Method for constructing polarized bubble-shaped topological domain array
Calcium-doped bismuth ferrate film was prepared by pulsed laser deposition, and polarized bubble-like topological domain array was constructed using PFM needle tip electric field technology, which solved the problem of insufficient storage density of ferroelectric memory and difficult preparation process, and achieved high-density, stable and environmentally friendly memory devices.
Patent Information
- Application Number
- CN202411722012.5
- Authority / Receiving Office
- CN · China
- Patent Type
- Applications(China)
- Current Assignee / Owner
- Filing Date
- 2024-11-28
- Publication Date
- 2025-03-14
AI Technical Summary
The current storage density of ferroelectric memory is insufficient, which limits its development in large-scale storage applications. The traditional preparation process is difficult, and the PbTiO3 used contains volatile lead, which poses environmental protection and safety risks.
Calcium-doped bismuth ferrate film was deposited by pulsed laser deposition, and a polarized bubble-like topological domain array was constructed using piezoelectric power microscope (PFM) needle tip electric field writing and dot writing technology.
The construction of a polarized bubble-like topological domain array with high density and good stability is realized, which reduces the preparation cost and avoids the use of lead. It is suitable for the development of high-density ferroelectric random access memory.
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Figure CN119630271A_ABST
Abstract
Description
Technical Field
[0001] The invention belongs to the technical field of micro-nano structures, and in particular relates to a method for constructing a polarized bubble-shaped topological domain array. Background Art
[0002] With the continuous development of information technology, the performance requirements for memory are increasing. Ferroelectric RAM (FeRAM) has many advantages, such as low power consumption, fast writing, and extremely high erase and write times (more than 10^16 times at 3.3V), making it promising to become the next generation of non-volatile memory. However, the current storage density of ferroelectric memory is still insufficient, which limits its development in large-scale storage applications. Researchers predict that the size of a single vortex polarized topological domain can reach 3.2nm, and the storage density can theoretically reach 60Tb / inch2, which provides a new direction for high-density storage of ferroelectric memory. In addition, in addition to vortex polarization, research on different topological structures such as anti-vortex, bubble domain, and radial center type has also been carried out. At present, these topological defects have been detected in PbTiO3 / SrTiO3 multilayer films and PbTiO3 / SrTiO3 superlattices. However, these results have not been further studied for controllable flipping because there is no bottom electrode, and it is difficult to integrate them into devices in the form of capacitors, and the preparation process is very difficult. At the same time, PbTiO3 contains volatile lead, which is toxic, and finding lead-free or low-toxic alternative materials has become an important research direction.
[0003] For the characterization of polarization topological domains, the visualization of X-ray diffraction characterization is not strong; transmission electron microscopy can only provide polarization direction data of a certain cross section of the sample, and sample preparation and characterization are both destructive to the sample. In contrast, piezoelectric force microscopy (PFM) has shown great potential in the characterization and regulation of ferroelectric topological domains. PFM can simultaneously characterize out-of-plane and in-plane polarization and construct the three-dimensional structure of the domain. Summary of the invention
[0004] Purpose of the invention: The purpose of the present invention is to provide a simple and easy method for constructing a polarized bubble-shaped topological domain array.
[0005] Technical solution: A method for constructing a polarized bubble-shaped topological domain array of the present invention comprises:
[0006] S1, depositing a calcium-doped bismuth ferrite thin film by pulsed laser deposition;
[0007] S2, using PFM tip electric field writing to flip the out-of-plane polarization direction of the film, construct an out-of-plane single domain, and characterize it;
[0008] S3, write a point in the surface writing area once by using the PFM tip pulse voltage to flip the polarization and construct the in-plane central domain;
[0009] S4, a second point writing is performed in the point writing area by using a PFM tip pulse voltage to flip the polarization;
[0010] S5. Change the position of the PFM tip and repeat steps S3 and S4 to construct a bubble-shaped ferroelectric topological domain array.
[0011] Furthermore, step S1 includes: pulse laser deposition of a SrRuO3 bottom electrode on a SrTiO3 substrate, and pulse laser deposition of a ferroelectric layer of calcium-doped BiFeO3 (Ca-BFO) on a surface of the SrRuO3 bottom electrode.
[0012] Furthermore, the thickness of the SrRuO3 layer is 5 to 10 nm, and the thickness of the Ca-BFO layer is 30 to 35 nm.
[0013] Further, step S2 includes: first presetting a rectangular voltage pattern, which includes positive and negative DC voltages, and then using the PFM tip to scan and write according to the preset pattern, and then performing PFM characterization to confirm that the writing operation has achieved out-of-plane polarization reversal. Because it is considered a large-area writing in the present invention, it is called surface writing.
