A nano multi-iron heterojunction array, a preparation method and application thereof

Abstract

This invention relates to the fields of multiferroic materials and magnetoelectric technology, specifically to a nano-multiferroic heterojunction array, its fabrication method, and its applications. The method includes: sequentially growing La from bottom to top on the surface of a LaAlO3 single-crystal substrate. 0.67 Sr 0.33 A CCMO-BFO-LSMO multilayer epitaxial film was obtained by constructing a MnO3 layer, a BiFeO3 layer, and a Ca3CoMnO6 composite oxide layer. Polystyrene microspheres were then deposited on the surface of the CCMO-BFO-LSMO multilayer epitaxial film, and the microspheres were subjected to a diameter reduction process to obtain diameter-reduced polystyrene microspheres. Using the diameter-reduced polystyrene microspheres as an etching mask, a nanocylindrical array was formed through etching. Subsequently, the etching mask was removed to obtain a nano-multiferroic heterostructure array. This invention's fabrication method eliminates the need for precise alignment, avoids photolithography contamination, is compatible with epitaxial oxide systems, and enables large-area fabrication and supports the construction of size-controllable nano-multiferroic heterostructures.

Description

Technical Field

[0001] This invention relates to the fields of multiferroic materials and magnetoelectric technology, specifically to a nano-multiferroic heterojunction array, its preparation method, and its application. Background Technology

[0002] Multiferroic heterostructures (such as ferroelectric / ferromagnetic or ferroelectric / conductive oxide heterojunctions) introduce polarization degrees of freedom and magnetic order (or spin-polarized conductive channels) into the same structural unit, forming a coupling relationship between electricity, magnetism and transport. This enables effects such as electric field-controlled magnetism, magnetic field-controlled polarization, and electric field-controlled resistance, thus providing a material and structural basis for low-energy, multi-state programmable and non-volatile information storage.

[0003] The functionality of this type of heterostructure originates from the synergistic effect of multiple mechanisms, including interface exchange coupling, strain coupling, carrier modulation, and defect (such as oxygen vacancy) modulation. It can exhibit domain structure evolution, local strain release, and charge distribution characteristics different from planar thin films at the nanoscale, thereby bringing about enhanced or reconfigurable magnetoelectric response and multi-physics coupling behavior. Therefore, it has important application prospects in the fields of multistate non-volatile storage, spintronic devices, array sensing, and neuromorphic computing. In particular, it is necessary to carry out repeatable and statistical structural construction for arraying and high-density integration.

[0004] The current array fabrication of nanoscale multiferroic devices mostly relies on electron beam lithography or deep ultraviolet lithography, which typically involves multiple steps such as resist coating, exposure, development, etching / deposition, and resist removal. This process is characterized by expensive equipment, long processing cycles, complex alignment, and difficulty in consistently controlling nanometer linewidth and edge roughness. As a result, the array unit size becomes discrete over a large area, leading to increased defect rate and decreased yield. For epitaxial oxide multilayer films, photoresist residues and their degradation products, chemical etching during development / removal, and surface defects and oxygen vacancy changes induced by plasma cleaning or ion etching can also cause changes in interface chemical state, interlayer mixing, or uncontrolled stress state, thereby leading to adverse consequences such as increased leakage current, unstable ferroelectric switching, magnetic degradation, weakened magnetoelectric coupling, and poor device consistency. Summary of the Invention

[0005] To address the shortcomings of existing technologies, this invention provides a nano-multiferroic heterojunction array, its fabrication method, and its applications. First, a CCMO-BFO-LSMO multilayer epitaxial film is formed on the surface of a LaAlO3 single-crystal substrate. Then, a single-layer microsphere array mask is formed on the epitaxially grown CCMO-BFO-LSMO multilayer film. By performing plasma shrinkage treatment and ion etching pattern transfer on polystyrene microspheres, a nano-multiferroic heterojunction array is obtained. This invention, through a patterning route combining self-assembled microsphere masks with shrinkage and etching transfer, eliminates the need for precise alignment, avoids photolithography contamination, is compatible with epitaxial oxide systems, and enables large-area fabrication and supports size-controllable nano-multiferroic heterojunction structures. It not only overcomes the problems of expensive equipment, complex processes, and easy contamination inherent in traditional photolithography, but also solves the technical bottleneck of balancing epitaxial quality and array consistency. Furthermore, it enables the controllable fabrication of nano-multiferroic heterojunction arrays of multiple sizes and electrode connection configurations (topologies) on the same epitaxial film, providing a material platform for size-dependent physical property research and device integration.

