduce the energy consumption of the SOT device ultimately.

[0063] When the platinum-bismuth alloy thin film in the strong spin-orbit coupling structure is adjacent to the free layer in the MgO-CoFeB magnetic tunnel junction structure, during the annealing process, a small amount of bismuth will diffuse to the interface of the free layer CoFeB or into the free layer, thereby improving the magnetic properties of the free layer.

[0064] In a specific embodiment of the present invention, the method for preparing a strongly spin-orbit coupled structure includes the following steps:

[0065] 1. Provide a silicon substrate.

[0066] The silicon substrate selected is a flat silicon substrate with a surface roughness of 0.3 nm.

[0067] 2. Based on magnetron sputtering, a buffer layer of Ta 2nm / Pt 1nm is deposited on a silicon substrate.

[0068] In this embodiment, the magnetron sputtering process in each step is performed using a magnetron sputtering machine with a back-floor vacuum of less than 10. -5 Pa, the working gas for sputtering coating is argon, and the working pressure is 0.26 Pa.

[0069] 3. Based on magnetron sputtering, elemental bismuth and elemental platinum thin films were alternately deposited on the buffer layer using elemental bismuth and elemental platinum targets, respectively. The content of both elemental bismuth and platinum targets was 99.95%, resulting in a platinum-bismuth alloy thin film (PtBi). The thickness of Pt was 0.3 nm, and the thickness of Bi was 0.7 nm. The thickness of the PtBi alloy thin film was controlled by adjusting the number of growth cycles. 4. Based on magnetron sputtering, a capping layer was deposited on the platinum-bismuth alloy thin film. The capping layer was made of silicon and had a thickness of 5 nm.

[0070] 5. When performing post-vacuum annealing on platinum-bismuth alloy thin films, the annealing temperature is 100℃, the annealing time is 20 minutes, and the vacuum degree is less than 10. -4 Pa.

[0071] Figure 5 The image shown is an atomic force surface topography (AFM) image of the platinum-bismuth alloy thin film prepared in this embodiment. The scanning range is square, the scanning range (scale bar) is 5 micrometers, and the scanning height range (height bar) is only -1.5nm to 1.5nm. There are no obvious grains on the surface, and its surface roughness is 0.3nm, which is the same as that of the silicon substrate, indicating that the surface of the thin film is smooth and flat.

[0072] Figure 6 The image shows the X-ray diffraction (XRD) patterns of the platinum-bismuth alloy thin films prepared in this embodiment. The thicknesses of the platinum-bismuth alloy thin films are 20 nm and 30 nm (labeled as 20 nm PtBi and 30 nm PtBi, respectively). The three characteristic peaks of each of the two films are consistent with those of PtBi2 with a stoichiometric ratio of 1:2 and a crystal structure of (P-3; no173). This indicates that a single-phase PtBi2 with a crystal structure of (P-3; no173) was prepared by this method, which means that this preparation method can obtain high-quality single-phase PtBi2 thin films.

[0073] Combination Figure 5 , Figure 6 As can be seen, the present invention can obtain large-area, atomically flat platinum-bismuth alloy thin films with precise and controllable composition.

[0074] This invention is based on magnetron sputtering and uses alternating growth of single bismuth and single platinum thin films on single bismuth and single platinum targets combined with an annealing process to prepare platinum-bismuth alloy thin films in a strong spin-orbit coupling structure. It has the advantages of large area, atomically flat surface, and precise controllable composition.

[0075] The strong spin-orbit coupling structure in this invention can be applied not only to the field of non-volatile memory, but also to the fields of photoelectric detection, condensed matter physics, and so on.

[0076] The specific embodiments described above with reference to the accompanying drawings are exemplary embodiments, but do not represent all embodiments that can be implemented or fall within the scope of the claims. The term "exemplary" as used throughout this specification means "serving as an example, instance, or illustration" and does not imply that it is "preferred" or "advantageous" compared to other embodiments. Specific details are included to provide an understanding of the described techniques. However, these techniques can be practiced without these specific details. In some instances, well-known structures and apparatuses are shown in block diagram form to avoid obscuring the concepts of the described embodiments.