[0014] Furthermore, the characterization uses a vector PFM method, with a vertical PFM (VPFM) tip frequency of 200 to 300 KHz and a lateral PFM (LPFM) tip frequency of 1.0 to 1.1 MHz.
[0015] Furthermore, the PFM tip has a Pt / Ir conductive coating.
[0016] Further, step S3 includes: first fixing the PFM tip in the surface writing area in step S2, applying a DC pulse voltage to the PFM tip for writing, and the direction of the pulse voltage is opposite to that of the surface writing. The effect of this operation is to construct a central domain. Because the tip is fixed at a point on the thin film sample, and it is the first time in the operation, it is called "one-point writing", and the subsequent second point writing is called "secondary point writing".
[0017] Further, step S4 includes: keeping the position of the PFM tip unchanged (ie, consistent with the position in step S3), applying a DC pulse voltage to the PFM tip for secondary point writing, and the direction of the pulse voltage is opposite to that of the primary point writing.
[0018] Furthermore, the pulse voltage of the first point writing is U1, and the pulse time is t1; the pulse voltage of the second point writing is U2, and the pulse time is t2; U2 is less than U1, and t2 is shorter than t1. Compared with the first point writing, the pulse time of the second point writing is shorter, and the purpose is to construct a bubble domain based on the central domain.
[0019] Furthermore, step S5 includes: by combining the data of two VPFM images with one LPFM image, the three-dimensional polarization vector distribution of the topological domain in space can be reconstructed.
[0020] Beneficial effects: Compared with the prior art, the present invention has the following significant advantages:
[0021] (1) By introducing acceptor ions to introduce oxygen vacancy defects, combined with pulsed laser deposition, calcium-doped bismuth ferrite thin films can be efficiently prepared. Compared with traditional lead-containing ferroelectric materials, the ferroelectric layer of the present invention has the characteristics of being green and environmentally friendly, having a high polarization value and good ferroelectricity. Pulsed laser deposition technology is mature, and no additional template assistance is required to prepare thin films.
[0022] (2) The PFM tip electric field is used to make the charged defects move in a directional manner (changing the tip electric field can adjust the domain structure). Through surface writing and two point writing operations, a high-density polarized bubble-shaped topological domain (bubble domain) array is formed to achieve precise construction of topological domains. The operation is simple and easy, which is convenient for large-scale and industrialized preparation, and reduces the manufacturing cost of future functional materials and high-density microelectronic device integration. The polarized topological domain array finally obtained has high density and good stability, which is suitable for the preparation of high-density ferroelectric random access memory, and is expected to effectively solve the technical bottleneck of low storage density faced by ferroelectric random access memory devices in the current market.
[0023] In summary, the present invention provides a new solution for the efficient construction of polarization topological domain arrays and has potential application value in high-density ferroelectric memories. BRIEF DESCRIPTION OF THE DRAWINGS
[0024] Figure 1 It is a flow chart of a method for constructing a polarized bubble-shaped topological domain array provided by an embodiment of the present invention;
[0025] Figure 2 is an X-ray diffraction diagram of a ferroelectric thin film in an embodiment of the present invention;
[0026] Figure 3 is a surface morphology diagram of a ferroelectric thin film in an embodiment of the present invention;
[0027] Figure 4 is a PFM piezoelectric loop diagram of a ferroelectric thin film in an embodiment of the present invention;
[0028] Figure 5It is the polarization bubble domain constructed by using the needle tip electric field in steps S2 to S5 of the embodiment of the present invention;
[0029] Figure 6 This is a diagram of a high-density bubble domain array constructed using the PFM tip electric field in steps S2 to S5 of an embodiment of the present invention. DETAILED DESCRIPTION
[0030] The present invention will be further described below in conjunction with the accompanying drawings.
[0031] like Figure 1 As shown, an embodiment of the present invention provides a method for constructing a polarized bubble-shaped topological domain array, comprising the following steps:
[0032] S1, depositing a calcium-doped bismuth ferrite thin film by pulsed laser deposition;
[0033] According to step S1, firstly, a high-quality, high-performance epitaxial ferroelectric film is prepared. The pulsed laser deposition method uses a stoichiometric BFO ceramic target and prepares the ferroelectric film by strictly controlling parameters such as oxygen pressure, temperature, laser energy density, and pulse frequency. X-ray diffraction θ-2θ scanning characterization, such as Figure 2 As shown, the results clearly give the epitaxial structure of STO / SRO / Ca-BFO, and the lattice constant calculated from the out-of-plane (001) peak shows that the Ca-BFO film has a rhombohedral phase structure.
