A method for constructing a central polarization topological domain

Abstract

The present invention discloses a method for constructing a central polarization topological domain, comprising the following steps: S1: preparing a BiFeO3 thin film sample by pulsed laser deposition, S2: using AAO as a template, an argon ion beam passing through the holes of the AAO template to etch the thin film, obtaining a high-density conical "volcano-shaped" nanodot array, and performing phase characterization; S3: using PFM characterization to obtain spontaneous polarization topological domains from the nanodot array, flipping the polarization direction by writing the PFM needle tip electric field, and performing vector PFM characterization on the result; S4: reorganizing the polarization vector to obtain three different types of central polarization topological domains. The present invention constructs three different types of smaller topological domains to obtain higher storage density, and this high-density, stable, and controllable polarization topological domain array can be used for ferroelectric random access memory devices.

Description

Technical Field

[0001] The present invention relates to the technical field of micro-nano structures, and in particular to a method for constructing a central polarization topological domain. Background Art

[0002] Polarized topological domains can improve storage density due to their small size, and can solve the technical bottleneck of low storage density faced by ferroelectric random access memory, which has attracted widespread attention. Polarized topological domains include topological structures such as vortex, anti-vortex, skyrmion, radial center type, etc., all of which are at the nanoscale, inspiring people to look for them in real materials. For example, vortex structures and their periodic arrays have been prepared by researchers in PbTiO3 / SrTiO3 multilayer films (YLTang et al., Science 348, 547, 2015) and PbTiO3 / SrTiO3 superlattices (AKYadav et al., Nature 530, 198, 2016), but the superlattice preparation methods involved have high requirements for experimental equipment, and often require the growth of many layers. For central polarized topological domains, people have obtained central domain structures in thin films and nanodot arrays, but they require a needle tip electric field to write at a specific domain wall position, and they cannot be high-density and arrayed, making them difficult to integrate into devices.

[0003] In the high-density, arrayed nanodot arrays of smaller sizes, the traditional standard ferroelectric lead zirconate titanate (PZT) nanodot arrays are epitaxially grown using anodized aluminum oxide (AAO) as a template, but the lead contained is toxic and vortex domains only exist in individual nanodots (BJ Rodriguez et al. Nano Lett. 9, 1127, 2009), which cannot be used commercially; for the green lead-free ferroelectric material BiFeO3 (BFO), the high-density nanodot array prepared by ion beam etching of thin films using AAO as a template can form an arrayed central topological domain structure (ZW Li et al., Sci. Adv. 3, e1700919, 2017). Driven by the market demand for device miniaturization, whether new smaller central polarization topological domains can be obtained is still worth exploring.

[0004] In addition, the invention patent (201910307724.3) previously applied by the applicant disclosed the construction of topological structures in thin films, but these topological structures will be affected by the spontaneous polarization in the thin film and are not independently addressable, causing crosstalk, which is somewhat different from the requirements of memory device applications. In addition, the invention patent (201910743213.6) previously applied for can only obtain a single-center topological structure, which cannot further reduce the size and cannot meet the market's high demand for device miniaturization.

[0005] Therefore, it is urgent to solve the above problems. Summary of the invention

[0006] Purpose of the invention: The purpose of the present invention is to provide a construction method for obtaining three different types of central polarization topological domains.

[0007] Technical solution: To achieve the above objectives, the present invention discloses a method for constructing a central polarization topological domain, comprising the following steps:

[0008] S1: Preparation of BiFeO3 thin film samples by pulsed laser deposition.

[0009] S2: Using AAO as a template, an argon ion beam passes through the holes of the AAO template to etch the film, obtaining a high-density conical "volcano-shaped" nanodot array, and performing phase characterization;

[0010] S3: PFM characterization is used to obtain spontaneous polarization topological domains of nanodot arrays, the polarization direction is flipped by PFM tip electric field writing, and the results are characterized by vector PFM;

[0011] S4: Reorganize the polarization vector to obtain three different types of central polarization topological domains.

[0012] In the step S1, the pulse laser deposition method uses SrTiO3

[100] as a substrate, SrRuO3 as a bottom electrode, BiFeO3 as a ferroelectric layer, the thickness of SrRuO3 is 10 to 15 nm, and the thickness of BiFeO3 is 80 to 100 nm.

