Transition metal sulfide thin films and methods of making the same, resistive random access memory and methods of making the same
By using ammonium thiomolybdate and other precursors and a specific liquid-phase coating annealing process, the problem of poor film quality in two-dimensional resistive switching memory was solved, and high-quality, large-area, few-layer two-dimensional films were prepared, which are suitable for the integration of resistive switching memory.
Patent Information
- Authority / Receiving Office
- CN · China
- Patent Type
- Patents(China)
- Current Assignee / Owner
- JIMEI UNIV
- Filing Date
- 2022-10-27
- Publication Date
- 2026-05-12
AI Technical Summary
Existing technologies for fabricating two-dimensional resistive switching memories (SSNs) suffer from challenges such as poor quality of two-dimensional material films, loss of sulfur during high-temperature annealing, and difficulties in controlling the uniformity and grain size of multilayer films, which affect device performance and integration.
Using ammonium thiomolybdate, ammonium thiotungstate, and ammonium thiovanadate as precursors, and combined with a specific liquid-phase coating annealing process, high-quality, large-area, few-layer two-dimensional transition metal sulfide films were prepared by controlling the replenishment of sulfur elements through annealing in a hydrogen-containing atmosphere, and then applied to resistive switching memory.
This method enables the fabrication of high-quality, large-area, few-layer two-dimensional thin films, improving device performance and tolerance to subsequent processes. It avoids the loss of sulfur during high-temperature annealing, ensuring the uniformity and crystal quality of the film, and is suitable for the integration of resistive switching memory.
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Figure CN115802876B_ABST
Abstract
Description
Technical Field
[0001] This invention belongs to the field of thin film materials technology, specifically relating to a transition metal sulfide thin film and its preparation method, and a resistive switching memory and its preparation method. Background Technology
[0002] As the semiconductor industry's demands for computer storage capacity and computing speed increase, traditional storage devices are gradually approaching their theoretical limits in terms of process technology and speed. Currently, academia and the semiconductor industry are continuously exploring a new information storage technology that is high-speed, low-power, high-density integrated, and has the potential for in-memory computing. Among them, two-dimensional resistive random access memory (RRAM), as a novel storage technology, combines non-volatility and high-speed access, and is expected to integrate the technological advantages of mainstream dynamic memory (DRAM) and flash memory (Flash), achieving in-memory computing, optimizing and upgrading computer hierarchical architecture, and thus playing an important role in the future information field and national industrial system.
[0003] Two-dimensional materials used in RRAM include boron nitride (hBN), two-dimensional perovskite crystals, black phosphorus composites, and transition metal dichalcogenides (TMDs). A series of studies on two-dimensional material RRAMs reveals a seesaw effect between material preparation, device performance, and integration processes. Specifically, for example, liquid phase exfoliation can prepare large-area black phosphorus and graphene films, but its ability to control the number of layers and defect density is poor; mechanical exfoliation produces TMDs with excellent performance and good heat resistance (340℃), but the film size is typically on the order of 10 micrometers, making it unsuitable for large-area integration; chemical vapor deposition (CVD) can achieve the preparation of large-area monolayer two-dimensional materials, but controlling crystallinity remains a challenging task, and the issues of film uniformity and grain size need to be resolved. In contrast, liquid phase coating processes hold promise for preparing high-quality, large-area, few-layer two-dimensional films, thus balancing the excellent performance of two-dimensional materials with the film's resilience in subsequent processes.