[0006] To achieve the above objectives, the technical solution adopted by the present invention is as follows: The first objective of this invention is to provide a method for fabricating a nano-multiferroic heterojunction array, comprising the following steps: S1. La AlO3 single crystal substrate is grown sequentially from bottom to top on the surface of the La AlO3 single crystal substrate. 0.67 Sr 0.33 A CCMO-BFO-LSMO multilayer epitaxial film was obtained by forming a MnO3 layer (LSMO), a BiFeO3 layer (BFO), and a Ca3CoMnO6 composite oxide layer (CCMO).

[0007] S2. Polystyrene (PS) microspheres are deposited on the surface of CCMO-BFO-LSMO multilayer epitaxial films and formed into a single-layer microsphere array mask by spin coating self-assembly or interface transfer.

[0008] S3. The polystyrene microspheres are subjected to a diameter reduction treatment to reduce their diameter and form openings between them, resulting in diameter-reduced polystyrene microspheres.

[0009] S4. Using shrunken polystyrene microspheres as an etching mask, the opening pattern is transferred to a CCMO-BFO-LSMO multilayer epitaxial film through etching to form a nanocylindrical array. Then, the etching mask is removed to obtain a nano-multiferroic heterojunction array.

[0010] Preferred, La 0.67 Sr 0.33The thickness of the MnO3 layer is 2nm~80nm, the thickness of the BiFeO3 layer is 2nm~80nm, and the thickness of the Ca3CoMnO6 composite oxide layer is 2nm~80nm. This thickness range is beneficial for obtaining better photovoltaic performance. If it exceeds this range, it will be difficult to achieve the expected effect.

[0011] Preferably, plasma is used for diameter reduction treatment. The conditions for diameter reduction treatment are: oxygen plasma or a mixed plasma of oxygen and inert gas is used, and the treatment is carried out for 5s to 600s under the condition of power of 10W to 300W, so that the diameter of polystyrene microspheres is reduced to 50nm to 900nm.

[0012] Preferably, the etching process ends when the etching penetrates the Ca3CoMnO6 composite oxide layer, the BiFeO3 layer, and the La. 0.67 Sr 0.33 A MnO3 layer is formed and the surface of the LaAlO3 single-crystal substrate is exposed to form an electrically isolated nanocylindrical heterojunction array; or etching is performed to penetrate the Ca3CoMnO6 composite oxide layer and the BiFeO3 layer to the LaAlO3 substrate. 0.67 Sr 0.33 The upper surface of the MnO3 layer retains continuous La 0.67 Sr 0.33 The MnO3 layer serves as the bottom electrode, forming a nanocylindrical heterojunction array-continuous bottom electrode structure.

[0013] Preferably, a segmented etching process is performed on a rotating stage using ion etching, which is selected from ion beam etching (IBE), reactive ion etching (RIE), or inductively coupled plasma etching (ICP) to reduce sidewall redeposition and multilayer interface damage and obtain nanocylinders with approximately vertical sidewalls.

[0014] Preferably, the diameter reduction process includes partitioning and at least two diameter reduction processes to form at least two different diameter nanocylinder array regions on the same CCMO-BFO-LSMO multilayer epitaxial film.

[0015] Preferably, the LaAlO3(001) single crystal substrate is prepared according to the following steps: the LaAlO3(001) single crystal is ultrasonically cleaned with acetone, isopropanol and deionized water in sequence, dried, and then pre-annealed at 30℃~40℃ in an oxygen atmosphere to obtain the LaAlO3(001) single crystal substrate.

[0016] A second objective of this invention is to provide a nano-multiferroic heterojunction array prepared by the above-described method.

[0017] Preferably, the nano-multiferroic heterojunction array includes a LaAlO3 single crystal substrate and a nanocylindrical heterojunction array located on the surface of the LaAlO3 single crystal substrate, wherein the nanocylinders maintain a CCMO / BFO / LSMO layered heterostructure from top to bottom in the vertical direction.