[0013] Preferably, the deposition conditions of the pulse laser deposition method in step S1 are: laser pulse energy of 140-145 mJ, deposition rate of 2 Hz, substrate temperature of 605-610° C., and oxygen pressure of deposited SrRuO 3 and BiFeO 3 layers of 1.0-1.2 Pa.

[0014] Furthermore, in step S2, the diameter of the AAO template is ∼60 nm, the height is ∼300 nm, and the etching time is 24 to 25 min.

[0015] Furthermore, the thickness of the single nanodot obtained in step S2 is ∼20 nm, and the diameter is ∼60 nm.

[0016] Preferably, in step S3, there is a conductive coating Pt / Ti on the PFM tip, and the PFM characterization uses vector PFM technology. First, a PFM image of a certain area of ​​the sample in the vertical direction is collected, and the PFM tip driving frequency of the vertical PFM characterization is 360-400KHz; then the sample is subjected to horizontal PFM characterization, and the PFM tip driving frequency of the horizontal PFM characterization is 0.8-1.0MHz.

[0017] Furthermore, the specific method of flipping the polarization direction by PFM tip electric field writing in step S3 is: first use DC PFM tip electric field scanning to write a rectangular pattern to achieve polarization flipping, and then use vector PFM technology to characterize the writing result.

[0018] Preferably, the flipping of the polarization direction is consistent with the vector electric field of the PFM tip, that is, the flipping occurs both out-of-plane and in-plane.

[0019] Furthermore, the specific method for reorganizing the polarization vector in step S4 is: combining the PFM images in the horizontal and vertical directions to obtain a three-dimensional polarization vector, and obtaining three different types of central polarization topological domains.

[0020] Furthermore, the three different types of central polarization topological domains in step S4 include a single central topological domain, a dual central topological domain, and a triple central topological domain.

[0021] Beneficial effects: Compared with the prior art, the present invention has the following significant advantages:

[0022] (1) The present invention obtains higher concentration of defective ions under lower oxygen pressure conditions while still ensuring that the thin film sample is free of impurities. The present invention increases the thickness of the bottom electrode and the ferroelectric layer, which facilitates the subsequent long-term etching on the ferroelectric layer to obtain smaller nanodots, avoiding the problem of being unable to obtain nanodot samples due to the ferroelectric layer being too thin and etching away the ferroelectric layer;

[0023] (2) The present invention uses a longer etching time to obtain a nanodot array of smaller size, thereby improving the storage density. In addition, the nanodot arrays are separated from each other and can meet the application requirements of independently addressable devices;

[0024] (3) The flipping of the polarization direction in the present invention is consistent with the vector electric field of the needle tip, that is, flipping occurs both out-of-plane and in-plane; at the same time, this operation is also a write / read operation of the storage device, indicating that the sample prepared by the preparation method of the present invention is suitable for storage devices;

[0025] (4) The frequency used in the present invention is further away from the resonant frequency of the needle tip, which can avoid the crosstalk of the morphology fluctuation on the PFM signal, where the morphology fluctuation is due to the unevenness of the nanodots compared with the film;

[0026] (5) Three central domains appeared in a single nanodot with a diameter of 60 nm in the present invention, indicating that it is possible to try to make nanodots of smaller size and obtain central ferroelectric polarization topological domains, providing a technical reference for further improving the storage density of the device;

[0027] (6) The present invention constructs three different types of smaller topological domains to achieve higher storage density. This high-density, stable, and controllable polarization topological domain array can be used in ferroelectric random access memory devices. BRIEF DESCRIPTION OF THE DRAWINGS

[0028] Figure 1 is a flowchart illustrating a method for constructing a central topological domain according to Embodiment 1 of the present invention;

[0029] Figure 2 is a flow chart of the preparation of the ferroelectric nanodot array in step S2 of Example 1 of the present invention;

[0030] Figure 3 is the XRD pattern of the ferroelectric nanodot array in step S2 of Example 1 of the present invention;