[0004] Liquid-phase coating and annealing (LPGA) for the preparation of TMDs refers to a liquid-phase chemical method that thermally decomposes precursors containing metals and chalcogen elements to form thin films. In 2011, Italian researcher Claudia Altavilla proposed using this method to prepare TMD nanosheets. Subsequently, the team led by Lain-Jong Li at TSMC's R&D department synthesized continuous TMD films on silicon oxide and sapphire surfaces using a similar method. The steps are briefly summarized as follows: an organic solution containing the precursor is coated onto a growth substrate (spin-coating or dip-coating), followed by baking and vacuum high-temperature annealing, ultimately yielding a film of uniform thickness. The coating quality, solution formulation, and annealing process determine the morphology and crystallinity of the film; the size of the substrate, in principle, determines the lateral dimensions of the film. The research team of S. Lee in South Korea borrowed this method, using pulsed laser heating to heat the precursor film coated on a 4-inch wafer to achieve patterned few-layer TMD films with a carrier mobility of 6.39 cm⁻¹. 2 V -1 s -1 It is evident that although this method successfully prepared large-area continuous few-layer TMDs films, their quality still lags significantly behind that of chemical vapor deposition. The loss of chalcogens during prolonged high-temperature vacuum annealing (and the ineffectiveness of replenishing chalcogens during annealing), and the insufficient heating temperature of pulsed lasers to form high-quality crystals, are likely reasons for the poor quality of the few-layer TMDs films. Summary of the Invention
[0005] To address the shortcomings of existing technologies, this invention provides a transition metal sulfide thin film and its preparation method, as well as a resistive switching memory and its preparation method.
[0006] Specifically, the present invention provides the following technical solutions:
[0007] This invention first provides a method for preparing transition metal sulfide thin films, comprising:
[0008] A precursor is provided, wherein the precursor is selected from at least one of ammonium thiomolybdate, ammonium thiotungstate, and ammonium thiovanadate;
[0009] The precursor and organic solvent are mixed to obtain a precursor solution;
[0010] The precursor solution is coated onto a first substrate and dried to form a precursor film.
[0011] A second substrate having a sulfide film formed thereon is provided;
[0012] A first substrate having a precursor film formed thereon and a second substrate having a sulfide film formed thereon are laminated in such a way that the precursor film and the sulfide film are in contact to form a laminate.
[0013] The laminate is annealed in a hydrogen-containing atmosphere to obtain a first substrate having a transition metal sulfide film formed thereon.
[0014] The synthesis of high-quality transition metal chalcogenides (TMDs) requires precise feed control of both the transition metals and sulfur precursors. One of the most challenging aspects of TMD quality control is the abundance of sulfur vacancies in most synthesized films. Over the past decade, significant efforts have been made to develop controllable sulfur supply methods, such as using elemental sulfur or sulfur compounds (e.g., heating sulfur powder, using hydrogen sulfide gas, or ammonium sulfide). However, the sulfur supply control has not been ideal. This is because, under normal conditions, monomeric states only exist at very high temperatures (>2500 K). Therefore, at typical TMD growth temperatures (generally <1300 K), most methods using sulfur supplementation cannot generate sufficient monomers.
[0015] This invention discovers that by using ammonium thiomolybdate, ammonium thiotungstate, and ammonium thiovanadate as precursors and combining them with a specific liquid-phase coating annealing process, the sulfur supply control effect is good, which can effectively avoid the loss of S element during the conversion of (NH4)2MOS4 to MoS2 in the high-temperature annealing process. This results in the preparation of high-quality, large-area, few-layer two-dimensional thin films that take into account the excellent performance of two-dimensional materials and the film's tolerance in subsequent processes, and are not affected by the uniformity and grain size issues of multilayer films.
[0016] Preferably, the organic solvent includes dimethylformamide, n-butylamine, and ethanolamine; more preferably, the mass ratio of dimethylformamide, n-butylamine, and ethanolamine is 4–6:1.5–2.5:1. This invention has found that using the above-mentioned organic solvent and ratio results in better dissolution of ammonium thiomolybdate, leading to a more uniform precursor film surface. Other ratios often result in the ammonium thiomolybdate failing to dissolve or exhibiting poor dissolution.
[0017] More preferably, the mass ratio of dimethylformamide, n-butylamine, and ethanolamine is 5:2:1. Experiments have shown that ammonium thiomolybdate exhibits the best solubility at a ratio of 5:2:1.
[0018] Preferably, the concentration of the precursor solution is 1 wt% to 6 wt%. A precursor solution concentration of 1 wt% to 6 wt% is acceptable; different concentrations result in different film thicknesses. Experiments have shown that when the concentration is below 1 wt%, the precursor film is too thin after the baking process, and the formed molybdenum disulfide film is prone to discontinuity; when the concentration is above 6 wt%, ammonium thiomolybdate is not completely dissolved, resulting in an uneven precursor film and the presence of undissolved particulate matter on the surface.
