Ferroelectric functional film and preparation method thereof

Abstract

The invention relates to the technical field of ferroelectric materials, in particular to a ferroelectric functional film and a preparation method thereof. The method comprises the following steps: depositing a target material on a substrate by adopting a pulse laser method, and carrying out post-treatment after deposition to obtain a ferroelectric functional film; the target material comprises one or two of a lead titanate target material and a strontium titanate target material. By changing technological parameters in the pulse laser deposition process, accurate control over the domain structure of the ferroelectric film is achieved, and the high-density and small-size domain structure is formed. The high-density domain structure obviously improves the physical properties such as piezoelectricity, dielectricity and the like of the ferroelectric film, and provides a material basis for high-performance electronic devices. The ferroelectric film prepared by the technology has potential application value in the field of high-density information memories, and is expected to solve the problem of density limitation in the existing storage technology.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of ferroelectric materials, and particularly relates to a ferroelectric functional film and a preparation method thereof. BACKGROUND

[0002] Ferroelectric materials have wide application prospects in the fields of non-volatile memory, sensors, actuators and the like due to their unique spontaneous polarization characteristics and reversibility. In recent years, the domain structure regulation of information has become a research hotspot. By controlling the domain size and density in the ferroelectric film, the electrical properties can be significantly improved, providing a new technical approach for high-density information storage. The commonly used growth methods for ferroelectric functional films mainly include sol-gel method (Sol-Gel), metal organic chemical vapor deposition (MOCVD), chemical vapor deposition (CVD), magnetron sputtering method (Magnetron Sputtering), pulsed laser deposition method (PLD) and molecular beam epitaxy (Molecular Beam Epitaxy) and the like.

[0003] However, the above methods are difficult to achieve controllable preparation of high-density and small-size domain structures by precise regulation of growth parameters. For example, in the mainstream growth processes such as sol-gel method and magnetron sputtering, the correlation mechanism between growth parameters and domain structure is not clear, which makes it difficult to precisely control the domain size and density, and most processes are prone to form large-size and low-order ferroelectric domains. Some studies attempt to obtain small-size domains by introducing doping or interface regulation, but often have the problem of insufficient stability of domain structure, which is prone to domain merging and growing during repeated electric field polarization reversal or long-term use, resulting in attenuation of ferroelectric properties of the film. At the same time, the existing methods are difficult to balance the high-density distribution of small-size domains and excellent electrical properties, and often have problems such as decrease of breakdown strength and increase of leakage current, which restricts their application in high-density storage and other high-end devices.

[0004] The pulsed laser deposition method can realize the consistency of the stoichiometric ratio of the film and the target material under appropriate conditions; the film quality is high; the growth parameters are easy to control; and especially in the preparation of multi-component complex oxide (such as ferroelectric, ferromagnetic and superconducting materials) films and multi-layer films, it has irreplaceable advantages. Based on this, how to precisely control the key parameters in the deposition process to realize the precise regulation of high-density and small-size domain structure of ferroelectric functional film preparation technology, and solve the problems of poor controllability of domain structure, insufficient performance synergy and difficulty in scale compatibility in the existing process have become technical problems to be solved in the field. SUMMARY

[0005] The purpose of this invention is to provide a ferroelectric functional thin film and its preparation method. By precisely controlling key parameters in the pulsed laser deposition process, such as laser energy, oxygen pressure, and growth temperature, a ferroelectric thin film with high density and small-sized domain structure can be obtained.

[0006] To achieve the above-mentioned objectives, the present invention provides the following technical solution: This invention provides a method for preparing ferroelectric functional thin films, comprising the following steps: A target material was deposited on a substrate using a pulsed laser method. After deposition, post-processing was performed to obtain a ferroelectric functional thin film. The target material includes one or both of lead titanate and strontium titanate targets.

[0007] Optionally, the substrate includes a neodymium scandate substrate or a gadolinium scandate substrate; the lead titanate target is a sintered ceramic target with a lead excess of 3 mol%.

[0008] Optionally, the lead titanate target needs to be pre-sputtered before deposition.

[0009] Optionally, the laser energy of the pre-sputtering is 300~350mJ, and the time is 30min.

[0010] Optionally, the deposition temperature is 750~780℃, the oxygen pressure is 55~75mTorr, and the time is 5~180min.

[0011] Optionally, the process parameters of the pulsed laser method are as follows: the laser source is a KrF laser with a wavelength of 248 nm, the laser energy is 370~390 mJ, the pulse frequency is 4~8 Hz, and the laser energy density is 2 J / cm². 2 .

[0012] Optionally, the post-processing includes: after deposition, oxygen is introduced to bring the pressure to 180~240 Torr, the temperature is maintained at 650~750℃ for 5~10 min, then the temperature is reduced to 200℃ at a cooling rate of 5~100℃ / min, and finally cooled to room temperature.