[0034] The morphology of the thin film samples was characterized by atomic force microscopy ( Figure 3 (a)), the roughness (rms, root mean square value) of the film is 0.5nm, indicating that the film surface is flat. Figure 3 (b) and Figure 3 (d)) It can be seen that the out-of-plane polarization of the film has a bubble-like domain structure, while the in-plane polarization is a smaller domain structure. Figure 3 (c) and Figure 3 (e)) It can be seen that the out-of-plane polarization of the film is more upward, and the in-plane polarization includes head-to-head and tail-to-tail distributions.
[0035] S2, using PFM tip electric field writing to flip the out-of-plane polarization direction of the film, construct an out-of-plane single domain, and characterize it;
[0036] According to step S2, in order to flip the out-of-plane polarization of the film, the PFM piezoelectric loop of the sample is characterized using the "piezoelectric-hysteresis" mode. Figure 4 (a) is the normalized PFM amplitude diagram, which is a butterfly-shaped piezoelectric loop. Figure 4(b) shows that the PFM phase diagram has changed by 180°. The PFM piezoelectric loop shows that the coercive field of the BFO film is ±4V and very symmetrical, indicating that the sample has good ferroelectricity. This provides a very good platform for constructing polarization topological domains through needle-tip electric field writing.
[0037] like Figure 5 As shown in (a), a bias voltage (exceeding the coercive field) template is prepared in advance, consisting of two rectangular patterns, with the light color (or white) set to +6V and the dark color (or black) set to -6V. Then, according to this bias template, a voltage is applied to the needle tip to scan the thin film sample. The process is as follows Figure 5 (a) is shown as the white line with arrows. Finally, PFM characterization is performed to test the results of the needle tip electric field writing, such as Figure 5 (d) and Figure 5 (g) is shown. Figure 5 In (d), the area on the left where +6V is written becomes a uniform light contrast, and the area on the right where -6V is written becomes a uniform dark contrast. Figure 5 In (g), the area on the left where +6V is written becomes a stripe-like contrast distribution, and the area on the right where -6V is written becomes a mostly dark contrast distribution.
[0038] S3, write a point in the surface writing area once by using the PFM tip pulse voltage to flip the polarization and construct the in-plane central domain;
[0039] According to step S3, Figure 5 As shown in (b), the PFM tip is fixed on the ferroelectric thin film sample in advance, precisely at the left side of the +6V (-6V) writing area in step S2. Then a -8V (+8V) voltage is applied to the tip to write, and this step is followed from left to right for three times. The process is as follows Figure 5 Finally, PFM characterization is performed to verify the result of the needle tip electric field writing, as shown in Figure 5 (e) and Figure 5 (h) is shown. Figure 5 In (e), the area on the left that was written once with -8V becomes a uniform dark contrast, and the area on the right that was written once with +8V becomes a uniform light contrast. Figure 5 In (h), the area on the left where -8V is written becomes a left dark, right bright contrast distribution, and the area on the right where +8V is written becomes a right bright / left dark contrast distribution.
[0040] S4, a second point writing is performed in the point writing area by using a PFM tip pulse voltage to flip the polarization;
[0041] According to step S4, Figure 5As shown in (c), the PFM tip is fixed on the ferroelectric thin film sample in advance, precisely at the left side of the -8V (+8V) writing area in step S3. Then a +8V (-8V) voltage is applied to the tip to write, and this step is followed from left to right for three times. The process is as follows Figure 5 Finally, PFM characterization is performed to verify the result of the needle tip electric field writing, as shown in Figure 5 (f) and Figure 5 (i) is shown. Figure 5 In (f), the area where the +8V secondary dot writing is turned into a uniform light contrast, and the area where the -8V secondary dot writing is turned into a uniform dark contrast. Figure 5 In (i), the left area still has the left dark and right bright contrast distribution, and the right area still has the right bright / left dark contrast distribution.
[0042] S5. Change the position of the PFM tip and repeat steps S3 and S4 to construct a bubble-shaped ferroelectric topological domain array.
