A ferroelectric topological domain structure and a preparation method thereof

Abstract

The present invention relates to a preparation method of a ferroelectric topological domain structure, which comprises the following steps: S1: depositing a layer of SRO conductive layer as a bottom electrode on a (001)-oriented STO single crystal substrate by pulsed laser deposition; S2: spreading a monolayer of PS spheres as a mask template on the surface of the SRO bottom electrode obtained in step S1, then performing oxygen plasma etching treatment, and then placing it in an ion beam etching machine for etching, and finally removing the residual monolayer of PS sphere mask template to obtain an SRO nanodot array bottom electrode; S3: selecting a BFO target, and depositing a layer of rhombohedral BFO thin film on the SRO nanodot array bottom electrode obtained in step S2 by pulsed laser deposition to form a nanodot array, which is a central topological domain structure with an "X-shaped" buffer domain out of plane and a stripe domain with opposite contrast in plane. The ferroelectric topological domain structure prepared by the present invention has the advantages of high density, ordered arrangement, convenient regulation, etc.

Description

Technical Field

[0001] The present invention relates to the technical field of ferroelectric materials, and particularly to a ferroelectric topological domain structure and a preparation method thereof. Background Art

[0002] With the advent of the Internet and big data era, traditional storage devices are difficult to meet the growing storage needs, and new storage devices are constantly being explored. In recent years, memories have evolved from volatile memories to non-volatile memories and then to new non-volatile memories, and their storage performance has been greatly improved. Among them, storage devices based on ferroelectric materials have received extensive attention because of their excellent characteristics such as non-volatility, fast read and write, low power consumption, and high erasure and write times, and are expected to become candidates for the new generation of storage devices.

[0003] At present, the market's requirements for the storage density, integration degree, and miniaturization of storage devices are constantly increasing. It is crucial to realize the preparation and regulation of ordered high-density ferroelectric topological domains at the nanoscale to meet the current market requirements for storage devices. Therefore, the research on the topological domain structure in ferroelectric materials has become a research hotspot. The ferroelectric topological domain structure has characteristics such as high stability and small size, which can avoid being damaged by other external fields other than the regulated field and can greatly reduce the volume of storage devices. People have also tried many methods to prepare ferroelectric topological domain structures. For example, applying a point electric field to a thin film or bulk to induce a single vortex domain structure, but the topological domain density obtained by this method is extremely low, the difficulty of repeatedly regulating a single point is extremely high, and it is difficult to integrate. Another example is the method of obtaining topological domains by preparing ferroelectric superlattices, but this method is very complex and the obtained topological domains are difficult to regulate. Therefore, there are still huge challenges in the preparation and regulation of ferroelectric topological domain structures, and it is particularly crucial to find a topological domain with a simple preparation process and convenient regulation. Summary of the Invention

[0004] Based on this, the object of the present invention is to provide a preparation method of a ferroelectric topological domain structure, and the ferroelectric topological domain structure prepared by this preparation method has advantages such as high density, ordered arrangement, and convenient regulation.

[0005] The object of the present invention is achieved by the following technical solutions: A preparation method of a ferroelectric topological domain structure, comprising the following steps:

[0006] S1: Depositing a layer of SRO conductive layer as a bottom electrode on a (001)-oriented STO single crystal substrate by pulsed laser deposition;

[0007] S2: Spread a single layer of PS microspheres on the surface of the SRO bottom electrode prepared in step S1 as a mask, then perform oxygen plasma etching treatment, and then place it in an ion beam etching machine for etching. Finally, remove the remaining single-layer PS microsphere mask to obtain the SRO nanodot array bottom electrode;

[0008] S3: Select a BFO target, and deposit a rhombohedral BFO thin film on the SRO nanodot array bottom electrode prepared in step S2 by pulsed laser deposition to form a nanodot array, which is a central topological domain structure.