[0018] Preferably, the diameter of the nanocylinders is 50nm~1000nm, the height is 20nm~200nm, and the nanocylinders are regularly distributed with a spacing of 200nm~2000nm between adjacent nanocylinders.

[0019] A third objective of this invention is to provide the application of the above-described nano-multiferroic heterojunction array in the fabrication of photovoltaic devices.

[0020] Compared with the prior art, the beneficial effects of the present invention are as follows: 1. This invention also provides a method for preparing a nano-multiferroic heterojunction array, wherein La is grown sequentially from bottom to top on the surface of a LaAlO3 single crystal substrate. 0.67 Sr 0.33 A CCMO-BFO-LSMO multilayer epitaxial film was obtained by forming a MnO3 layer, a BiFeO3 layer, and a Ca3CoMnO6 composite oxide layer. Polystyrene microspheres were deposited on the surface of the CCMO-BFO-LSMO multilayer epitaxial film to form a single-layer microsphere array mask. The polystyrene microspheres were then subjected to a diameter reduction process to reduce their diameter and form openings between them, resulting in diameter-reduced polystyrene microspheres. Using the diameter-reduced polystyrene microspheres as an etching mask, the opening pattern was transferred to the CCMO-BFO-LSMO multilayer epitaxial film through etching to form a nanocylindrical array. Subsequently, the etching mask was removed to obtain a nano-multiferroic heterojunction array.

[0021] Compared to existing methods that rely on high-end lithography technologies such as electron beam lithography or deep ultraviolet lithography to achieve nanoarrays, this invention replaces the alignment, development, and resist removal processes of electron beam or stepper lithography with a self-assembled polystyrene microsphere template (self-assembly refers to the physicochemical interaction between polystyrene microspheres), diameter reduction, and etching transfer route. It eliminates the need for lithographic alignment, significantly reduces the probability of contamination of the epitaxial oxide interface in the process chain while ensuring array quality, and improves the throughput and consistency of large-area fabrication.

[0022] In this invention, LaAlO3 single crystal is selected as the substrate to obtain stable epitaxial orientation and low defect interface. Compared with the unit differences and uncontrollable interface caused by template etching on non-epitaxy or polycrystalline substrates, this invention uses "epitaxy quality" as a prerequisite for the consistency of array units, so that each unit after subsequent nano-sizing still has comparable crystallographic and electrical boundary conditions.

[0023] Compared to methods that only etch the top electrode or form a single-layer nanolattice, this invention uses a reduced-diameter polystyrene microsphere as an etching mask to perform ion etching, transferring the opening pattern to the CCMO / BFO / LSMO layer film, directly forming a multilayer bulk nanocylinder, and suppressing sidewall redeposition, tapering and interface mixing through the etching window, ensuring that the heterogeneous interface inside each pillar is clear and reproducible.

[0024] 2. The present invention provides a nano-multiferroic heterostructure array, comprising a LaAlO3 single crystal substrate and an array of nanocylinders located on the surface of the LaAlO3 single crystal substrate. The nanocylinders maintain a layered heterostructure of CCMO / BFO / LSMO from top to bottom in the vertical direction and are regularly distributed.

[0025] This invention employs pulsed laser deposition (PLD) to epitaxially grow LSMO-BFO-CCMO multilayer epitaxial films and cuts the entire film into a nanocylinder array. Unlike the mainstream approach of "planar heterojunction + metal nanoelectrode array" in the prior art, this invention obtains an array unit where "each pillar contains a complete three-layer vertical heterojunction", which is closer to the basic device unit that can be scalably integrated from a structural level.

[0026] Compared to the contact difficulties and yield reduction caused by completely isolating nanocylinders, this invention uses La 0.67 Sr 0.33 The MnO3 layer is defined as a conductive and magnetic bottom layer, and its continuity is selectively retained as a bottom electrode by etching endpoints. This selectively continuous bottom layer topology makes the array electrical readout more stable and provides a natural common electrode for subsequent top electrode fabrication and array statistical testing.

