Preparation method of bismuth ferrite nano island based on molecular beam epitaxy method and product and application of bismuth ferrite nano island

Abstract

The invention discloses a preparation method of a bismuth ferrite nano island based on a molecular beam epitaxy method, which comprises the following steps: a substrate is transferred into a molecular beam epitaxy (MBE) growth chamber, and the temperature of the substrate is greater than or equal to the evaporation temperature of a Bi source; the Bi source and the Fe source are heated to the evaporation temperature, and the beam sizes of the Bi source and the Fe source are adjusted; pure ozone is adopted as an epitaxial growth atmosphere, the pressure of the growth chamber is adjusted to 2 * 10 <-6 >-1 * 10 <-5 > Torr, and the Bi source and the Fe source are mixed in the growth chamber for epitaxial growth; and cooling to obtain the bismuth ferrite epitaxial nano island. The invention also discloses the bismuth ferrite nano island obtained by the preparation method and an application of the bismuth ferrite nano island in a ferroelectric memory. According to the preparation method, large-area integration of the bismuth ferrite epitaxial nano island can be realized, introduction of a second phase such as a template is avoided, the process steps are simple, and the preparation efficiency is high; the prepared bismuth ferrite nano island has a good integration level, the nano islands are relatively independent, information can be independently written and erased, the position of a domain wall can be effectively bound, and the service life of a memory is prolonged.

Description

Technical Field

[0001] The present invention belongs to the technical field of information storage, and in particular relates to a method for preparing bismuth ferrite nano-islands based on molecular beam epitaxy, and a product and application thereof. Background Art

[0002] With the rapid development of artificial intelligence and big data, people have put forward higher requirements for the density, read and write speed, power consumption and security of information storage. Ferroelectric memory can complete the writing and erasing of information by polarization flipping under the action of electric field, thus avoiding the heat dissipation generated by current reading and writing in traditional storage devices, and has great potential in the field of information storage. In recent years, various ferroelectric storage methods and new ferroelectric memories have been widely studied, such as ferroelectric domain memories directly prepared using ferroelectric materials, topological domain memories prepared by ferroelectric superlattices (Nature, 2016, 530, 198-201; Science, 2009, 320, 190-194), and conductive domain wall memories prepared by 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, when these ferroelectric memories are flipped under an external electric field, it is difficult for their domains and domain walls to be in the same position after each polarization switch, which can easily damage the device itself during use, thereby reducing the storage density of the ferroelectric memory and greatly affecting the service life of the ferroelectric memory. Therefore, how to stabilize the position of the domain wall, realize the repeated reading and writing of information, and improve the service life of the ferroelectric memory is an urgent problem to be solved in the current ferroelectric memory field. The current main solution is to prepare electrodes with relatively large sizes and use a higher flip voltage to ensure that the domain is completely flipped. The main problem it brings is that the memory density is reduced and the power consumption is greatly increased. Summary of the invention

[0004] The purpose of the present invention is to provide a method for preparing bismuth ferrite nano-islands based on molecular beam epitaxy. The prepared bismuth ferrite nano-islands are independent of each other, information can be independently written and erased, domain wall positions can be effectively bound, and the application in ferroelectric memory can improve its service life and non-volatility.

[0005] A method for preparing bismuth ferrite nano-islands based on molecular beam epitaxy, the preparation 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) Raising the temperature of the Bi source and the Fe source to the evaporation temperature, and adjusting the beam current of the Bi source and the Fe source;

[0008] (3) Use pure ozone as the epitaxial growth atmosphere and adjust the growth chamber pressure to 2×10 -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 nano-islands.

[0010] In the preparation method provided by the present invention, the epitaxial layer is prepared by controlling the growth environment and growth rate of the oxide molecular beam epitaxy method, and growing at a temperature equal to or slightly higher than the evaporation temperature of the Bi source; the growth environment of the epitaxial layer is a pure ozone atmosphere, and the ozone partial pressure should be able to fully oxidize Bi atoms without generating Bi2O 2.5 The heterophase 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 (110) oriented scandate single crystal substrate.

[0012] Preferably, in step (1), the bevel angle of the substrate is less than 0.2°, atomic steps with a width of 100-500 nm and arranged in parallel can be observed on the surface, and 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 epitaxy of the bismuth ferrite nano-islands is guaranteed.