[0009] Compared with the prior art, in the present invention, the bottom electrode SRO thin film is prepared by pulsed laser method, and then the SRO nanodot array bottom electrode is prepared by the method of polystyrene microsphere-assisted ion etching. Then, the rhombohedral BFO nanodot array is prepared by pulsed laser deposition, and the topological domain structure is induced by strain gradient. The ferroelectric topological domain (central topological domain) structure array prepared in the present invention reaches the nanometer level, is a high-density ordered nanodot array structure, and the ferroelectric topological domain structures are independent of each other, can be regulated by an external electric field, and have good thermal stability at the same time. The ferroelectric topological domains prepared in the present invention exhibit an ordered nanodot array distribution, and are expected to be applied to the design of a new type of ferroelectric memory with a vertical structure, which can endow the new type of ferroelectric memory with the advantages of high stability, high reading speed, and high density.

[0010] Further, in step S1, the thickness of the SRO conductive layer is 30 - 60 nm. Controlling the thickness of the SRO conductive layer and the etching depth within this range is easier to operate and can ensure good conductive effects.

[0011] Further, in step S2, the etching depth of the SRO nanodot array bottom electrode is 20 - 50 nm, and the etching depth is less than the thickness of the SRO conductive layer. Controlling the etching depth within this range is easier to operate, and it is necessary to ensure that the SRO is not etched through to maintain good conductivity, which is convenient for subsequent electrical characterization.

[0012] Further, in step S3, the thickness of the rhombohedral BFO thin film is 50 nm. At this thickness, BFO shows a uniform rhombohedral phase and can ensure a flat surface to ensure that the topological domain structure array can be obtained smoothly.

[0013] Further, step S2 includes the following steps:

[0014] S21: Drop a mixed solution of PS microspheres with a diameter of 500 nm and ethanol into a petri dish filled with deionized water, add a dispersant, and make the PS microspheres form a single-layer close arrangement on the surface of the deionized water;

[0015] S22: Treat the sample prepared in step S2 with oxygen plasma for 3 minutes;

[0016] S23: Use tweezers to place the processed sample under the monolayer of PS spheres, and then gently lift it horizontally; after the water naturally evaporates, a monolayer of closely arranged PS spheres forms on the surface of the SRO film.

[0017] S24: Place the SRO film sample with the PS sphere mask on the oxygen plasma etching machine for 40 - 50 minutes to reduce the diameter of the PS spheres and separate the closely arranged PS spheres.

[0018] S25: Place the sample obtained in step S24 in an ion beam etching machine for etching.

[0019] S26: Remove the remaining monolayer PS sphere mask to obtain an ordered SRO nanodot array bottom electrode.

[0020] In the above steps, polystyrene (PS) spheres are used as the etching template, which is convenient and time-saving for preparation; using the ion etching technology, direct etching is carried out to directly transfer the arrangement pattern of the polystyrene spheres to the substrate material without introducing a sacrificial layer structure, and the nanostructure is obtained in one step, with simple operation and process; at the same time, ion etching does not require the introduction of chemical reaction gases, will not introduce new impurity pollution to the film microstructure, the operating environment is non-toxic and harmless, the preparation cost is low, and the safety of the operator is high.

[0021] Preferably, in step S25, under a vacuum of 8.0×10 -4 Pa and at room temperature, keep the cathode current of the ion beam etching system at 17.8 A, the anode voltage at 50 V, the screen grid voltage at 300 V, the acceleration voltage at 250 V, the neutralization current at 13 A, and the bias voltage at 1.2 V, and carry out etching for 90 seconds.

[0022] Preferably, in step S26, place the sample obtained in step S25 in chloroform, alcohol, and deionized water respectively, soak it and clean it ultrasonically for 15 - 20 minutes, take it out and dry it with a nitrogen gun, and then clean the surface with low-power oxygen plasma for 4 - 6 minutes to obtain an ordered and clean SRO nanodot array bottom electrode.

[0023] Preferably, in step S1, the preparation parameters of the pulsed laser deposition method are: energy is 300 mJ / cm 3 , the pulse frequency is 8 Hz, the temperature is 660 °C, and the oxygen pressure is 15 Pa.

[0024] Preferably, in step S3, the preparation parameters of the pulsed laser deposition method are: energy is 300 mJ / cm 3 , the pulse frequency is 8 Hz, the temperature is 730 °C, and the oxygen pressure is 20 Pa.