[0027] This invention uses the BiFeO3 layer as a ferroelectric layer and utilizes nanocylindrical geometry to modulate the domain structure, strain release, and leakage channels. Compared to the situation in planar BFO films where the domain structure is difficult to independently adjust due to the influence of large-area clamping and defect distribution, the columnar geometry provides additional geometric degrees of freedom, enabling the polarization stability, switching behavior, and leakage characteristics to change controllably with size.

[0028] Unlike the conventional choice of using only an inert protective cap or metal electrode as the top layer in existing technologies, this invention introduces a Ca3CoMnO6 composite oxide layer as the top layer and uses it for both functional response and process buffering. This allows the top composite oxide layer to shield the BiFeO3 layer and withstand ion bombardment damage during the etching stage. When the nano-multiferroic heterojunction array is used in the device stage, it continues to participate in the electro- or magnetic transport regulation, thereby taking into account both processing reliability and functional layer design.

[0029] Compared to the disturbance of the chemical state of oxide surfaces caused by photoresist residues and the development / removal process, this invention uses polystyrene microspheres to self-assemble into a single-layer microsphere mask. This reduces the risk of organic residues entering the heterogeneous interface while forming a periodic array, and is suitable for obtaining uniform pore size distribution over large areas.

[0030] Unlike the discrete size selection in existing technologies where "selecting a microsphere of a fixed diameter = obtaining a single aperture", this invention uses plasma to controllably shrink the diameter of polystyrene microspheres to form adjustable openings, transforming size adjustment into continuous process parameters. This allows for precise scanning of the diameter and spacing of the nanocylinders. By combining partitioned masking with multiple diameter shrinking processes, multi-specification arrays can be obtained on the same CCMO-BFO-LSMO multilayer epitaxial film to establish a size-performance mapping. Attached Figure Description

[0031] Figure 1 Flowchart for the fabrication of nanoheterostructures.

[0032] Figure 2 This is a morphology diagram of a nanoheterostructure.

[0033] Figure 3 This is a current-voltage curve for nanodots. Detailed Implementation

[0034] The technical solution of the present invention will be clearly and completely described below with reference to the data in the embodiments of the present invention. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative effort are within the scope of protection of the present invention.

[0035] It should be noted that the technical terms used in this invention are only for the purpose of describing specific embodiments and are not intended to limit the scope of protection of this invention. Unless otherwise specified, all raw materials, reagents, instruments and equipment used in the following embodiments of this invention can be purchased on the market or prepared by existing methods.

[0036] Figure 1 A flowchart illustrating the fabrication process of the nano-multiferroic heterojunction array according to an embodiment of the present invention is shown. Figure 1 This drawing is not to scale; some details have been enlarged for clarity, and some details may have been omitted. The shapes, relative sizes, and positional relationships of the various regions and layers shown are for illustrative purposes only, and actual conditions may differ due to manufacturing tolerances or technical limitations. Furthermore, professionals in the field can design regions or layers of different shapes, sizes, and relative positions to meet specific needs.

[0037] LaAlO3(001) single crystal substrate was selected and ultrasonically cleaned with acetone, isopropanol and deionized water in sequence and then dried to ensure the cleanliness of the epitaxial starting interface. This treatment reduces the epitaxial island growth caused by particles and organic residues, and avoids significantly reducing the unit differences of the subsequent nanocylindrical array.

[0038] Pre-annealing the substrate in an oxygen atmosphere to obtain a stepped mesa morphology and remove adsorbed contaminants; compared to the discontinuity of steps and increased defect density caused by non-annealing, this step is beneficial to the uniformity of the thickness and the smoothness of the interface of the multilayer epitaxial film, thereby improving the consistency of the height and diameter of the pillars after etching.

[0039] PLD deposition of LSMO layers is used and controlled within the range of 20nm to 80nm to obtain continuous conductive channels and stable magnetism. Compared with the discontinuity caused by excessive thinness or the stress accumulation caused by excessive thickness, this thickness window is more conducive to serving as a common bottom electrode and maintaining array read / write consistency.

[0040] A BFO layer was deposited on the LSMO layer using PLD and controlled within the range of 20nm to 80nm to balance ferroelectric switching and leakage suppression. Compared to the polarization instability or increased leakage caused by improper thickness, this setting is more conducive to maintaining a measurable ferroelectric response after the formation of nanocylinders.

