A memristor based on vacuum-assisted drying film formation and its preparation method

Abstract

This invention discloses a memristor based on vacuum-assisted drying film formation and its preparation method, belonging to the field of oxide thin film preparation and memristor technology. The method involves precisely preparing BiFeO3 and NiFe2O4 precursor solutions, coating the precursors using a solution method, and, crucially, replacing traditional hot-stage direct drying with vacuum-assisted drying, combined with annealing and calcination processes. This first prepares dense and uniform n-type BiFeO3 films, p-type NiFe2O4 films, and BiFeO3 / NiFe2O4 heterojunctions. Then, electrodes are deposited based on these heterojunctions to complete the memristor fabrication. The memristor prepared by this method exhibits low reverse leakage current, stable switching characteristics, and excellent cycling performance, maintaining good electrical performance even after hundreds of cycles. It solves the problems of poor film quality, poor heterojunction interface bonding, and unstable memristor performance in traditional solution-based preparation methods, making it suitable for mass production of high-performance memristors.

Description

Technical Field

[0001] This invention relates to the field of oxide thin film and memristor fabrication technology, specifically to a memristor based on vacuum-assisted drying film formation and its fabrication method, realizing integrated fabrication from core thin film to functional device. Background Technology

[0002] Memristors, the fourth basic circuit element besides resistors, capacitors, and inductors, have shown great application potential in recent years in fields such as novel memories, neuromorphic computing, and neuromorphic devices. Their core function relies on memristor characteristics—the non-volatile memory behavior where resistance changes with the history of current flow. This characteristic gives memristors unique advantages in building low-power, high-density, non-volatile memory systems and artificial neural networks that simulate biological synaptic behavior. In the structural design of memristors, the quality of the oxide thin film and its heterojunction are key factors determining device performance. BiFeO3, as a typical multiferroic oxide material, possesses excellent ferroelectricity and antiferromagnetism, enabling multiferroic coupling effects that regulate magnetic order through an electric field; while NiFe2O4 is a semiconductor oxide with high spin polarization, exhibiting good conductivity and spin-dependent transport properties. The BiFeO3 / NiFe2O4 heterojunction is considered an ideal structure for fabricating high-performance memristors because its heterojunction creates a significant resistance switching effect, thereby achieving stable switching behavior and low power consumption.

[0003] However, the current fabrication technology of traditional memristors still faces many challenges, which limit the improvement of their electrical performance, reliability and stability and their practical application. Summary of the Invention

[0004] The following factors contribute to the poor performance of current traditional memristors: First, during the preparation of oxide thin films, traditional heating and drying processes easily induce the coffee ring effect and void defects, resulting in uneven film thickness and poor density, which in turn affects their electrical performance and stability. Second, heterojunction interfaces often suffer from interface defects and lattice mismatch, leading to insufficient interfacial bonding strength and reduced device reliability.

[0005] Therefore, to overcome the bottlenecks in current memristor fabrication technology, it is necessary to improve the uniformity and density of oxide thin films and enhance the bonding strength at heterojunction interfaces, thereby achieving high-performance and high-reliability memristor devices. The purpose of this invention is to overcome the shortcomings of poor quality and unstable performance in existing solution-based technologies for preparing oxide thin films, heterojunctions, and memristors, and to provide a method for preparing high-quality oxide thin films, heterojunctions, and memristors based on vacuum-assisted drying. This method achieves integrated high-quality fabrication from oxide thin films and heterojunctions to memristors by optimizing precursor formulations, innovating vacuum-assisted drying processes, and precisely controlling all process parameters, significantly improving the core performance of memristors.

[0006] The method provided by this invention uses a high-temperature resistant conductive substrate and an integrated process of "solution coating-vacuum drying-segmented calcination" to prepare a BiFeO3 / NiFe2O4 heterojunction, followed by electroplating to obtain a memristor. The oxide film is dense and uniform, the heterojunction interface is tightly bonded, the electrode contact is good, and it possesses excellent electrical properties.

[0007] Preferably, the p-type BiFeO3 thin film precursor contains 5% excess Bi to compensate for high-temperature volatilization losses; the n-type NiFe2O4 thin film precursor contains Ni... 2+ with Fe 3+ A molar ratio of 1:2 ensures crystal integrity. Both thin films undergo vacuum drying at a vacuum level ≤10 Pa to suppress defect formation. Silver electrode deposition utilizes a high-vacuum environment and precise rate control to guarantee electrode density and conductivity, forming good ohmic contact with the heterojunction.

