A method for preparing bismuth ferrite nanoislands based on molecular beam epitaxy, its products and applications

Abstract

The application discloses a preparation method of bismuth ferrite nanometer islands based on a molecular beam epitaxy method, and comprises the following steps: conveying a substrate into a molecular beam epitaxy (MBE) growth chamber, wherein the substrate temperature is greater than or equal to the evaporation temperature of a Bi source; heating the Bi source and a Fe source to the evaporation temperature, and adjusting the beam current of the Bi source and the Fe source; using pure ozone as an epitaxial growth atmosphere, adjusting the pressure of the growth chamber to 2*10 ‑6 -1*10 ‑ 5 Torr, mixing the Bi source and the Fe source in the growth chamber to perform epitaxial growth; and cooling to obtain bismuth ferrite epitaxial nanometer islands. The application further discloses the bismuth ferrite nanometer islands obtained by the preparation method and application of the bismuth ferrite nanometer islands in a ferroelectric memory. The preparation method can realize large-area integration of the bismuth ferrite epitaxial nanometer islands, does not introduce a second phase such as a template, has simple process steps, and is high in preparation efficiency; the prepared bismuth ferrite nanometer islands have good integration, are relatively independent between each nanometer island, can perform independent writing and erasing of information, can effectively bind a domain wall position, and improve the service life of the memory.

Description

Technical Field

[0001] This invention belongs to the field of information storage technology, and particularly relates to a method for preparing bismuth ferrite nanoislands based on molecular beam epitaxy, as well as its products and applications. Background Technology

[0002] With the rapid development of artificial intelligence and big data, people have placed higher demands on the density, read / write speed, power consumption, and security of information storage. Ferroelectric memories can write and erase information by polarization switching under the influence of an electric field, thus avoiding the heat dissipation caused by current reading and writing in traditional storage devices, and have great potential in the field of information storage. In recent years, various ferroelectric memory methods and novel ferroelectric memories have been extensively studied, such as ferroelectric domain memories fabricated directly using ferroelectric materials, topological domain memories fabricated using ferroelectric superlattices (Nature, 2016, 530, 198-201; Science, 2009, 320, 190-194), and conductive domain wall memories fabricated through defect engineering (Phys. Rev. Lett., 2018, 120, 137602; ACS Nano, 2021, 15, 13380-13388; Proc. Natl. Acad. Sci. USA, 2023, 12, e2213650120) and strain engineering (Nat. Commun., 2023, 14, 4178).

[0003] However, under the influence of an external electric field, the domains and domain walls of these ferroelectric memories are unlikely to remain in the same position after each polarization switch. This can easily damage the device during use, reducing its storage density and significantly impacting its lifespan. Therefore, stabilizing the domain wall positions, achieving reusable reading and writing of information, and improving the lifespan of ferroelectric memories are pressing issues in the field of ferroelectric memories. Current main solutions involve fabricating larger electrodes and employing higher switching voltages to ensure complete domain switching. The main drawbacks of this approach are reduced memory density and significantly increased power consumption. Summary of the Invention

[0004] The purpose of this invention is to provide a method for preparing bismuth ferrite nanoislands based on molecular beam epitaxy. The prepared bismuth ferrite nanoislands are independent of each other and can be independently written and erased. They can effectively confine the domain wall positions and improve the service life and non-volatility of ferroelectric memory.

[0005] A method for preparing bismuth ferrite nanoislands based on molecular beam epitaxy, the method comprising:

[0006] (1) The substrate is introduced into the molecular beam epitaxy (MBE) growth chamber, and the substrate temperature is ≥ the evaporation temperature of the Bi source;

[0007] (2) Heat the Bi source and Fe source to the evaporation temperature and adjust the beam current of the Bi source and Fe source;

[0008] (3) Pure ozone was used as the epitaxial growth atmosphere, and the pressure in the growth chamber was adjusted to 2×10⁻⁶. -6 -1×10 -5 Torr, Bi source and Fe source are mixed in the growth chamber for epitaxial growth;

[0009] (4) After the epitaxial growth is completed, the temperature is lowered to obtain bismuth ferrite epitaxial nanoislands.