[0013] Preferably, the substrate pre-treated by mechanical polishing and annealing is introduced into the MBE growth chamber, wherein the pre-treatment method comprises: rough polishing by sandpaper of 200 to 2000 mesh, and then fine polishing by polishing paste to make the surface roughness less than 1 nm; cleaning the polished substrate, and then heat-annealing at 900-1300°C.

[0014] Specifically, in the present invention, the DyScO3-(110) having clear atomic steps and a smooth surface pcSubstrate treatment process. A DyScO3 single crystal substrate with a (110) orientation bevel angle of <0.2° was selected, and 200, 400, 800, 1200, 1500, and 2000 grit sandpaper were used to rough-polish the substrate on one side. The polishing time for each step should exceed 20 minutes to fully remove the scratches generated in the previous step. The thickness of the substrate after rough polishing should be slightly greater than 0.5 mm. Polishing pastes of 1.5 μm, 0.5 μm, and 0.25 μm were used for fine polishing to make the surface roughness less than 1 nm. The samples after fine polishing were ultrasonically cleaned in sufficient acetone, isopropanol, acetone, isopropanol, and pure water for 15-30 minutes to remove the attached particles and organic cutting fluid on the substrate surface. After that, the samples were annealed at a temperature above 900°C for 0.5-2 hours to allow the ions to fully migrate on the substrate surface and obtain a substrate with a flat 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 temperature. The annealing time should allow the atoms to fully rearrange. Therefore, the annealing temperature is usually 1000-1200°C, and the annealing time is about 1 hour to obtain a substrate with good surface quality.

[0016] The step (1) further includes: cutting the processed substrate into a standard size that meets the molecular beam epitaxy sample tray slot (2×10×0.5mm or 10×10×0.5mm). Place the sample with the polished surface facing upward in the sample tray slot, cut the tantalum foil into strips with a width of about 1mm, and spot weld the two ends or four corners of the sample to fix it. The tantalum foil is covered with a thickness of about 0.5mm to ensure that the sample will not fall and has sufficient epitaxial area. Place the sample tray with the sample facing downward into the MBE growth chamber, and perform a heating and degassing operation in steps of 20-100°C. After each heating, 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 operation is 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°C, and the evaporation temperature of the Fe source is 1200-1500°C.

[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 the growth process, the beam current of Bi atoms should be much higher than the beam current of Fe atoms.

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

[0023] In step (4), the temperature is rapidly lowered during the annealing process to promote the nucleation process of the nano-islands.

[0024] The present invention also provides bismuth ferrite nano-islands obtained by the above preparation method.

[0025] The average height of the bismuth ferrite nano-islands is 1-10 nm, the average size is 50-200 nm, and the orientation is in the (001) direction; the overall roughness RMS of the integrated nano-islands is 1-4 nm, and the roughness RMS of a single nano-island is 0.4-1.0 nm.

[0026] The present invention also provides an application of the bismuth ferrite nano-islands in a ferroelectric memory.

[0027] Compared with the prior art, the present invention has the following beneficial effects:

[0028] The preparation method provided by the present invention regulates the temperature and oxygen pressure during the growth of bismuth ferrite (BFO) by molecular beam epitaxy (MBE), and controls the volatilization and atomic surface migration rate of bismuth oxide on a substrate (such as dysprosium scandate (DyScO3-(110) pc ) single crystal) is integrated with (001)-oriented bismuth ferrite nano-islands: the temperature and oxygen pressure of the bismuth ferrite material during its growth are controlled by oxide molecular beam epitaxy, and the kinetic conditions of the adsorption, desorption and surface migration of Bi and Fe are controlled to adjust the nucleation and migration processes of bismuth ferrite, thereby achieving high and large-area integration of bismuth ferrite epitaxial nano-islands, without the introduction of a second phase such as a template, with simple process steps and high preparation efficiency.