[0031] Figure 4 is the PFM piezoelectric loop of the sample in step S2 of Example 1 of the present invention;

[0032] Figure 5 is a topographical image of the ferroelectric nanodot array in step S2 of Example 1 of the present invention;

[0033] Figure 6 is a spontaneous domain structure diagram obtained by characterizing the nanodots by PFM in step S3 of Example 1 of the present invention;

[0034] Figure 7 is a PFM image of nanodots written by the needle tip electric field in step S3 of embodiment 1 of the present invention;

[0035] Figure 8 It is a vector PFM diagram of three central topological domains in step S4 in embodiment 1 of the present invention. DETAILED DESCRIPTION

[0036] The technical solution of the present invention is further described below in conjunction with the accompanying drawings.

[0037] like Figure 1 As shown, a method for constructing a central polarization topological domain of the present invention comprises the following steps:

[0038] S1: Preparation of BiFeO3 thin film samples by pulsed laser deposition.

[0039] The specific method of preparing BiFeO3 thin film samples by pulsed laser deposition is as follows: SrTiO3

[100] is used as substrate, SrRuO3 is used as bottom electrode, and BiFeO3 is used as ferroelectric layer; the deposition conditions are as follows: laser pulse energy is 140-145mJ, deposition rate is 2Hz, substrate temperature is 605-610℃, oxygen pressure of deposited SrRuO3 and BiFeO3 layers is 1.0-1.2Pa; SrRuO3 thickness is 10-15nm, BiFeO3 thickness is 80-100nm;

[0040] The present invention obtains higher concentration of defective ions under lower oxygen pressure conditions while still ensuring that the thin film sample is free of impurities. The present invention increases the thickness of the bottom electrode and the ferroelectric layer, making it easier to subsequently perform long-term etching on the ferroelectric layer to obtain smaller nanodots, thereby avoiding problems such as being unable to obtain nanodot samples due to the ferroelectric layer being too thin and etching away the ferroelectric layer;

[0041] S2: Using AAO as a template, the diameter of the AAO template is 60nm, and the height is 300nm. The argon ion beam passes through the holes of the AAO template to etch the film. The etching time is 24 to 25 minutes, and a high-density conical "volcano-shaped" nanodot array is obtained and phase characterization is performed. The present invention uses a longer etching time to obtain a smaller nanodot array, and the thickness of the nanodot is 20nm and the diameter is 60nm, thereby improving the storage density.

[0042] S3: PFM characterization is used to obtain spontaneous polarization topological domains of nanodot arrays, the polarization direction is flipped by PFM tip electric field writing, and the results are characterized by vector PFM;

[0043] There is a conductive coating Pt / Ti on the PFM tip. The PFM characterization uses the vector PFM technology. First, the PFM image of a certain area of ​​the sample in the vertical direction is collected. The PFM tip driving frequency of the vertical PFM characterization is 360-400KHz. Then the sample is subjected to horizontal PFM characterization. The PFM tip driving frequency of the horizontal PFM characterization is 0.8-1.0MHz.

[0044] The specific method of flipping the polarization direction by PFM tip electric field writing is: first use DC PFM tip electric field scanning to write a rectangular pattern to achieve polarization flipping, and then use vector PFM technology to characterize the writing results; the flipping of the polarization direction is consistent with the vector electric field of the PFM tip, that is, both out-of-plane and in-plane flipping occur.

[0045] The flipping of the polarization direction in the present invention is consistent with the vector electric field of the needle tip, that is, the flipping occurs both out-of-plane and in-plane; at the same time, this operation is also a write / read operation of the storage device, indicating that the sample prepared by the preparation method of the present invention is suitable for the storage device; the frequency used in the present invention is further away from the resonance frequency of the needle tip, which can avoid the crosstalk of the morphology fluctuation on the PFM signal, wherein the morphology fluctuation is because the nanodots are uneven compared with the film;

[0046] S4: Reorganize the polarization vector to obtain three different types of central polarization topological domains.