[0025] The present invention also provides a ferroelectric topological domain structure prepared by the above preparation method, which is a central topological domain structure with "cross-shaped" buffer domains out-of-plane and has a strip domain with opposite contrast in-plane. This ferroelectric topological domain structure exhibits an ordered nano-dot array distribution and is expected to be applied to the design of a new ferroelectric memory with a vertical architecture, enabling the new ferroelectric memory to have advantages such as high stability, high reading speed, and high density.

[0026] For better understanding and implementation, the present invention will be described in detail below with reference to the accompanying drawings. BRIEF DESCRIPTION OF THE DRAWINGS

[0027] Figure 1 Schematic diagram of the preparation method process for Example 1 and SEM images of corresponding steps

[0028] Figure 2 XRD schematic diagram of the rhombohedral phase BFO nano-dot array prepared in Example 1

[0029] Figure 3 RSM schematic diagram of the rhombohedral phase BFO nano-dot array prepared in Example 1

[0030] Figure 4 AFM schematic diagram of the rhombohedral phase BFO nano-dot array prepared in Example 1

[0031] Figure 5 SEM morphology characterization diagram of the rhombohedral phase BFO nano-dot array prepared in Example 1

[0032] Figure 6 Morphology, in-plane, and out-of-plane amplitude and phase diagrams of the rhombohedral phase BFO nano-dots prepared in Example 1 when placed at 0°, 45°, and 90°

[0033] Figure 7 In-plane and out-of-plane amplitude and phase diagrams of a single rhombohedral phase BFO nano-dot prepared in Example 1 when placed at 0°, 45°, and 90°

[0034] Figure 8 Out-of-plane phase diagram of the rhombohedral phase BFO nano-dots prepared in Example 1 under the action of an electric field

[0035] Figure 9 In-plane phase diagram of the rhombohedral phase BFO nano-dots prepared in Example 1 under the action of an electric field

[0036] Figure 10 Two-dimensional vector schematic diagram formed by the central topological domain structure of the present invention

[0037] Figure 11 Principle diagram of the formation of the topological domain structure of the present invention DETAILED DESCRIPTION OF THE EMBODIMENTS

[0038] The following are the best embodiments of the present invention. It should be noted that in Embodiment 1, the thickness of the SRO conductive layer in step S1 is 60 nm, and the etching depth of the bottom electrode of the SRO nanodot array in step S2 is 40 nm, both of which are better parameter values within the parameter ranges limited by the claims. In other embodiments, as long as the thickness of the SRO conductive layer is within the range of 30 - 60 nm, the etching depth of the bottom electrode of the SRO nanodot array is within the range of 20 - 50 nm, and the etching depth is less than the thickness of the SRO conductive layer, that is, ensuring that the SRO is not etched through, the invention object of the present invention can be achieved.

[0039] Embodiment 1

[0040] For the preparation method of the ferroelectric topological domain structure of this embodiment, please refer to Figure 1 , including the following steps:

[0041] S1: Prepare the SRO bottom electrode thin film by pulsed laser deposition method: Select a (001)-oriented STO (SrTiO 3 ) single crystal substrate, and deposit a 60-nm-thick SRO (SrRuO 3 ) conductive layer on the substrate by laser pulse deposition method as the bottom electrode. The preparation parameters of the pulsed laser deposition method are as follows:

[0042] <![CDATA[Energy (mJ / cm 3 )]]> Pulse frequency (Hz) Temperature (°C) Oxygen pressure (Pa) 300 8 660 15

[0043] S2: Preparation of the SRO bottom electrode nanodot structure, specifically including the following steps:

[0044] S21: Drop a mixed solution of PS spheres with a diameter of 500 nm and ethanol into a petri dish filled with deionized water, add a dispersant, and make the PS spheres form a monolayer close-packed arrangement on the surface of the deionized water;

[0045] S22: Treat the sample obtained in step S1 with oxygen plasma for 3 minutes;

[0046] S23: Use tweezers to place the treated sample under the monolayer of PS spheres, and then gently lift it horizontally; after the water naturally evaporates, a monolayer of close-packed PS spheres is formed on the surface of the SRO thin film;