[0041] A CCMO layer is deposited on the BFO layer using PLD and controlled at 20nm~80nm, which allows it to both participate in the function and buffer the processing. Compared with using metal or inert oxide as the top layer to only serve as an electrode / protection, this top layer can share the ion bombardment damage during etching and provide additional transport / magnetic control channels in the device.

[0042] After deposition, annealing under oxygen or higher oxygen partial pressure reduces oxygen vacancies and stabilizes the interface valence state. Compared to the random distribution of defects caused by non-annealing, this step makes the influence of defects introduced by subsequent etching more controllable, thus facilitating the distinction between structural factors and defect factors and enabling parameter repeatability.

[0043] Polystyrene microspheres with a diameter of about 1 μm were deposited on the surface of the film, and a single-layer microsphere array mask was obtained by spin coating self-assembly or interface transfer. Compared with the uneven pore size and random etching masking caused by multi-layer stacking, the single-layer array is more conducive to forming regular periodicity and uniform columns.

[0044] The diameter of PS microspheres is reduced by using oxygen plasma or a mixture of oxygen and inert gas plasma, and the reduction amount is controlled by power and time. Compared with directly replacing microspheres of different diameters to achieve size changes, this continuous reduction method can achieve finer-grained size scanning and improve the comparability between samples.

[0045] When multiple sizes of arrays are required on the same sheet, the diameter is reduced twice or multiple times after local areas are masked. Compared with the epitaxial batch differences caused by preparing multiple samples of different sizes separately, this method of multiple sizes on the same sheet can compare the size effect under the same material baseline and significantly improve the screening efficiency.

[0046] Using shrunken polystyrene microspheres as a mask, ion beam etching or plasma etching is used to transfer patterns and form a nanocylinder array. Compared with the process of forming a metal nanoelectrode array only on the surface, the etching target is the multilayer epitaxial film body, so that the internal structure of the nanocylinder naturally maintains the vertical CCMO / BFO / LSMO layer heterojunction structure.

[0047] A rotating stage is introduced during the etching process to improve etching uniformity and reduce directional shading; compared with the sidewall skew and array anisotropy caused by fixed-direction etching, this measure can improve the symmetry of the column and reduce the geometric discrepancies within the array range.

[0048] The etching process employs segmented etching and intermittent cooling to reduce heat accumulation and redeposition. Compared to the tapering, sidewall roughness, and interface mixing caused by continuous high-power etching, this process is more conducive to obtaining near-straight walls and maintaining clear multilayer interfaces.

[0049] By controlling the etching depth, two endpoints can be achieved: etching through the CCMO layer, BFO layer, LSMO layer to LAO to form a completely isolated nanocylindrical array, or stopping at the upper surface of the LSMO layer to retain a continuous bottom electrode; compared with conventional etching, which can only obtain a single structure, this invention can switch the structural topology in the same template system to adapt to different application targets.

[0050] After etching, the PS microspheres are dissolved and removed using organic solvents such as toluene or tetrahydrofuran, and then cleaned and dried. Compared with strong desizing or strong oxidation treatments that cause secondary damage to the chemical state of the oxide surface, this gentle removal method is more conducive to maintaining the surface and interface state and reducing uncontrollable defects.

[0051] After deballing, the device undergoes oxidation annealing in an oxygen atmosphere or under controlled oxygen partial pressure to repair or regulate the oxygen vacancy gradient introduced by etching and stabilize device characteristics. If necessary, a top electrode is further deposited or dielectric filling and planarization are performed before fabricating the top electrode to form a measurable or addressable array. Compared to simply staying at the material display stage, this step provides a standardized interface for array statistical testing and device packaging and improves engineering portability.

[0052] To enable those skilled in the art to more clearly understand the technical solution of the present invention, the following will provide a detailed description in conjunction with specific embodiments: Example 1 A method for fabricating a nano-multiferroic heterojunction array, the specific fabrication process is illustrated in the schematic diagram below. Figure 1 As shown, it includes the following steps: S1. The LaAlO3(001) single crystal was ultrasonically cleaned with acetone, isopropanol and deionized water in sequence. After drying, it was pre-annealed at 40°C in an oxygen atmosphere to obtain the LaAlO3(001) single crystal substrate.