[0019] Preferably, the first substrate and the second substrate are each independently selected from a sapphire substrate or a SiO2 substrate.
[0020] Preferably, the precursor solution is coated onto the first substrate using a process selected from the group consisting of spraying, spin coating, brush coating, dip coating, and roll coating; more preferably, spin coating is performed at 2000-3000 r / min. Experiments have shown that using the above spin coating process results in a precursor film with uniform and easily controllable thickness.
[0021] Preferably, the drying temperature is 120–150°C and the drying time is 2–5 minutes.
[0022] Preferably, the sulfide film is a zinc sulfide film, which is deposited on the surface of the second substrate by an electron beam deposition machine.
[0023] Preferably, the hydrogen content of the hydrogen-containing atmosphere is 15-25%, and the remaining atmosphere in the hydrogen-containing atmosphere is an inert atmosphere, preferably argon.
[0024] Preferably, the laminate is annealed in a hydrogen-containing atmosphere, which includes annealing at a temperature of 800°C to 850°C for 20 to 40 minutes.
[0025] Taking ammonium tetrathiomolybdate as the precursor, the following reactions mainly occur during the annealing process:
[0026] A. At 120–360 °C, (NH4)2MOS4 is converted into MoS3.
[0027] B. Annealing at higher temperatures, such as above 800°C, converts MoS3 to MoS2.
[0028] C. The conversion of (NH4)2MoS4 to MoS2 in an H2 gas environment requires a temperature as low as ~450℃, as shown in Formula 3. This invention uses 800-850℃ because this results in a higher quality MoS2 film. Under high temperature and low pressure, sulfur vacancies easily form in MoS2. If sulfur is not replenished in time, the quality of the formed MoS2 film will be low. The sulfide film on the second substrate plays a role in continuously replenishing sulfur during the annealing process.
[0029] (NH4)2MoS4→2NH3+H2S+MoS3 (1)
[0030] MoS3→MoS2+S (2)
[0031] (NH4)2MoS4+H2→2NH3+2H2S+MoS2 (3)
[0032] The present invention also provides a transition metal sulfide thin film, which is prepared by the above preparation method.
[0033] The present invention also provides a resistive switching memory, comprising, from bottom to top, a substrate, a bottom electrode, a resistive switching layer, and a top electrode; the resistive switching layer comprises the above-described transition metal sulfide thin film or the transition metal sulfide thin film prepared by the above-described preparation method.
[0034] Preferably, the thickness of the transition metal sulfide film is in the range of 2–6 nm.
[0035] Preferably, the substrate is made of Si and / or SiO2.
[0036] Preferably, the materials of the bottom electrode and the top electrode are each independently selected from Au, TiN, TaN, Ir, Al, Ru or Pd, and the thickness of the bottom electrode and the top electrode ranges from 20 to 50 nm.
[0037] The present invention also provides a method for fabricating the above-mentioned resistive random access memory, comprising:
[0038] Provide substrate;
[0039] A bottom electrode is formed on the substrate;
[0040] A resistive switching layer is formed on the side of the bottom electrode opposite to the substrate;
[0041] A top electrode is formed on the side of the resistive switching layer opposite to the substrate.
[0042] Preferably, the specific steps for forming the resistive switching layer on the side of the bottom electrode opposite to the substrate are as follows:
[0043] PMMA is coated onto the first substrate having a transition metal sulfide film formed thereon and dried to form a PMMA film;
[0044] The PMMA film was adhered using heat-release adhesive tape.
[0045] The first substrate was etched with an alkaline solution to obtain a transition metal sulfide film / PMMA film / thermal release tape.
[0046] The side of the transition metal sulfide film / PMMA film / heat release tape with the transition metal sulfide film is attached to the side of the bottom electrode away from the substrate.
[0047] Heating causes the heat-release tape to lose its stickiness, and then the heat-release tape is peeled off.