[0010] In the preparation method provided by this invention, the epitaxial layer is prepared by controlling the growth environment and growth rate of oxide molecular beam epitaxy at a temperature equal to or slightly higher than that of the Bi source evaporation temperature; the growth environment of the epitaxial layer is a pure ozone atmosphere, and the ozone partial pressure should be sufficient to fully oxidize Bi atoms without producing Bi2O. 2.5 Impurity phase, which ensures that the atomic beam has a long mean free path while providing sufficient oxidation capacity.

[0011] Preferably, in step (1), the substrate is a scandium single crystal substrate with (110) orientation.

[0012] Preferably, in step (1), the bevel angle of the substrate is less than 0.2°, and parallel atomic steps with a width of 100-500 nm can be observed on the surface. The overall surface roughness of the substrate is less than 0.4 nm. By limiting the overall surface roughness of the substrate, the quality of the bismuth ferrite nanoisland epitaxy is ensured.

[0013] Preferably, the substrate, after being pretreated by mechanical polishing and annealing, is introduced into the MBE growth chamber. The pretreatment method includes: rough polishing with 200-2000 grit sandpaper, followed by fine polishing with polishing paste to make the surface roughness less than 1 nm; cleaning the polished substrate, and then annealing at 900-1300℃.

[0014] Specifically, in this invention, the DyScO3-(110) having clear atomic steps and a smooth surface is described. pcSubstrate processing. A DyScO3 single-crystal substrate with a (110) orientation and a bevel angle <0.2° was selected. One side of the substrate was coarsely polished successively using 200, 400, 800, 1200, 1500, and 2000 grit sandpaper. Each polishing step should last more than 20 minutes to thoroughly remove scratches generated in the previous step. The thickness of the substrate after coarse polishing should be slightly greater than 0.5 mm. Fine polishing was then performed successively using 1.5 μm, 0.5 μm, and 0.25 μm polishing pastes to achieve a surface roughness of less than 1 nm. After fine polishing, the sample was ultrasonically cleaned for 15-30 minutes in sufficient amounts of acetone, isopropanol, acetone, isopropanol, and pure water to remove adhering particles and organic cutting fluid from the substrate surface. Subsequently, the substrate was annealed at a temperature above 900℃ for 0.5-2 hours to allow ions to fully migrate on the substrate surface, resulting in a substrate with a smooth surface and clear atomic steps.

[0015] Preferably, the annealing temperature should meet the energy required for atomic migration on the substrate surface but prevent the substrate from deteriorating at high temperatures. The annealing time should allow for sufficient atomic rearrangement. Therefore, the annealing temperature is usually 1000-1200℃ and the annealing time is about 1 hour to obtain a substrate with good surface quality.

[0016] Step (1) further includes: cutting the processed substrate into standard dimensions (2×10×0.5mm or 10×10×0.5mm) that conform to the molecular beam epitaxy sample tray. The sample is placed with the polished side facing up in the sample tray. Tantalum foil is cut into strips approximately 1mm wide and spot-welded to both ends or four corners of the sample. The tantalum foil covers the sample with a thickness of approximately 0.5mm to ensure the sample does not fall out and provides sufficient epitaxial area. The sample tray, with the sample side facing down, is transferred into the MBE growth chamber. Degassing is performed in increments of 20-100℃. After each temperature increase, the pressure at the top of the chamber should be reduced to 10℃. -9 After reaching the Torr level, the next step of heating and degassing will be carried out.

[0017] In step (1), the substrate temperature is 600-750°C.

[0018] In step (2), the evaporation temperature of the Bi source is 600-700℃, and the evaporation temperature of the Fe source is 1200-1500℃.

[0019] Preferably, the Bi source is Bi metal particles and the Fe source is Fe metal particles.

[0020] In step (2), the growth rate of the epitaxial layer is controlled by controlling the beam size of Fe atoms.

[0021] Preferably, the beam current ratio of the Bi source to the Fe source is greater than or equal to 4:1. To compensate for the desorption and volatilization of bismuth oxide during growth, the beam current of Bi atoms should be much higher than that of Fe atoms.