[0008] This invention provides a method for preparing a memristor based on vacuum-assisted drying film formation, comprising the following steps:

[0009] S1 BiFeO3 precursor solution and NiFe2O4 precursor solution were prepared;

[0010] S2 The BiFeO3 precursor solution is transferred onto a conductive substrate to form a thin film, and vacuum pre-drying is performed to remove the solvent from the film.

[0011] After vacuum pre-drying, annealing is performed to eliminate internal stress in the film;

[0012] After the annealed film is naturally cooled, it is calcined in stages to decompose and crystallize the BiFeO3 precursor in stages, resulting in a dense and uniform BiFeO3 film. This avoids the formation of pores in the BiFeO3 film and improves the bonding force between the BiFeO3 film and the conductive substrate.

[0013] S3 transfers the NiFe2O4 precursor solution onto the BiFeO3 film to form a film, and performs vacuum pre-drying to remove the solvent from the film.

[0014] After vacuum pre-drying, annealing is performed to eliminate internal stress in the film;

[0015] After the annealed film is naturally cooled, it is calcined in stages to decompose and crystallize the NiFe2O4 precursor in stages, resulting in a dense and uniform NiFe2O4 film. This avoids the formation of pores in the NiFe2O4 film, enhances the bonding force between the NiFe2O4 film and the BiFeO3 film, and forms a BiFeO3 / NiFe2O4 heterojunction.

[0016] S4 An electrode is disposed on the surface of a NiFe2O4 thin film, and the electrode and the conductive substrate are used as the two ports of the memristor.

[0017] Preferably, in S2 and S3, the vacuum pre-drying temperature is 70°C and the vacuum environment pressure is ≤10Pa.

[0018] Preferably, in S2 and S3, the annealing process specifically includes: placing the sample in an annealing table and annealing at 300°C for 5 minutes;

[0019] Preferably, in S2, the segmented calcination specifically includes: in a muffle furnace, first holding at 300°C for 10 minutes, then raising the temperature to 500°C at a rate of 10°C / min, calcining for 30 minutes, and then naturally cooling to room temperature.

[0020] Preferably, in S3, the segmented calcination specifically includes: in a muffle furnace, first holding at 300°C for 10 minutes, then raising the temperature to 520°C at a rate of 10°C / min, calcining for 60 minutes, and then naturally cooling to room temperature.

[0021] Preferably, in step S2, the transfer of the BiFeO3 precursor solution onto the conductive substrate to form a thin film specifically includes:

[0022] The BiFeO3 precursor solution was dropped onto the center of the pretreated FTO substrate.

[0023] Place the solution in a spin coater, set the speed to 500 r / min, and hold for 5 seconds to allow the solution to spread evenly on the substrate surface. Then increase the speed to 3000 r / min and hold for 30 seconds to form a uniformly coated film.

[0024] Preferably, in S3, the transfer of the NiFe2O4 precursor solution onto the BiFeO3 film to form a film specifically includes: dropping the NiFe2O4 precursor solution onto the BiFeO3 film;

[0025] Place the solution in a spin coater, set the speed to 500 r / min, and hold for 5 seconds to allow the solution to spread evenly on the substrate surface. Then increase the speed to 3000 r / min and hold for 30 seconds to form a uniformly coated film.

[0026] Preferably, in the BiFeO3 precursor solution, Bi element is in 5% excess relative to Fe element to compensate for the volatilization loss of Bi element during high-temperature calcination; the concentration of Fe element is in the range of 0.1~1 mol / L.

[0027] In the NiFe2O4 precursor solution, Ni 2+ with Fe 3+Prepared at a stoichiometric ratio of 1:2, with a Ni concentration ranging from 0.1 to 1 mol / L.

[0028] Preferably, for both the BiFeO3 precursor solution and the NiFe2O4 precursor solution, the solvent is a mixture of ethylene glycol methyl ether and glacial acetic acid, with the volume ratio of ethylene glycol methyl ether to glacial acetic acid ranging from 10:1 to 4:1.

[0029] The present invention also provides a memristor prepared using the described preparation method.

[0030] Compared with existing technologies, the core advantage of this invention lies in the construction of an integrated and precise fabrication system for "thin film-heterojunction-memristor", with the following specific beneficial effects:

[0031] This innovative method employs vacuum-assisted drying instead of traditional hot-stage drying. By controlling the vacuum level to ≤10Pa, the solvent evaporation rate is homogenized, fundamentally eliminating the coffee ring effect and significantly reducing internal pore defects in the film. Characterization was performed using SEM (e.g., ...). Figure 3 As shown), the BiFeO3 thin film prepared by this method still has no obvious pores, while the traditional hot-stage dried film (such as...) Figure 2 As shown in the figure, a large number of pores are visible, and the film density is significantly improved.