[0013] The present invention also provides ferroelectric functional thin films prepared by the above-described preparation method.

[0014] Compared with the prior art, the present invention has the following beneficial effects: 1. Domain structure control method: By changing the process parameters during pulsed laser deposition, the domain structure of ferroelectric thin films can be precisely controlled to form high-density, small-size domain structures.

[0015] 2. Performance Enhancement: The high-density domain structure significantly improves the piezoelectricity, dielectricity and other physical properties of ferroelectric thin films, providing a material basis for high-performance electronic devices.

[0016] 3. Application prospects: Ferroelectric thin films prepared by this technology have potential application value in the field of high-density information storage, and are expected to solve the density limitation problem faced by existing storage technologies. Attached Figure Description

[0017] Figure 1 The images show the planar morphology of the PbTiO3 films prepared in Examples 1-3. Figure 2 These are high-resolution high-angle annular dark-field images prepared in Examples 1-3; Figure 3 The images show the cross-sectional morphology of the PbTiO3 films prepared in Examples 4-6. Detailed Implementation

[0018] Various exemplary embodiments of the present invention will now be described in detail. This detailed description should not be considered as a limitation of the present invention, but rather as a more detailed description of certain aspects, features, and embodiments of the present invention.

[0019] It should be understood that the terminology used in this invention is merely for describing particular embodiments and is not intended to limit the invention. Furthermore, with respect to numerical ranges in this invention, it should be understood that each intermediate value between the upper and lower limits of the range is also specifically disclosed. Every smaller range between any stated value or intermediate value within a stated range, and any other stated value or intermediate value within said range, is also included in this invention. The upper and lower limits of these smaller ranges may be independently included or excluded from the range.

[0020] Unless otherwise stated, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art. While only preferred methods and materials have been described herein, any methods and materials similar or equivalent to those described herein may be used in the implementation or testing of this invention. All references to this specification are incorporated by way of citation to disclose and describe methods and / or materials associated with those references. In the event of any conflict with any incorporated reference, the content of this specification shall prevail.

[0021] Various modifications and variations can be made to the specific embodiments described in this specification without departing from the scope or spirit of the invention, as will be apparent to those skilled in the art. Other embodiments derived from this specification will also be obvious to those skilled in the art. This application specification and embodiments are merely exemplary.

[0022] The terms “include,” “including,” “have,” “contain,” etc., used in this article are all open-ended terms, meaning that they include but are not limited to.

[0023] All raw materials used in this invention can be obtained commercially or prepared using existing technologies.

[0024] This invention provides a method for preparing ferroelectric functional thin films, comprising the following steps: A target material was deposited on a substrate using a pulsed laser method. After deposition, post-processing was performed to obtain a ferroelectric functional thin film. The target material includes one or both of lead titanate and strontium titanate targets.

[0025] In this invention, the lead titanate (PbTiO3) target is a sintered ceramic target with lead excess of 3 mol%.

[0026] In some embodiments of the present invention, the lead titanate (PbTiO3) target material was purchased from Hefei Kejing Materials Technology Co., Ltd.

[0027] In this invention, the substrate includes a neodymium scandate substrate and a gadolinium scandate substrate; the neodymium scandate (NbScO3) and gadolinium scandate (GdScO3) substrates are commercially available single-crystal high-precision polished substrates.

[0028] In some embodiments of the present invention, the substrate needs to be ultrasonically cleaned in acetone for 10 to 20 minutes to remove surface contaminants.

[0029] In some embodiments of the present invention, the substrate is heated to 850~1000°C and held for about 5~15 minutes in a pulsed laser deposition apparatus before deposition to clean contaminants on the substrate surface, and then cooled to 750~780°C for deposition.

[0030] In this invention, the lead titanate target needs to be pre-sputtered before deposition.

[0031] In this invention, the laser energy of the pre-sputtering is 300~350mJ, for example, it can be 300mJ, 320mJ, 340mJ or 350mJ, etc., and the time is 30min. The purpose of the pre-sputtering is to clean the surface of the target material.

[0032] In this invention, the deposition temperature is 750~780℃, for example, 750℃, 760℃, 770℃ or 780℃, the oxygen pressure is 55~75mTorr, for example, 55mTorr, 60mTorr, 65mTorr, 70mTorr or 75mTorr, and the time is 5~180min, for example, 5min, 10min, 15min, 20min, 50min, 100min, 150min or 180min.

[0033] In this invention, the process parameters of the pulsed laser method are as follows: the laser source is a KrF laser with a wavelength of 248 nm; the laser energy is 370~390 mJ, for example, 370 mJ, 380 mJ, or 390 mJ; the pulse frequency is 4~8 Hz, for example, 4 Hz, 6 Hz, or 8 Hz; and the laser energy density is 2 J / cm². 2 .