[0053] S2. Pulsed laser deposition (PLD) was performed on the surface of a LAO(001) single-crystal substrate at an oxygen partial pressure of 15 Pa and a laser energy density of 1.5 J / cm². 2 LSMO was deposited for 15 min at a pulse frequency of 5 Hz and a deposition temperature of 750 °C to form an LSMO layer with a thickness of 30 nm. Then, PLD was performed on the LSMO layer at an oxygen partial pressure of 10 Pa and a laser energy density of 1.2 J / cm². 2 BFO was deposited for 20 min at a pulse frequency of 5 Hz and a deposition temperature of 650 °C to form a BFO layer with a thickness of 20 nm. Then, PLD deposition was performed on the BFO layer at an oxygen partial pressure of 12 Pa and a laser energy density of 1.4 J / cm². 2 CCMO composite oxide was deposited for 12 min at a pulse frequency of 5 Hz and a deposition temperature of 700 ℃ to form a CCMO composite oxide layer with a thickness of 30 nm, resulting in a CCMO-BFO-LSMO multilayer epitaxial film with a total thickness of 80 nm.

[0054] S3. Polystyrene microspheres with a diameter of 1 μm were deposited on the surface of a CCMO-BFO-LSMO multilayer epitaxial film, and a single-layer microsphere array mask was formed on the surface of the CCMO-BFO-LSMO multilayer epitaxial film by spin coating self-assembly. The polystyrene microspheres were subjected to a diameter reduction treatment for 120 s using oxygen plasma at a plasma power of 100 W, so that the diameter of the polystyrene microspheres was reduced from 1 μm to 300 nm, and an opening was formed between adjacent polystyrene microspheres to obtain the diameter-reduced polystyrene microspheres.

[0055] S4. Using reduced-diameter polystyrene microspheres as an etching mask, the opening pattern is transferred to the CCMO-BFO-LSMO multilayer epitaxial film by ion etching at a depth of 80 nm until the multilayer epitaxial film in the area not covered by the polystyrene microspheres is completely etched away until the substrate surface is exposed, thereby forming a nanocylinder array. Finally, the etching mask is removed by cleaning with acetone to obtain a nano multiferroic heterojunction array.

[0056] Example 2 A method for fabricating a nano-multiferroic heterojunction array, the specific fabrication process is illustrated in the schematic diagram below. Figure 1 As shown, it includes the following steps: S1. The LaAlO3(001) single crystal was ultrasonically cleaned with acetone, isopropanol and deionized water in sequence. After drying, it was pre-annealed at 30°C in an oxygen atmosphere to obtain the LaAlO3(001) single crystal substrate.

[0057] S2. Pulsed laser deposition (PLD) was performed on the surface of a LAO(001) single-crystal substrate at an oxygen partial pressure of 15 Pa and a laser energy density of 1.5 J / cm². 2 LSMO was deposited for 15 min at a pulse frequency of 5 Hz and a deposition temperature of 750 °C to form a 20 nm thick LSMO layer. Then, PLD was performed on the LSMO layer at an oxygen partial pressure of 10 Pa and a laser energy density of 1.2 J / cm². 2 BFO was deposited for 20 min at a pulse frequency of 5 Hz and a deposition temperature of 650 °C to form a BFO layer with a thickness of 50 nm. Then, PLD deposition was performed on the BFO layer at an oxygen partial pressure of 12 Pa and a laser energy density of 1.4 J / cm². 2 CCMO composite oxide was deposited for 12 min at a pulse frequency of 5 Hz and a deposition temperature of 700 ℃ to form a CCMO composite oxide layer with a thickness of 20 nm, resulting in a CCMO-BFO-LSMO multilayer epitaxial film with a total thickness of 90 nm.

[0058] S3. Polystyrene microspheres with a diameter of 1 μm were deposited on the surface of a CCMO-BFO-LSMO multilayer epitaxial film, and a single-layer microsphere array mask was formed on the surface of the CCMO-BFO-LSMO multilayer epitaxial film by spin coating self-assembly. The polystyrene microspheres were subjected to a diameter reduction treatment for 5 s using oxygen plasma at a plasma power of 300 W, so that the diameter of the polystyrene microspheres was reduced from 1 μm to 50 nm, and an opening was formed between adjacent polystyrene microspheres to obtain the diameter-reduced polystyrene microspheres.