[0077] The foregoing description of this disclosure is provided to enable any person skilled in the art to implement or use this disclosure. Various modifications to this disclosure will be apparent to those skilled in the art, and the general principles applicable herein can be applied to other variations without departing from the scope of this disclosure. Therefore, this disclosure is not limited to the examples and designs described herein, but is consistent with the widest scope of the principles and novel features disclosed herein.

Claims

1. A method for preparing a strongly spin-orbit coupled structure, characterized in that, The preparation method includes the following steps: Provide a silicon substrate; Based on the magnetron sputtering process, elemental bismuth thin films and elemental platinum thin films are alternately deposited on the silicon substrate using elemental bismuth targets and elemental platinum targets, respectively, to obtain platinum-bismuth alloy thin films; A capping layer was deposited on a platinum-bismuth alloy thin film using magnetron sputtering; wherein... Liquid nitrogen was used for cooling during the deposition of platinum-bismuth alloy thin films to suppress the crystallization of elemental bismuth films during growth. A metal layer is deposited between a bismuth film and a platinum film. The metal layer is made of an alloy of one or more of manganese, chromium, cobalt, iron, nickel, neodymium, europium, gadolinium, terbium, dysprosium, and holmium.

2. The method for preparing the strong spin-orbit coupling structure according to claim 1, characterized in that, The preparation method further includes: In-situ annealing is performed during the platinum-bismuth alloy thin film deposition process at temperatures ranging from 50°C to 250°C; and / or, After platinum-bismuth alloy thin film deposition, vacuum annealing is performed at a temperature of 100℃ to 500℃ and a vacuum degree of less than or equal to 10. -3 Pa.

3. The method for preparing a strongly spin-orbit coupled structure according to claim 1, characterized in that, The alternating deposition period of the elemental bismuth film and elemental platinum film is 2 to 100, and the sum of the thicknesses of the elemental bismuth film and elemental platinum film in each period is 0.1 nm to 5 nm.

4. The method for preparing the strong spin-orbit coupling structure according to claim 3, characterized in that, The content of the elemental bismuth target and the elemental platinum target is greater than or equal to 99.9%; and / or, the atomic percentage of bismuth in the platinum-bismuth alloy film is 20% to 80%.

5. The method for preparing a strongly spin-orbit coupled structure according to claim 1, characterized in that, The thickness of the metal thin layer is 0.01 nm to 1 nm.

6. The method for preparing a strongly spin-orbit coupled structure according to claim 1, characterized in that, The material of the capping layer is one or more of silicon oxide, silicon nitride, aluminum oxide, tantalum, ruthenium, platinum, chromium, titanium, molybdenum, and tungsten; and / or the thickness of the capping layer is 0.1 nm to 20 nm.

7. The method for preparing a strongly spin-orbit coupled structure according to claim 1, characterized in that, The preparation method further includes: A buffer layer is deposited on a silicon substrate using a magnetron sputtering process. The buffer layer is made of one or more of silicon, tantalum, tantalum nitride, ruthenium, and platinum; the thickness of the buffer layer is 0.1 nm to 10 nm, and the surface roughness is less than or equal to 0.5 nm.

8. The method for preparing a strongly spin-orbit coupled structure according to claim 1, characterized in that, In the magnetron sputtering process, the vacuum level is less than or equal to 10. -5 Pa, the working gas is an inert gas, and the working gas pressure is 0.13Pa~1.3Pa.

9. A strong spin-orbit coupling structure, characterized in that, The strong spin-orbit coupling structure is prepared by the preparation method according to any one of claims 1 to 8.

10. An SOT device, characterized in that, The SOT device includes a strong spin-orbit coupling structure and a magnetic tunnel junction structure, wherein the strong spin-orbit coupling structure is the strong spin-orbit coupling structure of claim 9.

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