[0022] Further preferably, the beam ratio of the Bi source to the Fe source is 4-16:1.

[0023] In step (4), rapid cooling is performed during annealing to promote the nucleation process of nanoislands.

[0024] The present invention also provides a bismuth ferrite nanoisland obtained by the above preparation method.

[0025] The bismuth ferrite nanoislands have an average height of 1-10 nm, an average size of 50-200 nm, and an orientation of (001). The overall roughness RMS of the integrated nanoislands is 1-4 nm, and the roughness RMS of a single nanoisland is 0.4-1.0 nm.

[0026] The present invention also provides an application of the above-mentioned bismuth ferrite nanoislands in ferroelectric memory.

[0027] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0028] The preparation method provided by this invention uses molecular beam epitaxy (MBE) to regulate the temperature and oxygen pressure during the growth of bismuth ferrite (BFO). By controlling the volatilization and atomic surface migration rate of bismuth oxide, the growth is achieved on a substrate (such as dysprosium scandate (DyScO3-(110))). pc Bismuth ferrite nanoislands with (001) orientation are integrated on a single crystal. The temperature and oxygen pressure of the bismuth ferrite material during the growth process are controlled by oxide molecular beam epitaxy, which controls the adsorption, desorption and surface migration kinetics of Bi and Fe to adjust the nucleation and migration process of bismuth ferrite. This achieves high-level and large-area integration of bismuth ferrite epitaxial nanoislands, without the introduction of a second phase such as a template. The process steps are simple and the preparation efficiency is high.

[0029] The bismuth ferrite nanoislands prepared by this invention are highly uniform and have good integration. The bismuth ferrite nanoislands are independent of each other, and information can be written and erased independently. This can effectively confine the domain wall positions, improve the service life and non-volatility of the ferroelectric memory. Attached Figure Description

[0030] Figure 1 This is a schematic diagram of the BiFeO3 nanoisland model provided in an embodiment of the present invention;

[0031] Figure 2 X-ray diffraction pattern of BiFeO3 nanoislands provided in an embodiment of the present invention;

[0032] Figure 3 Atomic force microscopy morphology images of BiFeO3 nanoislands at different scales provided in embodiments of the present invention:

[0033] Figure 4 A height measurement diagram of BiFeO3 nanoislands provided in an embodiment of the present invention;

[0034] Figure 5 A piezoelectric microscopy image of a BiFeO3 nanoisland provided for an embodiment of the present invention;

[0035] Figure 6 AFM morphology images of nanoislands under different oxygen pressures and beam ratios provided in embodiments of the present invention;

[0036] Figure 7 The morphology of BFO grown at different temperatures is provided for embodiments of the present invention;

[0037] Figure 8 A piezoelectric microscopy image of BiFeO3 nanoislands flipping outwards, provided in an embodiment of the present invention. Detailed Implementation

[0038] Various other advantages and benefits will become apparent to those skilled in the art upon reading the following detailed description of preferred embodiments.

[0039] Specifically, the method for preparing bismuth ferrite nanoislands by molecular beam epitaxy provided in this embodiment of the invention includes:

[0040] (1) Surface treatment process for dysprosium scandium substrate, specifically including:

[0041] A DyScO3 single-crystal substrate with a (110) orientation and a bevel angle of 0.1° was selected. The substrate was coarsely polished on one side using 200, 400, 800, 1200, 1500, and 2000 grit sandpaper, with each polishing step lasting 30 minutes, to thoroughly remove scratches generated in the previous step. The substrate thickness after coarse polishing should be 0.6 mm. Fine polishing was then performed using polishing pastes of 1.5 μm, 0.5 μm, and 0.25 μm to achieve a surface roughness of less than 1 nm. After fine polishing, the sample was ultrasonically cleaned for 20 minutes in sufficient amounts of acetone, isopropanol, acetone, isopropanol, and pure water to remove adhering particles and organic cutting fluid from the substrate surface. Subsequently, the substrate was annealed at 1100℃ for 1 hour to allow ions to migrate fully on the substrate surface, resulting in a smooth substrate with clear atomic steps.