[0059] S4. Using reduced-diameter polystyrene microspheres as an etching mask, the opening pattern is transferred to the CCMO-BFO-LSMO multilayer epitaxial film by ion etching at a depth of 90 nm until the multilayer epitaxial film in the area not covered by the polystyrene microspheres is completely etched away until the substrate surface is exposed, thereby forming a nanocylinder array. Finally, the etching mask is removed by cleaning with acetone to obtain a nano multiferroic heterojunction array.

[0060] Example 3 A method for fabricating a nano-multiferroic heterojunction array, the specific fabrication process is illustrated in the schematic diagram below. Figure 1 As shown, it includes the following steps: S1. The LaAlO3(001) single crystal was ultrasonically cleaned with acetone, isopropanol and deionized water in sequence. After drying, it was pre-annealed at 40°C in an oxygen atmosphere to obtain the LaAlO3(001) single crystal substrate.

[0061] S2. Pulsed laser deposition (PLD) was performed on the surface of a LAO(001) single-crystal substrate at an oxygen partial pressure of 15 Pa and a laser energy density of 1.5 J / cm². 2 LSMO was deposited for 15 min at a pulse frequency of 5 Hz and a deposition temperature of 750 °C to form an LSMO layer with a thickness of 80 nm. Then, PLD was performed on the LSMO layer at an oxygen partial pressure of 10 Pa and a laser energy density of 1.2 J / cm². 2 BFO was deposited for 20 min at a pulse frequency of 5 Hz and a deposition temperature of 650 °C to form a BFO layer with a thickness of 80 nm. Then, PLD deposition was performed on the BFO layer at an oxygen partial pressure of 12 Pa and a laser energy density of 1.4 J / cm². 2 CCMO composite oxide was deposited for 12 min at a pulse frequency of 5 Hz and a deposition temperature of 700 ℃ to form a CCMO composite oxide layer with a thickness of 80 nm, resulting in a CCMO-BFO-LSMO multilayer epitaxial film with a total thickness of 240 nm.

[0062] S3. Polystyrene microspheres with a diameter of 1 μm were deposited on the surface of a CCMO-BFO-LSMO multilayer epitaxial film, and a single-layer microsphere array mask was formed on the surface of the CCMO-BFO-LSMO multilayer epitaxial film by spin coating self-assembly. The polystyrene microspheres were subjected to a diameter reduction treatment for 600 s using oxygen plasma at a plasma power of 10 W, so that the diameter of the polystyrene microspheres was reduced from 1 μm to 900 nm, and an opening was formed between adjacent polystyrene microspheres to obtain the diameter-reduced polystyrene microspheres.

[0063] S4. Using reduced-diameter polystyrene microspheres as an etching mask, the opening pattern is transferred to the CCMO-BFO-LSMO multilayer epitaxial film by ion etching at a depth of 120 nm until the multilayer epitaxial film in the area not covered by the polystyrene microspheres is completely etched away until the substrate surface is exposed, thereby forming a nanocylinder array. Finally, the etching mask is removed by cleaning with acetone to obtain a nano multiferroic heterojunction array.

[0064] Figure 2The results show that a large-area, highly uniform nanoscale multiferroic heterojunction array was successfully fabricated by combining a self-assembled template (monolayer microsphere array mask) with reactive ion etching. The nanoscale flat surface and steep sidewall morphology provide an ideal geometric basis for the subsequent uniform deposition of the top electrode, while the highly consistent nanopillar size distribution ensures the statistical uniformity of the array's electrical response.

[0065] Figure 3 The results show that the nano-multiferroic heterojunction array of the present invention exhibits significantly enhanced photovoltaic response and tunable photoelectric properties. In the LSMO / BFO / CCMO nano-heterojunction, the ferroelectric BFO layer absorbs visible light to generate electron-hole pairs, and its intrinsic polarization provides a strong depolarization field to drive carrier separation. LSMO and CCMO serve as the bottom electrode and top electrode, respectively, collecting opposite charges. The coupling between the bulk photovoltaic effect and the ferroelectric polarization enables the device to exhibit an open-circuit voltage higher than the bandgap limit and a reversible photocurrent direction.