[0032] Precise optimization of precursor solution formulation: In response to the volatile nature of Bi element, 5% excess Bi element is introduced into the BiFeO3 precursor, and the volatilization loss is compensated by stoichiometric control to ensure the uniformity of film composition; a mixed solvent of ethylene glycol methyl ether and glacial acetic acid is used to improve the solubility of metal nitrates and solution stability, and avoid film defects caused by precursor agglomeration.

[0033] Memristors exhibit excellent overall performance and significant application value: Memristors fabricated using this method show excellent IV cycle curves (e.g., Figure 4 As shown, a clear bistable resistance transition is observed. The current rises sharply after the forward voltage exceeds 0.6 V, while the reverse leakage current is controlled between -0.02 A and -0.06 A. After 200 cycles, the high-to-low resistance ratio remains at 10. 3 The switching characteristics show no significant degradation, solving the problems of poor stability and short lifespan of traditional memristors and meeting the mass production requirements of high-performance memory devices. Attached Figure Description

[0034] Figure 1 : Schematic diagram of the overall structure of the memristor prepared by this invention;

[0035] Figure 2 SEM image of BiFeO3 thin film prepared by traditional hot-stage drying process, showing a large number of pore defects;

[0036] Figure 3SEM image of BiFeO3 thin film prepared by the vacuum-assisted drying process of this invention; the film is dense and has no obvious defects.

[0037] Figure 4 The IV cycle curve of the BiFeO3 memristor prepared in this invention shows excellent switching stability.

[0038] Figure 5 The IV cycle curve of the BiFeO3 / NiFe2O4 heterojunction memristor prepared by this invention shows that the high-quality pn junction exhibits an extremely low Set threshold voltage.

[0039] Explanation of reference numerals in the attached figures:

[0040] 1-FTO substrate, 2-n-type BiFeO3 thin film, 3-p-type NiFe2O4 thin film, 4-silver electrode. Detailed Implementation

[0041] The following is in conjunction with the appendix Figures 1-4 The specific embodiments of the present invention will be described in detail below, but it should be understood that the scope of protection of the present invention is not limited to the specific embodiments. All other embodiments obtained by those skilled in the art based on the embodiments of the present invention without inventive effort are within the scope of protection of the present invention.

[0042] like Figure 1 As shown, in the memristor structure prepared by the present invention, 1 is an FTO substrate, 2 is a p-type BiFeO3 thin film, 3 is an n-type NiFe2O4 thin film, and 4 is a silver electrode, forming a complete "substrate-thin film-heterojunction-electrode" functional structure. The layers are tightly bonded, providing a reliable guarantee for the performance of the memristor.

[0043] The preparation method of the present invention will be described in detail below with reference to specific operational details. All reagents used in this embodiment are of analytical grade, and all instruments and equipment have been calibrated to ensure accurate parameters.

[0044] (1) FTO substrate pretreatment: Select FTO glass substrate 1 and place it in detergent solution, deionized water and ethanol in sequence. Each solution is ultrasonically cleaned for 5 minutes to remove oil, dust and other impurities from the substrate surface through multi-stage cleaning. Remove surface oil and impurities. After cleaning, place it in an oven at 80°C to dry. Then place the dried FTO glass substrate 1 in a plasma cleaner for 10 minutes to further remove residual organic matter on the surface, improve the hydrophilicity of the substrate surface, and ensure good wettability between the precursor solution and the substrate.

[0045] (2) Preparation of precursor solution: This step is the core of thin film quality control. Accurate weighing and thorough stirring are used to ensure that the solution is uniform and stable. Specifically, it includes:

[0046] ① BiFeO3 precursor solution: Using analytical grade reagents, accurately weigh 0.7635 g of bismuth nitrate pentahydrate and add it to a beaker containing a mixture of 4.5 ml ethylene glycol methyl ether and 0.5 ml glacial acetic acid. Stir magnetically for 30 minutes until completely dissolved. Then add 0.606 g of ferric nitrate nonahydrate and continue stirring magnetically until the solution is clear and transparent to prepare a 0.3 mol / L BiFeO3 precursor solution. The bismuth nitrate pentahydrate is in 5% excess to compensate for the volatilization loss of Bi element during subsequent calcination and to ensure accurate stoichiometry of the thin film.