[0042] (2) Bismuth ferrite epitaxial films prepared by molecular beam epitaxy specifically include:

[0043] The prepared substrate is cut into standard dimensions of 2×10×0.5mm. The sample, polished side up, is placed in the sample holder slot. Tantalum foil is cut into strips approximately 1mm wide and spot-welded to both ends or four corners of the sample, with a tantalum foil thickness of approximately 0.5mm to ensure the sample does not fall out and provides sufficient epitaxial area. The sample holder, sample side down, is transferred into the MBE growth chamber, and degassing is performed in 50°C increments. After each temperature increase, the pressure at the top of the chamber should be reduced to 10. -9 After reaching the Torr level, the next step of heating and degassing is performed, ultimately raising the temperature to 620℃. The Bi source is heated to 620℃ and the Fe source to 1370℃, and the beam current of each source is measured using a quartz microbalance (QCM) to ensure a beam current ratio of 7:1. Pure ozone is used as the epitaxial growth atmosphere. The micro-leak valve connected to the ozone generator is opened, and the pressure in the growth chamber is adjusted to 4 × 10⁻⁶. -6 The process involves simultaneously opening the baffles of both the Bi and Fe sources to allow the two beams to mix uniformly within the chamber. Then, the substrate baffle is opened for epitaxial growth. The film thickness is controlled by calculating the Fe beam current and time. After growth, the growth chamber baffles are closed, ozone is removed, and the film is cooled in a vacuum at a rate of 1-5 °C / min until it reaches room temperature, yielding a substrate with epitaxial nanoislands.

[0044] Figure 1 This is a schematic diagram of the BiFeO3 nanoisland model according to an embodiment of the present invention; Figure 1 The application methods of nanoisland memory are given. Information storage and retrieval can be achieved by writing and erasing individual nanoislands using methods such as needle tip force or needle tip voltage.

[0045] Figure 2 X-ray diffraction patterns of BiFeO3 nanoislands prepared according to embodiments of the present invention: (a) X-ray diffraction pattern of BiFeO3 epitaxial nanoislands, (b) X-ray reciprocal space diagram (RSM); from Figure 2 It can be seen from this that the nano islands have good crystallinity and good epitaxial properties;

[0046] Figure 3 Atomic force microscopy (AFM) images of BiFeO3 nanoislands at different scales prepared for embodiments of the present invention: (a)-(b) are height maps, (c)-(d) are amplitude maps, (e)-(f) are phase maps, and (g)-(h) are three-dimensional morphologies; from Figure 3 It can be seen that the nano islands have a high density and are highly uniform.

[0047] Figure 4 Height measurement images of BiFeO3 nanoislands prepared in this embodiment of the invention: (a) AFM height map, (b) height measurement curve of the underlined portion in (a); from Figure 4It can be seen that the height of the nano islands is about 4 nm, and the width is between 100-150 nm;

[0048] Figure 5 Piezoelectric microscopy images of a BiFeO3 nanoisland prepared in an embodiment of the present invention: (a) height map, (b) three-dimensional morphology map, (c) amplitude map, and (d) phase map; from Figure 5 It can be seen that the BFO nano islands are single domains, and the domain walls are bound at the island boundaries.

[0049] In this embodiment, an array of (001)-oriented bismuth ferrite nanoislands with a height of 4 nm and an average size of approximately 10 nm × 10 nm was epitaxially grown on a (110)-oriented dysprosium scandate (DyScO3) substrate, exhibiting good integration (more than 50,000 per square millimeter). The overall roughness RMS of the integrated nanoislands is 2.8 nm, and the roughness RMS of a single nanoisland is 0.51 nm.