[0047] The specific method for reorganizing the polarization vector is: combining the horizontal and vertical PFM images to obtain a three-dimensional polarization vector, and obtaining three different types of central polarization topological domains, including a single central topological domain, a dual central topological domain, and a triple central topological domain; three central domains appear in a single nanodot with a diameter of ~60nm in the present invention, indicating that it is possible to try to make smaller nanodots and obtain central ferroelectric polarization topological domains, providing a technical reference for further improving the storage density of the device.

[0048] The present invention epitaxially grows a green lead-free ferroelectric material BFO ​​on a substrate, and sets the oxygen pressure condition of the pulsed laser deposited BFO film to a low oxygen pressure (1.0-1.2 Pa), which can ensure that the sample contains high-concentration ion defects, such as oxygen vacancies, etc.; the diameter of the AAO template in the present invention is 60nm and the height is 300nm, and the appropriate diameter ensures that the obtained nanodot array has a higher density than before, and the appropriate height can ensure the uniformity of the nanodot array; the argon ion beam passes through the holes of the template to etch the film, and the desired nanodot array sample can be obtained by controlling the etching time. If the etching time is too short, anti-nanodots (nano-sized holes) may be formed, or there may be residual connections between the nanodots and they are not completely independent. If the etching time is too long, the template and the ferroelectric layer may be completely etched and the ferroelectric nanodot array sample cannot be obtained. Therefore, the etching time of the present invention is 24-25 minutes.

[0049] The present invention cuts the surface into smaller discrete nanocapacitors to construct a thinner ferroelectric nanodot array (~20nm). Using vector PFM technology, single-center and dual-center polarization topological domains are obtained, and a new type of topological domain with three centers is obtained. Writing operations are performed on these topological domains to achieve flipping control of the polarization topological state. Reading over a period of 30 days shows that these topological states are stable. In summary, the construction method of the present invention constructs three different types of smaller topological domains to obtain higher storage density. This high-density, stable, and controllable polarization topological domain array can be used for ferroelectric random access memory devices.

[0050] The present invention has a promoting effect on improving ferroelectric storage density. The topological domains constructed by the present invention are stable during the entire one-month measurement period. The top-down manufacturing strategy is suitable for large-scale production and low cost, and nanocapacitors less than 100nm can be easily obtained. The present invention can form smaller topological domains by cutting the surface into smaller discrete nanocapacitors, thereby obtaining a higher storage density. This high-density, stable, and controllable polarized topological domain array can be used for ferroelectric random access memory devices.

[0051] Example 1

[0052] like Figures 1 to 8 As shown, Figure 1 This is a flow chart of a method for constructing a central polarization topological domain of the present invention. Figure 2 to Figure 8 , this embodiment is described in detail.

[0053] A method for constructing a central polarization topological domain in Example 1 comprises the following steps:

[0054] S1: Preparation of BiFeO3 thin film samples by pulsed laser deposition.

[0055] The specific method of preparing BiFeO3 thin film samples by pulsed laser deposition is as follows: SrTiO3

[100] is used as substrate, SrRuO3 is used as bottom electrode, and BiFeO3 is used as ferroelectric layer; the deposition conditions are as follows: laser pulse energy is 140mJ, deposition rate is 2Hz, substrate temperature is 605℃, oxygen pressure of deposited SrRuO3 and BiFeO3 layers is 1.0Pa; SrRuO3 thickness is 10nm, BiFeO3 thickness is 80nm;

[0056] S2, using AAO as a template, the diameter of the AAO template is 60nm, the height is 300nm, the argon ion beam passes through the holes of the AAO template to etch the film, the etching time is 25min, and a high-density conical "volcano-shaped" nanodot array is obtained and phase characterization is performed; the present invention uses a longer etching time to obtain a smaller nanodot array, the thickness of the nanodot is 20nm, and the diameter is 60nm, thereby improving the storage density;

[0057] S3, using PFM to characterize the spontaneous polarization topological domains of the nanodot array, flipping the polarization direction by PFM tip electric field writing, and performing vector PFM characterization on the results;

[0058] There is a conductive coating Pt / Ti on the PFM tip. The PFM characterization uses the vector PFM technology. First, the PFM image of a certain area of ​​the sample in the vertical direction is collected. The PFM tip driving frequency of the vertical PFM characterization is 380KHz. Then the sample is subjected to horizontal PFM characterization. The PFM tip driving frequency of the horizontal PFM characterization is 0.9MHz.