[0047] ② NiFe2O4 precursor solution: Accurately weigh 0.291 g of nickel nitrate hexahydrate (Ni(NO3)2·6H2O), dissolve it in a mixed solvent of 4.5 ml ethylene glycol methyl ether and 0.5 ml glacial acetic acid, stir magnetically to dissolve, add 0.808 g of ferric nitrate nonahydrate, and continue stirring magnetically until the solution is clear and free of precipitate, thus preparing a 0.2 mol / L NiFe2O4 precursor solution, in which Ni 2+ with Fe 3+ The molar ratio is controlled at 1:2.

[0048] (3) Preparation of n-type BiFeO3 thin film: The BiFeO3 precursor solution prepared in step (2) ① is dropped onto the center of the pretreated FTO substrate 1; the substrate is placed in a spin coater, the rotation speed is set to 500 r / min, and it is held for 5s to make the solution spread evenly on the substrate surface. Then the rotation speed is increased to 3000 r / min and held for 30s to achieve uniform coating of the film; after spin coating, the substrate is placed in a vacuum drying oven, the vacuum degree is set to ≤10 Pa, the temperature is 70℃, and it is pre-dried for 15 minutes. The solvent in the film was removed and allowed to evaporate slowly and uniformly to suppress the coffee ring effect. After removing the substrate, it was placed in an annealing station and annealed at 300°C for 5 minutes to eliminate the internal stress caused by solvent evaporation. After the film cooled to room temperature, it was placed in a muffle furnace and held at 300°C for 10 minutes. Then, the temperature was increased to 500°C at a rate of 10°C / min and calcined for 30 minutes to fully decompose and crystallize the precursor. After naturally cooling to room temperature, a dense and uniform BiFeO3 film was obtained.

[0049] (4) Preparation of p-type NiFe2O4 thin film: Take 40 μl of the clarified NiFe2O4 precursor solution in step (2) ② and drop it onto the surface of the BiFeO3 thin film 2 prepared in step (3). Coat it with the same spin coating parameters as the BiFeO3 thin film to ensure that the thickness of the two films is matched. After spin coating, vacuum pre-dry at 70℃ for 15 min and vacuum degree ≤10Pa and anneal at 300℃ for 5 min, and cool to room temperature. Place the substrate in a muffle furnace, keep it at 300℃ for 10 minutes to avoid the interface cracking caused by the sudden temperature rise, and then raise the temperature to 520℃ at a heating rate of 10℃ / min and calcine for 60 minutes to make the NiFe2O4 precursor crystallize and form a good heterojunction interface with the BiFeO3 thin film. After naturally cooling to room temperature, a BiFeO3 / NiFe2O4 heterojunction structure is obtained. The interface of the two films in the heterojunction is tightly bonded and there is no obvious element interdiffusion phenomenon.

[0050] (5) Silver electrode preparation: A portion of the BiFeO3 / NiFe2O4 heterojunction film was scraped off with a blade, exposing a portion of the FTO substrate as the electrode contact area; the treated substrate was placed in a point electrode mask, and after fixing its position, it was placed in a vacuum deposition machine to ensure accurate electrode deposition position; the vacuum level was set to deposit silver electrode 4; after deposition was completed, the substrate was removed after the chamber temperature dropped to room temperature, resulting in a high-quality oxide heterojunction memristor based on vacuum-assisted drying. The IV cycle curve of the BiFeO3 / NiFe2O4 heterojunction memristor prepared in this invention is shown below. Figure 5 As shown, its high-quality pn junction exhibits an extremely low Set threshold voltage.

[0051] Preferably, vacuum-assisted drying in steps (3) and (4) is a key process to ensure film quality. The vacuum environment pressure is strictly controlled at ≤10 Pa. Under this pressure condition, the solvent evaporation rate is uniform and controllable, which can effectively avoid the coffee ring effect and pore defects caused by the solvent evaporation gradient in traditional hot-plate drying, while reducing the residual solvent inside the film and reducing the risk of cracking in the subsequent calcination process.

[0052] Preferably, the muffle furnace heating process in steps (3) and (4) adopts programmed temperature control, with a heating rate error of ≤ ±1℃ / min and a calcination temperature fluctuation range of ≤ ±5℃. The segmented calcination process (low-temperature holding + high-temperature calcination) can decompose the precursor in stages, avoid the violent release of gas caused by rapid heating and the formation of pores, and at the same time improve the bonding force between the film and the substrate, and between the films.

[0053] Performance test results show that, in the core structure of the memristor prepared by this invention, the ratio of high to low resistance states remains stable at 10 within a ±1 V sweep voltage range. 3After 200 cycles of testing, the on / off ratio attenuation rate was ≤5%, and the reverse leakage current remained below 0.06 A, demonstrating excellent structural stability and electrical performance. This method exhibits good process repeatability, with controllable parameters at each step, making it suitable for industrial mass production and providing strong technical support for the practical application of memristors.