[0048] The PMMA film is removed using acetone to obtain the final product.
[0049] Compared with the prior art, the present invention has the following advantages:
[0050] (1) The method for preparing transition metal sulfide thin films provided by the present invention can prepare high-quality large-area few-layer two-dimensional thin films, thereby taking into account the excellent performance of two-dimensional materials and the tolerance of the film in subsequent processes, and is not affected by the uniformity and grain size of multilayer films.
[0051] (2) The method for preparing transition metal sulfide films provided by the present invention, compared with the prior art, adopts a unique coating annealing method that can effectively avoid the loss of S element during the conversion of (NH4)2MOS4 to MoS2 during high-temperature annealing, and ensure the crystal quality of the corresponding transition metal sulfide films (currently, both MoS2 and WS2 films can be prepared using this method). Attached Figure Description
[0052] Figure 1 Optical image of the MOS2 thin film fabricated in Example 1.
[0053] Figure 2 The Raman spectrum of the MOS2 thin film prepared in Example 1 is shown; in the figure, the horizontal axis represents the Raman shift in cm. -1 The vertical axis represents the intensity of the Raman scattered light.
[0054] Figure 3 This is a schematic flowchart of the fabrication method of the resistive switching memory provided in Example 2.
[0055] Figure 4 The resistive switching curve is shown in Example 2 for the resistive switching memory.
[0056] Figure 5 The optical images of the (NH4)2MOS4 thin film fabricated in Comparative Example 1 are compared with those of the (NH4)2MOS4 thin film fabricated in Example 1; wherein, the left image is the optical image of the (NH4)2MOS4 thin film fabricated in Comparative Example 1, and the right image is the optical image of the (NH4)2MOS4 thin film fabricated in Example 1. Detailed Implementation
[0057] The specific embodiments of the present invention will be described in further detail below with reference to the accompanying drawings and examples.
[0058] The following examples are for illustrative purposes only and are not intended to limit the scope of protection of the present invention.
[0059] Unless otherwise specified, the experimental methods used in the following examples are conventional methods.
[0060] Unless otherwise specified, all materials and reagents used in the following examples are commercially available.
[0061] Example 1
[0062] A method for preparing a transition metal sulfide thin film specifically includes the following steps:
[0063] S1. Prepare a mixture of dimethylformamide (DMF), n-butylamine, and 2-aminoethanol in a ratio of 5:2:1, and sonicate for 20 minutes, avoiding overheating during the process.
[0064] S2. Add 0.25g of 99.99% pure ammonium thiomolybdate (NH4)2MOS4 to 20ml of the prepared mixture to form a 1.25wt% solution. Before use, sonicate for at least 30 minutes, avoiding overheating during the process. The (NH4)2MOS4 solution is now ready.
[0065] S3. Clean the insulating substrate (sapphire or 300nm SiO2), bake it on the board at 80℃ for 10 minutes, and treat it with ultraviolet ozone for 1 minute.
[0066] S4. Spin-coat the prepared (NH4)2MOS4 solution onto the treated sapphire or 300nm SiO2 at a speed of 2000-3000 / mins.
[0067] S5. Place the substrate on a hot plate at 120-150 degrees Celsius and bake for 3 minutes to solidify the film. That is, coat the substrate with a (NH4)2MOS4 film with a thickness of about 30nm.
[0068] S6. Take substrate 2 (sapphire) and place it in an electron beam coating machine to deposit a zinc sulfide film with a thickness of 500nm.
[0069] S7. Place the prepared substrate 1 (sapphire) / (NH4)2MOS4 film and substrate 2 (sapphire) / zinc oxide film face to face in a CVD tube furnace and anneal for 30 mins at Ar / H2 flow rate = 4 / 1, gas pressure 1 torr, and temperature 800℃ to obtain substrate 1 (sapphire) / MOS2 thin film sample.