[0047] S24: Place the SRO thin film sample with the PS sphere mask on the oxygen plasma etching machine and treat it for 40 - 50 minutes to reduce the diameter of the PS spheres and separate the closely packed PS spheres;

[0048] S25: Place the sample obtained in step S24 in an ion beam etching machine for etching: At a vacuum degree of 8.0×10 - 4Pa. At room temperature, keep the cathode current of the ion beam etching system at 17.8 A, the anode voltage at 50 V, the screen grid voltage at 300 V, the acceleration voltage at 250 V, the neutralization current at 13 A, and the bias voltage at 1.2 V, and perform etching for 90 seconds. The ion etching machine model used in this example is MIBE-150C;

[0049] S26: Remove the residual monolayer PS microsphere mask to obtain an ordered SRO nanodot array: Immerse the sample obtained in step S25 in chloroform, alcohol, and deionized water respectively, and ultrasonically clean for 15 - 20 minutes. After taking it out, dry it with a nitrogen gun, and then clean the surface with low-power oxygen plasma for 4 - 6 minutes to obtain an ordered and clean SRO nanodot array bottom electrode on the surface.

[0050] S3: Preparation of BFO nanodot array: Deposit a 50-nm-thick BFO (BiFeO 3 ) nanodot array on the SRO conductive layer nanodot array obtained in the step described in S2 by pulsed laser deposition. The preparation parameters of the pulsed laser deposition method are as follows:

[0051] <![CDATA[Energy (mJ / cm 3 )]]> Pulse frequency (Hz) Temperature (°C) Oxygen pressure (Pa) 300 8 730 20

[0052] Wait until the temperature drops to room temperature to obtain a large-area ordered rhombohedral phase BFO nanodot array as shown in Figures 4 - 5 which is a central-type topological domain structure.

[0053] Test results

[0054] Figure 1 Figure 1 is a schematic diagram of the specific process flow of the preparation method of Example 1 and the corresponding SEM image. It can be clearly seen from the SEM image that the arrangement from the microsphere template, the SRO bottom electrode array to the BFO nanodots is of high density and high order.

[0055] Figure 2 and Figure 3 are the XRD and RSM schematic diagrams obtained by using X-ray diffraction to characterize the basic crystal structure of the nanodot array. The results show that the nanodot array is rhombohedral phase BFO, and its lattice constant is obtained by RSM calculation

[0056] Figures 4 - 5 Figure 3 is a schematic diagram of the AFM and SEM morphology characterization of the rhombohedral phase BFO nanodot array prepared in Example 1. It can be seen from the figure that the average height of the BFO nanodots is 50 nm, the surface has no impurities, and a high-density ordered nanodot array is presented.

[0057] Piezoelectric response force microscopy is a technique for testing microscopic electro-induced deformation based on contact mode atomic force microscopy. This mode is mainly used to characterize the domain structure of ferroelectric materials, and its principle is to detect the mechanical deformation induced by an electric field through the inverse piezoelectric effect. By applying an alternating voltage to the conductive tip, the tested sample generates vibrational deformation, which is then fed back to the tip vibration. The signal reflected by the cantilever is detected by a photodiode detector, thus achieving detection. In this embodiment, piezoelectric response force microscopy is used to characterize the initial domain structure of rhombohedral BFO nanodots. By performing multi-angle tests on in-plane domains and out-of-plane domains, it can be determined that the nanodot array is a central-type topological domain structure, and the domain structure of rhombohedral BFO nanodots under electric field control is characterized.

[0058] As Figure 6 shown, the initial domain structure of rhombohedral BFO nanodots is a central-aggregated domain structure, which appears under piezoelectric response force microscopy as follows: when the sample is placed at 0° and 90°, the in-plane domains are both divided into left and right halves with a "cross-shaped buffer domain"; when the sample is placed at 45°, there are obvious changes in the in-plane Amplitude with two contrasts; the out-of-plane initial domain is in a polarization state where the "cross-shaped" buffer domain is upward and the four-quadrant domains are downward. According to the principle of the formation of central-type topological domains, it can be determined that the nanodot array is a central-type topological domain structure.