[0066] It should be noted that when numerical ranges are involved in this invention, it should be understood that both endpoints of each numerical range, as well as any value between the two endpoints, can be selected. Since the steps and methods used are the same as in the embodiments, preferred embodiments are described here to avoid redundancy. Although preferred embodiments of the invention have been described, those skilled in the art, upon learning the basic inventive concept, can make other changes and modifications to these embodiments. Therefore, the appended claims are intended to be interpreted as including the preferred embodiments as well as all changes and modifications falling within the scope of this invention.

Claims

1. A method for fabricating a nano-multiferroic heterojunction array, characterized in that, Includes the following steps: La were grown sequentially from bottom to top on the surface of a LaAlO3 single crystal substrate. 0.67 Sr 0.33 A CCMO-BFO-LSMO multilayer epitaxial film was obtained by forming a MnO3 layer, a BiFeO3 layer, and a Ca3CoMnO6 composite oxide layer. Polystyrene microspheres were deposited on the surface of CCMO-BFO-LSMO multilayer epitaxial films to form a single-layer microsphere array mask; Polystyrene microspheres are subjected to a diameter reduction process to decrease their diameter and create openings between them, resulting in diameter-reduced polystyrene microspheres. Using shrunken polystyrene microspheres as an etching mask, the opening pattern was transferred to a CCMO-BFO-LSMO multilayer epitaxial film through etching to form a nanocylindrical array. Subsequently, the etching mask was removed to obtain a nano-multiferroic heterojunction array.

2. The preparation method according to claim 1, characterized in that, La 0.67 Sr 0.33 The thickness of the MnO3 layer is 2nm~80nm, the thickness of the BiFeO3 layer is 2nm~80nm, and the thickness of the Ca3CoMnO6 composite oxide layer is 2nm~80nm.

3. The preparation method according to claim 1, characterized in that, The diameter reduction process is carried out using plasma. The conditions for the diameter reduction process are: oxygen plasma or a mixed plasma of oxygen and inert gas is used, and the process is carried out for 5s to 600s at a power of 10W to 300W, so that the diameter of polystyrene microspheres is reduced to 50nm to 900nm.

4. The preparation method according to claim 1, characterized in that, The etching process ends when the etching penetrates the Ca3CoMnO6 composite oxide layer, the BiFeO3 layer, and the La. 0.67 Sr 0.33 MnO3 layers are layered and the surface of the LaAlO3 single crystal substrate is exposed to form an electrically isolated nanocylindrical heterojunction array; Or etching through the Ca3CoMnO6 composite oxide layer and BiFeO3 layer to La 0.67 Sr 0.33 The upper surface of the MnO3 layer retains continuous La 0.67 Sr 0.33 The MnO3 layer serves as the bottom electrode, forming a nanocylindrical heterojunction array-continuous bottom electrode structure.

5. The preparation method according to claim 4, characterized in that, The etching process is performed using ion etching, which is selected from ion beam etching (IBE), reactive ion etching (RIE), or inductively coupled plasma etching (ICP).

6. The preparation method according to claim 1, characterized in that, The diameter reduction process includes zone masking and at least two diameter reduction processes.

7. A nano-multiferroic heterojunction array prepared by the preparation method according to any one of claims 1 to 6.

8. The nano-multiferroic heterojunction array according to claim 7, characterized in that, The nano-multiferroic heterojunction array includes a LaAlO3 single crystal substrate and a nano-cylindrical heterojunction array located on the surface of the LaAlO3 single crystal substrate. The nano-cylinders maintain a CCMO / BFO / LSMO layered heterostructure from top to bottom in the vertical direction.

9. The nano-multiferroic heterojunction array according to claim 8, characterized in that, The nanocylinders have a diameter of 50nm~1000nm and a height of 20nm~200nm, and the nanocylinders are regularly distributed with a spacing of 200nm~2000nm between adjacent nanocylinders.

10. The application of the nano-multiferroic heterojunction array of claim 7 in the fabrication of photovoltaic devices.

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