[0050] Figure 6 AFM morphology images of nanoislands under different oxygen pressures and beam ratios provided in the embodiments of the present invention: (a) at oxygen pressure 1×10 -5 Torr, beam ratio 7:1, (b) oxygen pressure 1×10 -5 Torr, beam ratio 15:1, (c) is oxygen pressure 4.5 × 10 -6 Torr, beam ratio 7:1, (d) is oxygen pressure 4.5 × 10 -6 Torr, beam ratio 15:1; from Figure 6 As can be seen, the morphology and density of BFO nanoislands can be controlled by adjusting the oxygen pressure and beam current ratio.

[0051] Figure 7 The embodiments of this invention provide BFO morphology images grown at different temperatures: (a) is the AFM height map of BFO grown at 400℃ without islands, and (b) is the AFM height map of BFO nanoislands grown at 620℃; from Figure 7 The results show that at low temperatures, BFO has a smooth morphology without island formation, while at 620℃, the increased nucleation rate promotes the formation of nano-islands.

[0052] Figure 8 The piezoelectric microscopy images of BiFeO3 nanoislands exhibiting out-of-plane flipping provided in this embodiment of the invention are as follows: (a)-(g) show the domain distribution after continuously increasing the out-of-plane voltage, and (h) shows the domain distribution after the voltage is removed and stabilized. Figure 8 It can be seen that the flip voltage of nanoislands is much higher than that of other regions, that is, the formation of nanoislands has a binding effect on domain walls, which can improve the lifetime and non-volatility of memory.

[0053] In summary, this invention integrates bismuth ferrite nanoislands on a dysprosium scandate substrate by controlling the growth temperature of bismuth ferrite using molecular beam epitaxy. The nanoislands exhibit excellent epitaxial quality, low surface roughness, and high integration density. Furthermore, the nanoislands are independent of each other, enabling independent writing and erasing of information, and the domain walls are confined within the nanoislands, improving the memory's write / erase cycle characteristics.

[0054] Although preferred embodiments of the invention have been disclosed for illustrative purposes, those skilled in the art will recognize that various modifications, additions, and substitutions are possible, and therefore the scope of the invention should not be limited to the embodiments described above.

Claims

1. A method for preparing BiFeO3 nanoislands based on molecular beam epitaxy, characterized in that, The preparation method comprises: (1) transferring a substrate into a molecular beam epitaxy (MBE) growth chamber, the substrate temperature being greater than or equal to the evaporation temperature of a Bi source; (2) heating the Bi source and a Fe source to the evaporation temperature and adjusting the beam current of the Bi source and the Fe source; (3) Pure ozone was used as the epitaxial growth atmosphere, the pressure of the growth chamber was adjusted to 2 x 10 -6 -1 x 10 -5 Torr, the Bi source and the Fe source were mixed in the growth chamber for epitaxial growth; (4) obtaining BiFeO3 epitaxial nano-islands after cooling after the epitaxial growth; In step (1), the substrate is a (110)-oriented scandate single crystal substrate, the substrate has an off-cut angle less than 0.2°, the surface has atom steps with a width of 100-500 nm and parallel arrangement, and the surface overall roughness of the substrate is less than 0.4 nm; In step (2), the beam current ratio of the Bi source and the Fe source is greater than or equal to 4:

1.

2. The method for preparing BiFe03 nanoislands based on molecular beam epitaxy according to claim 1, wherein In step (1), the substrate temperature is 600-750 o C.

3. The method of claim 1, wherein the method is characterized by: In step (2), the evaporation temperature of the Bi source is 600-700 o C, and the evaporation temperature of the Fe source is 1200-1500 o C.

4. The method of claim 1, wherein the method is characterized by: The beam current ratio of the Bi source and the Fe source is 4-16:

1.

5. A BiFeO3 nano-island obtained by the preparation method according to any one of claims 1-4.

6. The BiFeO3 nanoisland of claim 5, wherein, The average height of the BiFeO3 nano-island is 1-10 nm, the average size is 50-200 nm, and the orientation is (001) direction.

7. The BiFeO3 nanoisland of claim 5, wherein, The integrated nano-island has an overall roughness of RMS=1-4 nm, and a single nano-island has a roughness of RMS=0.4-1.0 nm.

8. Use of the BiFeO3 nano-island according to claim 5 in a ferroelectric memory.

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