[0059] Figure 7 are randomly selected representative BFO nanodots from Figure 6 , and the in-plane and out-of-plane phase diagrams and amplitude diagrams when the samples are placed at 0°, 45°, and 90° are given in detail. It can be seen from the figures that when the samples are placed at 0° and 90°, the in-plane domains are divided into halves with a small strip domain structure of opposite contrasts; when the sample is placed at 45°, there are obvious changes in the in-plane Amplitude with two contrasts; the out-of-plane initial domain is in a polarization state where the "cross-shaped" buffer domain is upward and the four-quadrant domains are downward. According to the principle of the formation of central-type topological domains, it can be determined that the nanodot array is a central-type topological domain structure. Compared with the central-type topological domain structure prepared by the existing technology, this central-type topological domain shows a novel domain structure in the out-of-plane direction, that is, due to different strain conditions during the growth process, a "cross-shaped" buffer domain structure with an initial upward direction is formed in the out-of-plane direction.

[0060] As Figure 8 and Figure 9 shown, under the control of an electric field, the out-of-plane domains of rhombohedral BFO nanodots undergo polarization reversal under the action of the electric field, and the "cross-shaped" buffer domain disappears; the in-plane domains become a structure divided into left and right halves under the action of the electric field, flipping from a central-converging type to a central-diverging type domain structure. It can be seen that the domain control of the electric field on the rhombohedral BFO nanodot array can achieve the change between "0" and "1", thus promising to fabricate a storage device with a single storage unit reaching the nanoscale.

[0061] Figure 10 It is a two-dimensional schematic diagram of randomly selected nanodots synthesized vectorially by a Matlab program. Using the vector PFM test results, that is, the in-plane phase (Lat-Phase) at 0° and 90° is measured by rotating the sample at different angles, and then the direction of different contrasts is determined by end-field writing. Finally, the two-dimensional schematic diagram of the in-plane domain structure shown is obtained by the method of vector synthesis with the Matlab program. Figure 10 As can be seen from the figure, the grown nanodot array presents a central-converging topological domain structure.

[0062] Figure 11 It is the schematic diagram of the formation principle of the topological domain structure in the nanodots of the present invention. During the deposition process of BFO, there are two interfaces (i) and (ii) with the bottom electrode SRO nanodot array. Through strain analysis, it is found that both interfaces are subjected to compressive strain. The BFO at the edge of the nanodot is simultaneously subjected to the compressive strain of the two interfaces, while the part near the center is only subjected to the compressive strain of interface (i). This makes the stress received by the entire BFO nanodot uneven during the growth process. The stress received by the part near the center is less than that received by the part near the edge, thus forming a central-type topological domain.

[0063] Compared with the prior art method of first depositing a BFO thin film and then preparing a rhombohedral BFO nanodot array by the method of assisted ion etching with polystyrene microspheres, the present invention first etches the bottom electrode SRO into a nanodot array, and then deposits BFO on the SRO nanodot array, and induces a central-type topological domain structure by strain gradient. That is, the present invention introduces a strain gradient from the BFO growth conditions and spontaneously forms a central-type topological domain during the growth process. Different from the topological domain structure obtained by the prior art method, the topological domain of the present invention is a central-type topological domain with a "cross-shaped" buffer domain out of the plane, and a strip domain with an opposite contrast will also appear in the plane. The ordered ferroelectric topological domain (central-type topological domain) structure array obtained by the present invention reaches the nanoscale and is a high-density ordered nanodot array structure. The ferroelectric topological domain structures are independent of each other and can be regulated by an external electric field. The ferroelectric topological domains obtained by the present invention exhibit an ordered nanodot array distribution and are expected to be applied to the design of a new type of ferroelectric memory with a vertical architecture, having the advantages of high stability, high reading speed, high density, etc.

[0064] The above-described embodiments merely represent one implementation mode of the present invention, and its description is relatively specific and detailed, but it should not be construed as a limitation to the scope of the invention patent. It should be noted that for those of ordinary skill in the art, without departing from the concept of the present invention, several deformations and improvements can still be made, and these all belong to the protection scope of the present invention.