[0034] In this invention, the post-processing includes: after deposition, oxygen is introduced to achieve a pressure of 180~240 Torr (e.g., 180 Torr, 200 Torr, 220 Torr, or 240 Torr, etc.), and the temperature is maintained at 650~750℃ (e.g., 650℃, 680℃, 700℃, 720℃, or 750℃, etc.) for 5~10 min (e.g., 5 min, 8 min, or 10 min, etc.), then the temperature is reduced to 200℃ at a cooling rate of 5~100℃ / min (5℃ / min, 15℃ / min, 30℃ / min, 50℃ / min, 60℃ / min, 70℃ / min, 80℃ / min, 90℃ / min, or 100℃ / min, etc.), and finally cooled to room temperature.

[0035] The present invention also provides ferroelectric functional thin films prepared by the above-described preparation method.

[0036] The technical solutions provided by the present invention will be described in detail below with reference to the embodiments, but they should not be construed as limiting the scope of protection of the present invention.

[0037] Example 1 Neodymium scanate (NbScO3) substrate pretreatment: Before deposition, the neodymium scanate (NbScO3) substrate was ultrasonically cleaned in acetone for 15 minutes to remove surface contaminants; then the substrate was heated to 900℃ in a pulsed laser deposition equipment and held for about 10 minutes to remove surface contaminants. Pretreatment of lead titanate (PbTiO3) sintered ceramic target: The lead titanate (PbTiO3) sintered ceramic target with an excess of 3 mol% lead (Pb) was pre-sputtered with a low-energy laser beam for 30 minutes to clean the target surface. PbTiO3 thin films were deposited on NbScO3 substrates using pulsed laser deposition (PLD). The deposition temperature was 750 °C, the oxygen pressure was 75 mTorr, a KrF laser with a wavelength of 248 nm, a laser energy of 370 mJ, a laser pulse frequency of 4 Hz, and a laser energy density of approximately 2 J / cm². 2The deposition time was 5 minutes. After the film deposition was completed, oxygen was introduced into the chamber to make the pressure reach 200 Torr. The oxygen pressure was kept constant, and the temperature was kept at 700℃ for 5 minutes. Then, the temperature was cooled to 200℃ at a cooling rate of 50℃ / min. Finally, the temperature was allowed to cool naturally to room temperature to obtain the ferroelectric functional film.

[0038] Example 2 The only difference from Example 1 is that the deposition time is 15 minutes, and the rest of the steps are the same as in Example 1.

[0039] Example 3 The only difference from Example 1 is that the deposition time is 20 minutes, while the remaining steps are the same as in Example 1.

[0040] Figure 1 The images show TEM planar morphology of PbTiO3 thin films grown on NbScO3 substrates by pulsed laser deposition in Examples 1-3, with a scale bar of 100 micrometers. Figure 1 (a) shows the PbTiO3 film of Example 1, with a thickness of 2 nm and a domain size of 5.2 nm; (b) shows the PbTiO3 film of Example 2, with a thickness of 5 nm and a domain size of 8.4 nm; and (c) shows the PbTiO3 film of Example 3, with a thickness of 8 nm and a domain size of 9.5 nm. As the film thickness decreases, the domain size becomes smaller and the density becomes higher. High-density domain structures offer significant advantages in performance regulation and information storage.

[0041] Figure 2 High-angle annular dark-field imaging (HAADF-STEM) images of domain structure cross-sections obtained with the same growth parameters for different film thicknesses are shown. (a) is a high-resolution HAADF-STEM image of the cross-section of a 2 nm thick film structure in Example 1, showing that the film surface is flat, the interface between the film and the substrate is straight, and no dislocations are formed. (b) is a high-resolution HAADF-STEM image of the cross-section of a 5 nm thick film structure in Example 2, showing that the film surface is flat, the interface between the film and the substrate is straight, and no dislocations are formed. (c) is a high-resolution HAADF-STEM image of the cross-section of an 8 nm thick film structure in Example 3, showing that the film surface is flat, the interface between the film and the substrate is straight, and no dislocations are formed.

[0042] Example 4 SrTiO3 and PbTiO3 films grown on gadolinium scandate (GdScO3): PbTiO3 films were deposited on gadolinium scandate (GdScO3) substrates using pulsed laser deposition at a temperature of 750℃ and an oxygen pressure of 75 mTorr. A KrF laser source with a wavelength of 248 nm and a laser energy of 370 mJ was used, with a laser pulse frequency of 4 Hz and a laser energy density of approximately 2 J / cm². 2 The deposition time was 150 min, and then SrTiO3 film was deposited under the same conditions for 30 min. After deposition, oxygen was introduced to make the chamber pressure reach 200 Torr. The oxygen pressure was kept constant, and the temperature was kept at 700℃ for 5 minutes. Then it was cooled to 200℃ at a cooling rate of 5℃ / min, and then naturally cooled to room temperature to obtain ferroelectric functional film.