[0029] The bismuth ferrite nano-islands prepared by the present invention are highly uniform and have good integration. The bismuth ferrite nano-islands are independent of each other, and information can be independently written and erased, which can effectively bind the domain wall position and improve the service life and non-volatility of the ferroelectric memory. BRIEF DESCRIPTION OF THE DRAWINGS

[0030] Figure 1 A schematic diagram of a BiFeO3 nano-island 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 microscope morphology images of BiFeO3 nanoislands at different scales provided in the embodiments of the present invention:

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

[0034] Figure 5 A piezoelectric force microscope image of a BiFeO3 nanoisland provided in an embodiment of the present invention;

[0035] Figure 6 AFM morphology images of nano-islands under different oxygen pressures and beam current ratios provided by the embodiments of the present invention;

[0036] Figure 7 The morphology of BFO grown at different temperatures provided in the embodiment of the present invention;

[0037] Figure 8 This is a piezoelectric force microscope image of the out-of-plane flipping of the BiFeO3 nano-island provided in an embodiment of the present invention. DETAILED DESCRIPTION

[0038] Various additional advantages and benefits will become apparent to those of ordinary skill in the art from a reading of the following detailed description of the preferred embodiments.

[0039] Specifically, the method for preparing bismuth ferrite nano-islands by molecular beam epitaxy provided in an embodiment of the present invention includes:

[0040] (1) Surface treatment process of dysprosium scandate 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 sandpapers. The polishing time for each step was 30 minutes to fully remove the scratches generated in the previous step. The thickness of the substrate after coarse polishing should be 0.6 mm. Polishing pastes of 1.5 μm, 0.5 μm, and 0.25 μm were used for fine polishing to make the surface roughness less than 1 nm. The samples after fine polishing were ultrasonically cleaned in sufficient acetone, isopropanol, acetone, isopropanol, and pure water for 20 minutes to remove the attached particles and organic cutting fluid on the substrate surface. After that, the samples were annealed at 1100°C for 1 hour to allow the ions to fully migrate on the substrate surface and obtain a substrate with a flat surface and clear atomic steps.

[0042] (2) Bismuth ferrite epitaxial film prepared by molecular beam epitaxy, specifically comprising:

[0043] Cut the processed substrate into a standard size of 2×10×0.5mm. Place the sample with the polished surface facing up in the sample holder slot, cut the tantalum foil into strips with a width of about 1mm, and spot weld the two ends or four corners of the sample. The thickness of the tantalum foil is about 0.5mm to ensure that the sample will not fall and has sufficient epitaxial area. Place the sample holder with the sample facing down into the MBE growth chamber, and perform a heating and degassing operation in steps of 50°C. After each heating, the pressure at the top of the chamber should be reduced to 10 -9 Torr level, the next step of heating and degassing was carried out, and the temperature was finally raised to 620℃. The Bi source was raised to 620℃, and the Fe source was raised to 1370℃. The beam sizes of the two sources were measured by quartz microbalance (QCM) to make the beam ratio equal to 7:1. Pure ozone was used as the epitaxial growth atmosphere, and the micro-leak valve connected to the ozone generator was opened, and the pressure in the growth chamber was adjusted to 4×10 -6 Torr, open the baffles of Bi source and Fe source at the same time, make the two beams mix evenly in the chamber, then open the substrate baffle for epitaxial growth. The film thickness is regulated by calculating the Fe beam and time. After the growth is completed, close the growth chamber baffle, remove the ozone in the growth chamber, cool the film at a rate of 1-5℃ / min in a vacuum environment, and obtain a substrate with epitaxial nano-islands after it cools to room temperature.

[0044] Figure 1 Schematic diagram of the BiFeO3 nano-island model according to an embodiment of the present invention; Figure 1 The application of nano-island memory is given. The single nano-island can be written and erased by needle tip force or needle tip voltage to realize information storage and reading.

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

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

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

[0048] Figure 5 Piezoelectric force microscopy images of a BiFeO3 nanoisland prepared in an embodiment of the present invention: (a) is a height image, (b) is a three-dimensional morphology image, (c) is an amplitude image, and (d) is a phase image; Figure 5 It can be seen that the BFO nanoisland is a single domain, and the domain wall is bounded at the island boundary;

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

[0050] Figure 6 AFM morphology images of nano-islands under different oxygen pressures and beam current ratios provided in the embodiment of the present invention: (a) oxygen pressure 1×10 -5 Torr, beam ratio 7:1, (b) oxygen pressure 1×10 -5 Torr, beam ratio 15:1, (c) oxygen pressure 4.5×10 -6 Torr, beam ratio 7:1, (d) oxygen pressure 4.5×10 -6 Torr, beam ratio 15:1; Figure 6 It can be seen that with the regulation of oxygen pressure and beam current ratio, the morphology and density of BFO nanoislands can be controlled.