[0070] Figure 1 This is an optical image of the MOS2 thin film fabricated in Embodiment 1 of the present invention. Figure 2 The image shows the Raman spectrum of the MOS2 thin film fabricated in Example 1 of this invention. The optical spectrum of the film demonstrates its excellent uniformity, and the characteristic peak at 382 cm⁻¹ is visible in the corresponding Raman spectrum. -1 (Half height and width: 4.78) and 407cm -1 (Half width at half maximum: 7.97) indicates that the prepared multilayer MoS2 film has high crystallinity.
[0071] Example 2
[0072] A method for fabricating resistive random access memory (RANM), the entire process of which can be partially referenced. Figure 3 This includes the following steps:
[0073] S1. Fabrication of the metal bottom electrode:
[0074] AZ5209 photoresist was dropped onto a 2cm×2cm Si / SiO2 (280nm) substrate and spin-coated for 1 minute at 600 rpm. The substrate was then placed on a hot plate and baked at 90°C for 2 minutes. Exposure was performed using a UV lithography machine (Karl Suss MA6) in soft-contact mode with the following parameters: exposure power 7.5mW, exposure time 5s, and a 15μm gap between the photoresist and the sample surface. The sample was then baked at 110°C for 50s on a hot plate and developed with developer for 95s. After the UV lithography process, a Ti / Au(Pt) (2 / 45nm) metal layer was deposited on the sample using electron beam evaporation, with Ti acting as an adhesion layer between the Au and the substrate. The sample was then soaked in acetone, left overnight, and then subjected to a lift-off process to complete the fabrication of the patterned interdigitated metal electrodes on the substrate layer (Si / SiO2 (280nm)), hereinafter referred to as substrate three.
[0075] Transfer of S2 and MoS2 thin films:
[0076] (1) Take PMMA A4 out of the refrigerator and let it stand for half an hour until it reaches room temperature; spin-coat it onto the substrate (sapphire) / MoS2 film prepared in Example 1 at a speed of 600 rpm for 1 min, and set the temperature of the heating stage to 120°C for 20 min + 150°C for 3 min; so that the substrate / MoS2 film and PMMA are fully bonded.
[0077] (2) Use a blade to scrape off the PMMA around the sapphire;
[0078] (3) Blow clean the PMMA surface and apply heat release tape; use a waste substrate to gently press the tape and sample together;
[0079] (4) Place the sample on the surface of KOH solution (the higher the concentration, the better) to etch the sapphire. It can be removed within one hour.
[0080] (5) Take out the MoS2 / PMMA / tape, rinse it in deionized water (three times), and float it on the water surface for 30 minutes to reduce impurities;
[0081] (6) Take out the sample, blow it dry and naturally attach it to the pre-cleaned substrate 3, and leave it for several hours;
[0082] (7) Place it in a vacuum chamber and evacuate the air. Make sure that the PMMA and the substrate are in close contact. Otherwise, when you peel off the heat release tape, the PMMA will be taken away.
[0083] (8) Bake: Heat release tape can be removed at 125℃~135℃;
[0084] (9) Bake sample: Bake at 120°C for 20 min to further adhere the PMMA film to the substrate.
[0085] (10) Place it in a vacuum chamber and evacuate for 1-2 hours to further ensure that the film adheres tightly to the substrate.
[0086] (11) Remove PMMA with acetone, 60°C (acetone should be preheated to the preset temperature, and the sample should also be preheated at 60°C for 1 min).
[0087] Finally, the prepared MOS2 thin film was transferred from sapphire substrate one to prepared substrate three (Si / SiO2 (280nm) / bottom electrode).
[0088] S3. Fabrication of the metal bottom electrode:
[0089] The substrate / bottom electrode / MoS2 film obtained in S2 is patterned again using ultraviolet lithography to create the top electrode. This results in a substrate / bottom electrode / MoS2 film / top electrode, which is the metal sulfide thin-film resistive switching memory (MIM) structure.
[0090] The obtained resistive switching curve of the resistive switching memory is as follows: Figure 4 As shown, the fabricated memristor device exhibits bipolar resistive switching behavior, where a positive bias sets the MoS2 resistive switching layer from a high-resistivity state to a low-resistivity state, and then a negative bias resets the MoS2 resistive switching layer from a low-resistivity state to a high-resistivity state. Figure 4 Cyclic testing with over 30 manual DC scans was demonstrated, confirming the device's robustness with an average window exceeding one order of magnitude. Over 150 DC scan cycles were also observed in some devices, and more cycles are expected with further measurements. Furthermore, the device's set voltage is approximately 0.5–0.8V, meeting the requirements for low power consumption.