[0043] According to step S5, the ferroelectric thin film is characterized by using the vector PFM method to obtain the out-of-plane ( Figure 6 (b, c)) and in-plane ( Figure 6 (d, e)) PFM images, using a pre-designed +6V / -6V rectangular pattern to perform polarization reversal and characterization on a large area (2.6μm*2.6μm) of the film, from the out-of-plane ( Figure 6 (c) The PFM phase image shows that the polarization in the vertical direction of the written area is completely flipped, with the left side downward and the right side upward. Figure 6 The PFM phase diagram of (e) is relatively complex, forming a domain structure of left dark / middle bright / right dark. The needle tip is fixed at a certain position on the film, and a pulse voltage is applied to the needle tip. The specific parameters are given in Table 1. Then, the vector PFM is characterized and analyzed after the pulse voltage is written. The out-of-plane PFM phase ( Figure 6 The solid circular image (c) shows that the writing effect of the tip is very good. The electric field of the tip is well coupled with the polarization of the ferroelectric film. More importantly, the in-plane PFM phase ( Figure 6 (e)) The half-to-half contrast distribution of left dark / right bright and left bright / right dark in the figure shows the head-to-head and tail-to-tail polarization distribution. Through polarization reconstruction, two polarization bubble domain arrays with center up and center down are finally obtained. Moreover, the existence of these two topological domains in the same figure shows that they can be controlled. The size of the topological domain has a corresponding relationship with the size and time of the pulse voltage. From the out-of-plane ( Figure 6 (b)) and in-plane ( Figure 6 (d) Amplitude diagram, we can see the one-to-one correspondence between it and the phase diagram. Figure 6(a) It can be seen that the topological domains constructed by the method of the present invention do not damage or affect the sample morphology. This stable and controllable polarization topological domain array can be used in high-density ferroelectric memory devices.
[0044] Table 1 PFM "point writing" pulse voltage size and time parameters
[0045]
Claims
1. A method for constructing a polarized bubble-shaped topological domain array, characterized in that: include: S1, depositing a calcium-doped bismuth ferrite thin film by pulsed laser deposition; S2, using PFM tip electric field writing to flip the out-of-plane polarization direction of the film, construct an out-of-plane single domain, and characterize it; S3, write a point in the surface writing area once by using the PFM tip pulse voltage to flip the polarization and construct the in-plane central domain; S4, a second point writing is performed in the point writing area by using a PFM tip pulse voltage to flip the polarization; S5. Change the position of the PFM tip and repeat steps S3 and S4 to construct a bubble-shaped ferroelectric topological domain array.
2. The method for constructing a polarized bubble-shaped topological domain array according to claim 1, characterized in that: Step S1 includes: pulse laser deposition of a SrRuO3 bottom electrode on a SrTiO3 substrate, and pulse laser deposition of a ferroelectric layer of calcium-doped BiFeO3 (Ca-BFO) on the surface of the SrRuO3 bottom electrode.
3. The method for constructing a polarized bubble-shaped topological domain array according to claim 2, characterized in that: The thickness of the SrRuO3 layer is 5 to 10 nm, and the thickness of the Ca-BFO layer is 30 to 35 nm.
4. The method for constructing a polarized bubble-shaped topological domain array according to claim 1, characterized in that: Step S2 includes: first presetting a rectangular voltage pattern, which includes positive and negative DC voltages, and then using a PFM tip to scan and write according to the preset pattern, and then performing PFM characterization to confirm that the writing operation has achieved out-of-plane polarization reversal.
5. The method for constructing a polarized bubble-shaped topological domain array according to claim 4, characterized in that: The characterization uses the vector PFM method, with the longitudinal PFM tip frequency of 200-300 KHz and the transverse PFM tip frequency of 1.0-1.1 MHz.
6. The method for constructing a polarized bubble-shaped topological domain array according to claim 4, characterized in that: The PFM tip has a Pt / Ir conductive coating.
7. The method for constructing a polarized bubble-shaped topological domain array according to claim 1, characterized in that: Step S3 includes: first fixing the PFM tip in the surface writing area in step S2, applying a DC pulse voltage to the PFM tip for writing, and the direction of the pulse voltage is opposite to that of the surface writing.
8. The method for constructing a polarized bubble-shaped topological domain array according to claim 7, characterized in that: Step S4 includes: keeping the position of the PFM needle tip unchanged, applying a DC pulse voltage to the PFM needle tip for secondary point writing, and the direction of the pulse voltage is opposite to that of the primary point writing.
9. The method for constructing a polarized bubble-shaped topological domain array according to claim 8, characterized in that: The pulse voltage of the first point writing is U1, and the pulse time is t1; the pulse voltage of the second point writing is U2, and the pulse time is t2; U2 is smaller than U1, and t2 is shorter than t1.
10. The method for constructing a polarized bubble-shaped topological domain array according to claim 1, characterized in that: Step S5 includes: reconstructing the three-dimensional polarization vector distribution of the topological domain in space by combining the data of one LPFM image with the two VPFM images.