[0054] Although embodiments of the invention have been shown and described, it will be understood by those skilled in the art that various changes, modifications, substitutions, and alterations can be made to these embodiments without departing from the principles and spirit of the invention. The scope of protection of the invention is defined by the appended claims and their equivalents.

Claims

1. A method for preparing a memristor based on vacuum-assisted drying film formation, characterized in that, Includes the following steps: S1 BiFeO3 precursor solution and NiFe2O4 precursor solution were prepared; S2 The BiFeO3 precursor solution is transferred onto a conductive substrate to form a thin film, and vacuum pre-drying is performed to remove the solvent from the film. After vacuum pre-drying, annealing is performed to eliminate internal stress in the film; After the annealed film is naturally cooled, it is calcined in stages to decompose and crystallize the BiFeO3 precursor in stages, resulting in a dense and uniform BiFeO3 film. This avoids the formation of pores in the BiFeO3 film and improves the bonding force between the BiFeO3 film and the conductive substrate. S3 transfers the NiFe2O4 precursor solution onto the BiFeO3 film to form a film, and performs vacuum pre-drying to remove the solvent from the film. After vacuum pre-drying, annealing is performed to eliminate internal stress in the film; After the annealed film is naturally cooled, it is calcined in stages to decompose and crystallize the NiFe2O4 precursor in stages, resulting in a dense and uniform NiFe2O4 film. This avoids the formation of pores in the NiFe2O4 film, enhances the bonding force between the NiFe2O4 film and the BiFeO3 film, and forms a BiFeO3 / NiFe2O4 heterojunction. S4 An electrode is disposed on the surface of a NiFe2O4 thin film, and the electrode and the conductive substrate serve as the two ports of the memristor; In S2 and S3, the vacuum environment pressure is ≤10Pa.

2. The preparation method according to claim 1, characterized in that, In S2 and S3, the vacuum pre-drying temperature is 70°C.

3. The preparation method according to claim 1, characterized in that, In S2 and S3, the annealing process specifically includes: placing the sample in an annealing station and annealing it at 300°C for 5 minutes.

4. The preparation method according to claim 1, characterized in that, In S2, the segmented calcination specifically includes: in a muffle furnace, first holding at 300°C for 10 minutes, then raising the temperature to 500°C at a rate of 10°C / min, calcining for 30 minutes, and then naturally cooling to room temperature.

5. The preparation method according to claim 1, characterized in that, In S3, the segmented calcination specifically includes: in a muffle furnace, first holding at 300°C for 10 minutes, then raising the temperature to 520°C at a rate of 10°C / min, calcining for 60 minutes, and then naturally cooling to room temperature.

6. The preparation method according to claim 1, characterized in that, In S2, the BiFeO3 precursor solution is transferred onto a conductive substrate to form a thin film, specifically including: The BiFeO3 precursor solution was dropped onto the center of the pretreated FTO substrate. Place the solution in a spin coater, set the speed to 500 r / min, and hold for 5 seconds to allow the solution to spread evenly on the substrate surface. Then increase the speed to 3000 r / min and hold for 30 seconds to form a uniformly coated film.

7. The preparation method according to claim 1, characterized in that, In S3, the transfer of the NiFe2O4 precursor solution onto the BiFeO3 film to form a film specifically includes: dropping the NiFe2O4 precursor solution onto the BiFeO3 film; Place the solution in a spin coater, set the speed to 500 r / min, and hold for 5 seconds to allow the solution to spread evenly on the substrate surface. Then increase the speed to 3000 r / min and hold for 30 seconds to form a uniformly coated film.

8. The preparation method according to claim 1, characterized in that, In the BiFeO3 precursor solution, Bi element is in 5% excess relative to Fe element to compensate for the volatilization loss of Bi element during high-temperature calcination; the concentration range of Fe element is 0.1~1 mol / L. In the NiFe2O4 precursor solution, Ni 2+ with Fe 3+ Prepared at a stoichiometric ratio of 1:2, with a Ni concentration ranging from 0.1 to 1 mol / L.

9. The preparation method according to claim 8, characterized in that, For both the BiFeO3 precursor solution and the NiFe2O4 precursor solution, the solvent is a mixture of ethylene glycol methyl ether and glacial acetic acid, with a volume ratio of ethylene glycol methyl ether to glacial acetic acid ranging from 10:1 to 4:

1.

10. A memristor prepared using the preparation method described in any one of claims 1 to 9.

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