[0091] Comparative Example 1
[0092] Compared with Example 1, the only difference is that in S1, the 5:2:1 mixture of dimethylformamide (DMF): n-butylamine: 2-aminoethanol is replaced with a single DMF.
[0093] The optical effect image of the (NH4)2MOS4 thin film coated in Comparative Example 1 is compared with that of the (NH4)2MOS4 thin film coated in Example 1. Figure 5 The precursor film formed in Example 1 has a more uniform surface.
[0094] Finally, the above are merely preferred embodiments of the present invention and are not intended to limit the scope of protection of the present invention. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the scope of protection of the present invention.
Claims
1. A method for preparing a transition metal sulfide thin film, characterized in that, include: A precursor is provided, wherein the precursor is selected from at least one of ammonium thiomolybdate, ammonium thiotungstate, and ammonium thiovanadate; The precursor and an organic solvent are mixed to obtain a precursor solution; the organic solvent includes dimethylformamide, n-butylamine, and ethanolamine; the mass ratio of dimethylformamide, n-butylamine, and ethanolamine is 4~6:1.5~2.5:1; The precursor solution is coated onto a first substrate and dried to form a precursor film. A second substrate having a sulfide film formed thereon is provided; A first substrate having a precursor film formed thereon and a second substrate having a sulfide film formed thereon are laminated in such a way that the precursor film and the sulfide film are in contact to form a laminate. The laminate is annealed in a hydrogen-containing atmosphere to obtain a first substrate having a transition metal sulfide film formed thereon.
2. The preparation method according to claim 1, characterized in that, The mass ratio of dimethylformamide, n-butylamine, and ethanolamine is 5:2:
1.
3. The preparation method according to claim 1 or 2, characterized in that, The concentration of the precursor solution is 1 wt% to 6 wt%.
4. The preparation method according to claim 1 or 2, characterized in that, The precursor solution is coated onto the first substrate using a process selected from the group consisting of spraying, spin coating, brush coating, dip coating, and roll coating.
5. The preparation method according to claim 4, characterized in that, The coating is selected from spin coating at 2000-3000 r / min.
6. The preparation method according to claim 1 or 2, characterized in that, The sulfide film is a zinc sulfide film, which is deposited on the surface of the second substrate by electron beam deposition.
7. The preparation method according to claim 3, characterized in that, The laminate was annealed in a hydrogen-containing atmosphere at a temperature of 800°C to 850°C for 20 to 40 minutes.
8. A transition metal sulfide thin film, characterized in that, It is prepared by the preparation method according to any one of claims 1-7.
9. A resistive random access memory, characterized in that, From bottom to top, it includes a substrate, a bottom electrode, a resistive switching layer, and a top electrode; The resistive switching layer comprises the transition metal sulfide thin film of claim 8.
10. A method for fabricating a resistive random access memory as described in claim 9, characterized in that, include: Provide substrate; A bottom electrode is formed on the substrate; A resistive switching layer is formed on the side of the bottom electrode opposite to the substrate; A top electrode is formed on the side of the resistive switching layer opposite to the substrate.
11. The preparation method according to claim 10, characterized in that, The specific steps for forming the resistive switching layer on the side of the bottom electrode opposite to the substrate are as follows: PMMA is coated onto the first substrate having a transition metal sulfide film formed thereon and dried to form a PMMA film; The PMMA film was adhered using heat-release adhesive tape. The first substrate was etched with an alkaline solution to obtain a transition metal sulfide film / PMMA film / thermal release tape. The side of the transition metal sulfide film / PMMA film / heat release tape with the transition metal sulfide film is attached to the side of the bottom electrode away from the substrate. Heating causes the heat-release tape to lose its stickiness, and then the heat-release tape is peeled off; The PMMA film is removed using acetone to obtain the final product.