[0051] Figure 7 The morphology of BFO grown at different temperatures provided in the embodiment of the present invention: (a) is an AFM height image of BFO grown without islands at 400°C, and (b) is an AFM height image of BFO nano-islands grown at 620°C; Figure 7 It can be seen that at low temperature, the morphology of BFO is flat and no islands are generated, while when the temperature reaches 620℃, the increase in nucleation rate promotes the formation of nano-islands.

[0052] Figure 8 Piezoelectric force microscopy images of the out-of-plane flipping of BiFeO3 nanoislands provided in the embodiment of the present invention: (a)-(g) are the domain distribution after increasing the out-of-plane voltage, and (h) is the domain distribution after the voltage is removed and stabilized; Figure 8 It can be seen that the flipping voltage of the nano-island is much higher than that of other regions, that is, the formation of the nano-island has a binding effect on the domain wall, which can improve the life and non-volatility of the memory.

[0053] In summary, the present invention uses molecular beam epitaxy to control the growth temperature of bismuth ferrite and integrates bismuth ferrite nano-islands on a dysprosium scandate substrate: the epitaxial quality of the nano-islands is good, the surface roughness of each nano-island is low, and the integration is high. The nano-islands are independent of each other, and information can be written and erased independently, and the domain wall can be confined in the nano-islands, thereby improving the erase and write cycle characteristics of the memory.

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

Claims

1. A method for preparing bismuth ferrite nano-islands based on molecular beam epitaxy, characterized in that: The preparation method comprises: (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; (2) Raising the temperature of the Bi source and the Fe source to the evaporation temperature, and adjusting the beam current of the Bi source and the Fe source; (3) Use pure ozone as the epitaxial growth atmosphere and adjust the growth chamber pressure to 2×10 -6 -1×10 -5 Torr, Bi source and Fe source are mixed in the growth chamber for epitaxial growth; (4) After the epitaxial growth is completed, the temperature is lowered to obtain bismuth ferrite epitaxial nano-islands.

2. The method for preparing bismuth ferrite nano-islands based on molecular beam epitaxy according to claim 1, characterized in that: In step (1), the substrate is a (110) oriented scandate single crystal substrate, the bevel angle of the substrate is less than 0.2°, the surface has atomic steps with a width of 100-500nm and arranged in parallel, and the overall surface roughness of the substrate is less than 0.4nm.

3. The method for preparing bismuth ferrite nano-islands based on molecular beam epitaxy according to claim 1, characterized in that: In step (1), the substrate temperature is 600-750°C.

4. The method for preparing bismuth ferrite nano-islands based on molecular beam epitaxy according to claim 1, characterized in that: In step (2), the evaporation temperature of the Bi source is 600-700°C, and the evaporation temperature of the Fe source is 1200-1500°C.

5. The method for preparing bismuth ferrite nano-islands based on molecular beam epitaxy according to claim 1, characterized in that: In step (2), the beam ratio of the Bi source to the Fe source is greater than or equal to 4:

1.

6. The method for preparing bismuth ferrite nano-islands based on molecular beam epitaxy according to claim 5, characterized in that: The beam ratio of the Bi source to the Fe source is 4-16:

1.

7. A bismuth ferrite nanoisland obtained by the preparation method according to any one of claims 1 to 6.

8. The bismuth ferrite nano-island according to claim 7, characterized in that: The bismuth ferrite nano-islands have an average height of 1-10 nm, an average size of about 50-200 nm, and are oriented in the (001) direction.

9. The bismuth ferrite nano-island according to claim 7, characterized in that The overall roughness of the integrated nano-islands is RMS = 1-4 nm, and the roughness of a single nano-island is RMS = 0.4-1.0 nm.

10. Use of the bismuth ferrite nano-islands according to claim 7 in ferroelectric memory.

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