Claims

1. A preparation method of a ferroelectric topological domain structure, characterized in that: It includes the following steps: S1: Deposit a layer of SRO conductive layer as the bottom electrode on a (001)-oriented STO single crystal substrate by pulsed laser deposition; S2: Spread a monolayer of PS microspheres on the surface of the SRO bottom electrode prepared in step S1 as a mask template, then perform oxygen plasma etching treatment, and then place it in an ion beam etching machine for etching. Finally, remove the remaining monolayer PS microsphere mask template to obtain an SRO nanodot array bottom electrode; S3: Select a BFO target, and deposit a layer of rhombohedral BFO thin film on the SRO nanodot array bottom electrode prepared in step S2 by pulsed laser deposition to form a nanodot array, which is a central-type topological domain structure.

2. The preparation method of the ferroelectric topological domain structure according to claim 1, characterized in that: In step S1, the thickness of the SRO conductive layer is 30 - 60 nm.

3. The preparation method of the ferroelectric topological domain structure according to claim 2, characterized in that: In step S2, the etching depth of the SRO nanodot array bottom electrode is 20 - 50 nm, and the etching depth is less than the thickness of the SRO conductive layer.

4. The preparation method of the ferroelectric topological domain structure according to claim 1, characterized in that: In step S3, the thickness of the rhombohedral BFO thin film is 50 nm.

5. The preparation method of the ferroelectric topological domain structure according to claim 1, characterized in that: Step S2 includes the following steps: S21: Drop a mixed solution of PS microspheres with a diameter of 500 nm and ethanol into a petri dish filled with deionized water, add a dispersant to make the PS microspheres arrange tightly in a monolayer on the surface of the deionized water; S22: Treat the sample prepared in step S2 with oxygen plasma for 3 minutes; S23: Use tweezers to place the treated sample under the monolayer of PS microspheres, and then gently lift it horizontally; after the water naturally evaporates, a monolayer of tightly arranged PS microspheres is formed on the surface of the SRO thin film; S24: Place the SRO thin film sample with the PS microsphere mask template in an oxygen plasma etching machine for 40 - 50 minutes to reduce the diameter of the PS microspheres and separate the tightly arranged PS microspheres; S25: Place the sample obtained in step S24 in an ion beam etching machine for etching; S26: Remove the remaining monolayer PS microsphere mask template to obtain an ordered SRO nanodot array bottom electrode.

6. The preparation method of the ferroelectric topological domain structure according to claim 5, characterized in that: In step S25, at a vacuum degree of 8.0×10 -4 Pa, under room temperature conditions, maintain the cathode current of the ion beam etching system at 17.8 A, the anode voltage at 50 V, the screen grid voltage at 300 V, the acceleration voltage at 250 V, the neutralization current at 13 A, and the bias voltage at 1.2 V, and perform etching for 90 seconds.

7. The preparation method of the ferroelectric topological domain structure according to claim 5, characterized in that: In step S26, place the sample obtained in step S25 in chloroform, alcohol, and deionized water respectively, soak and ultrasonically clean for 15 - 20 minutes, take it out, dry it with a nitrogen gun, and then clean the surface with low-power oxygen plasma for 4 - 6 minutes to obtain an ordered and clean SRO nanodot array bottom electrode.

8. The preparation method of the ferroelectric topological domain structure according to claim 1, characterized in that: In step S1, the preparation parameters of the pulsed laser deposition method are: the energy is 300 mJ / cm 3 , the pulse frequency is 8 Hz, the temperature is 660 °C, and the oxygen pressure is 15 Pa.

9. The preparation method of the ferroelectric topological domain structure according to claim 1, characterized in that: In step S3, the preparation parameters of the pulsed laser deposition method are: the energy is 300 mJ / cm 3 , the pulse frequency is 8 Hz, the temperature is 730 °C, and the oxygen pressure is 20 Pa.

10. A ferroelectric topological domain structure, characterized in that: it is prepared by using the preparation method described in any one of claims 1 to 9, and it is a central topological domain structure with an "X-shaped" buffer domain in the out-of-plane direction and has a stripe domain with opposite contrast in the in-plane direction.

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