[0059] The specific method of flipping the polarization direction by PFM tip electric field writing is: first use DC PFM tip electric field scanning to write a rectangular pattern to achieve polarization flipping, and then use vector PFM technology to characterize the writing results; the flipping of the polarization direction is consistent with the vector electric field of the PFM tip, that is, both out-of-plane and in-plane flipping occur.

[0060] S4, reorganize the polarization vector to obtain three different types of central polarization topological domains,

[0061] The specific method for reorganizing the polarization vector is: combining the horizontal and vertical PFM images to obtain a three-dimensional polarization vector, and obtaining three different types of central polarization topological domains, including a single-center topological domain, a dual-center topological domain, and a triple-center topological domain.

[0062] like Figure 2 As shown, the first step is to prepare high-quality, high-performance epitaxial ferroelectric nanodot arrays.

[0063] like Figure 3 As shown, the structure was characterized by X-ray diffraction θ~2θ scanning, and the results clearly gave the epitaxial structure of STO / SRO / BFO. The lattice constant calculated from the out-of-plane (002) peak showed that the BFO film was a rhombohedral phase structure.

[0064] like Figure 4 As shown in Figure 2, the PFM piezoelectric loop of the sample is characterized using the “piezoelectric-hysteresis” mode. Figure 5 As shown, the PFM phase diagram ( Figure 5 a) A 180° change, normalized PFM amplitude diagram ( Figure 5 b) is a butterfly-shaped piezoelectric loop, which shows that the coercive field of the BFO film is ±4V and very symmetrical, indicating that the sample has good ferroelectric properties.

[0065] like Figure 5 As shown, scanning electron microscopy and atomic force microscopy characterization give the morphology, nanodot height ( Figure 5 d) is 20nm, the diameter is 60nm, and the cross-sectional view of the morphology ( Figure 5 c) Prove that the conical "volcano-shaped" nanodot array can be prepared according to the parameters provided by the present invention; the corresponding storage density can reach 100Gbit / inch 2 .

[0066] like Figure 6 As shown, the ferroelectric nanodots were then characterized by vector PFM to obtain a spontaneous topological domain structure.

[0067] like Figure 7 As shown, the polarized topological domain array is controlled by needle-tip electric field writing.

[0068] like Figure 8 As shown, through polarization reconstruction, a single-center topological domain, a dual-center topological domain, and a triple-center topological domain are finally obtained.

[0069] According to step S1, the pulsed laser deposition method uses a stoichiometric BFO ceramic target to prepare a STO / SRO / BFO structure film containing a certain concentration of defects by strictly controlling parameters such as oxygen pressure, temperature, laser energy density, and pulse frequency (see Table 1). The pulsed laser is a laser with a wavelength of 248nm emitted by a KrF excimer laser. The preparation parameters for depositing the BFO film are given in Table 1:

[0070] Table 1 Pulsed laser deposition parameters

[0071] Energy (mJ) Pulse frequency (Hz) Temperature(℃) Oxygen pressure (Pa) 140 2 605 1.0

[0072] After the BFO film was deposited, it was cooled to room temperature under an oxygen pressure of 1.0 Pa.

[0073] According to step S2, an argon ion beam passes through the holes of the template to etch the film, and the etching time is 25 minutes.

[0074] According to step S3, the initial state of the nanodot array is characterized by the vector PFM method, and the out-of-plane ( Figure 6 b) and in-plane ( Figure 6 d) PFM phase image, using the pre-designed +8V / -8V rectangular pattern to flip the polarization of the nanodot array and characterize it from the out-of-plane ( Figure 7 b) The PFM phase image can confirm that the polarization in the vertical direction of the written area is completely flipped, with the polarization direction being downward on the left and upward on the right; the in-plane ( Figure 7 d) The PFM phase diagram shows that the area written at +8 V has a left-bright / right-dark contrast distribution, while the area written at -8 V has a left-dark / right-bright contrast distribution. This indicates that the nanodots have a head-to-head and tail-to-tail polarization distribution within the surface, which may form an unusual topological structure. At the same time, the existence of these two domain structures in the same image shows that they can be controlled.

[0075] According to step S4, the polarization topological domain structure is analyzed and reconstructed using a vector PFM method, such as Figure 8 Specifically, it refers to the three-dimensional domain structure in the ferroelectric nanodots obtained by combining the in-plane (horizontal direction) polarization and the out-of-plane (vertical direction) polarization, which is a single-center, dual-center, and triple-center polarization topological domain ( Figure 8 For type I, we can determine that the nanodots have an unusual domain structure, where all horizontal polarization vectors point radially toward the center, such as Figure 8 As shown in (aⅤ), the vertical component in the entire nanodot is upward. Type II, Figure 8 (bⅤ) shows that the out-of-plane phase image has a single bright contrast and the in-plane phase image has a dark / bright double contrast, which is a structure that diverges along the radial center. Type III, Figure 8 (cⅤ) shows that the out-of-plane phase image has double contrast and the in-plane phase image has triple contrast, that is, the left side is the radial center convergence area and the right side is the radial center divergence. Type IV, Figure 8 (dV) shows that the out-of-plane phase image has three contrasts and the in-plane phase image has a complex contrast distribution, namely, radial center convergence on the left, radial center divergence in the middle, and center convergence area on the right.

[0076] The topological domains constructed by the present invention are stable throughout the measurement period of one month, suitable for large-scale production and low cost, and can easily obtain nanocapacitors less than 100nm. The present invention cuts the surface into smaller discrete nanocapacitors, and the "volcano-like" depolarization field gradient appears at the vertices and slopes of the nanodots to form smaller topological domains, thereby obtaining a higher storage density, and obtaining a high-density, stable, and controllable polarization topological domain array that can be used in ferroelectric memory devices.

Claims

1. A method for constructing a central polarization topological domain, characterized in that: The steps include: S1: Prepare BiFeO3 thin film sample by pulse laser deposition method, wherein the pulse laser deposition method in step S1 uses SrTiO3[100] as substrate, SrRuO3 as bottom electrode, BiFeO3 as ferroelectric layer, SrRuO3 thickness is 10nm, BiFeO3 thickness is 80nm; the deposition conditions of the pulse laser deposition method in step S1 are: laser pulse energy is 140mJ, deposition rate is 2Hz, substrate temperature is 605℃, and oxygen pressure of depositing SrRuO3 and BiFeO3 layers is 1.0Pa; S2: Using AAO as a template, an argon ion beam passes through the holes of the AAO template to etch the film, obtaining a high-density conical "volcano-shaped" nanodot array, and performing phase characterization; the diameter of the AAO template in step S2 is 60nm, the height is 300nm, and the etching time is 25min; the thickness of a single nanodot obtained in step S2 is 20nm, and the diameter is 60nm; S3: PFM characterization is used to obtain spontaneous polarization topological domains on the nanodot array, the polarization direction is flipped by PFM tip electric field writing, and the result is characterized by vector PFM; in the step S3, there is a conductive coating Pt / Ti on the PFM tip, and the PFM characterization uses vector PFM technology. First, a PFM image of a certain area of ​​the sample in the vertical direction is collected, and the PFM tip driving frequency of the vertical PFM characterization is 380KHz; then the sample is characterized by horizontal PFM, and the PFM tip driving frequency of the horizontal PFM characterization is 0.9MHz; the specific method of flipping the polarization direction by PFM tip electric field writing in the step S3 is: first use DC PFM tip electric field scanning to write a rectangular pattern to achieve polarization flipping, and then use vector PFM technology to characterize the written result; the flipping of the polarization direction is consistent with the vector electric field of the PFM tip, that is, flipping occurs both out-of-plane and in-plane; S4: Reorganize the polarization vector to obtain three different types of central polarization topological domains. The specific method for reorganizing the polarization vector in step S4 is: combine the PFM images in the horizontal and vertical directions to obtain a three-dimensional polarization vector, and obtain three different types of central polarization topological domains. The three different types of central polarization topological domains include a single central topological domain, a dual central topological domain, and a triple central topological domain.

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