[0043] Example 5 The only difference from Example 4 is that the cooling rate is 15℃ / min, and the other steps are the same as in Example 4.

[0044] Example 6 The only difference from Example 4 is that the cooling rate is 30℃ / min, and the other steps are the same as in Example 4.

[0045] Figure 3 The figures show the cross-sectional TEM morphology of films with the same thickness but different growth parameters, with a scale bar of 100 nm. Figure (a) shows the TEM morphology of a 60 nm thick PbTiO3 film from Example 4. The film surface has some undulations, the film-substrate interface is smooth, and the film structure is mainly an a / c domain structure with the same orientation (

[011] ), with clear domain wall interfaces. Figure (b) shows the TEM morphology of a 60 nm thick PbTiO3 film from Example 5. The film surface and the film-substrate interface are relatively smooth, and the film structure is an a / c domain structure with more

[011] orientations and fewer

[011] orientations, with clear domain wall interfaces. Figure (c) shows the TEM morphology of a 60 nm thick PbTiO3 film from Example 6. The film surface and the film-substrate interface are relatively smooth, and the film structure is an a / c domain structure with

[011] orientations intersecting with

[011] orientations. The intersecting region forms a lattice-like domain structure distribution, and the domain wall interfaces are blurred. Figure 3 It can be seen that for films of the same thickness, as the cooling rate increases after film growth, the interaction between domain structures becomes more and more complex, and even new domain structure distribution characteristics are formed. These new domain structure characteristics significantly improve the domain density.

[0046] The dielectric constant and piezoelectric coefficient of the ferroelectric functional thin films prepared in Examples 1-6 were tested; Dielectric constant (εr) testing method: A 5 nm thick Cr adhesion layer and a 50 nm Au bottom electrode are deposited on one side of the film, and a 50 nm Au top electrode is deposited on the other side to form a parallel plate capacitor structure; at room temperature (25℃), the capacitance (C) and loss (tanδ) are measured using an impedance analyzer, with a frequency range of 100 Hz ~ 1 MHz and a signal amplitude of 50 mV, and the relative dielectric constant εr of the film is calculated.

[0047] piezoelectric response (d 33 (eff) Test: A circular Pt top electrode with a thickness of 100 nm was deposited on the surface of the prepared thin film. Using PFM (piezoelectric microscopy) in contact mode, an AC bias voltage was applied to the electrode region, and the longitudinal vibration amplitude of the probe was detected. The vibration amplitude was then calibrated to convert it into the effective piezoelectric coefficient d. 33 ,eff.

[0048] The test results are shown in Table 1.

[0049] Table 1 Performance test results of ferroelectric functional thin films in Examples 1-6

[0050] As shown in Table 1, the high-density domain structure significantly improves the piezoelectricity, dielectricity and other physical properties of ferroelectric thin films, providing a material basis for high-performance electronic devices.

[0051] The above description is only a preferred embodiment of the present invention. It should be noted that for those skilled in the art, several improvements and modifications can be made without departing from the principle of the present invention, and these improvements and modifications should also be considered within the scope of protection of the present invention.

Claims

1. A method for preparing a ferroelectric functional thin film, characterized in that, Includes the following steps: A target material was deposited on a substrate using a pulsed laser method. After deposition, post-processing was performed to obtain a ferroelectric functional thin film. The target material includes one or both of lead titanate and strontium titanate targets.

2. The preparation method according to claim 1, characterized in that, The substrate includes a neodymium scandate substrate or a gadolinium scandate substrate; the lead titanate target is a sintered ceramic target with lead excess of 3 mol%.

3. The preparation method according to claim 1, characterized in that, The lead titanate target needs to be pre-sputtered before deposition.

4. The preparation method according to claim 3, characterized in that, The laser energy for pre-sputtering is 300~350mJ, and the time is 30min.

5. The preparation method according to claim 1, characterized in that, The deposition temperature is 750~780℃, the oxygen pressure is 55~75mTorr, and the time is 5~180min.

6. The preparation method according to claim 1, characterized in that, The process parameters for the pulsed laser method are as follows: the laser source is a KrF laser with a wavelength of 248 nm, the laser energy is 370~390 mJ, the pulse frequency is 4~8 Hz, and the laser energy density is 2 J / cm². 2 .

7. The preparation method according to claim 1, characterized in that, The post-processing includes: after deposition, oxygen is introduced to bring the pressure to 180~240 Torr, and the temperature is maintained at 650~750℃ for 5~10 min. Then, the temperature is reduced to 200℃ at a cooling rate of 5~100℃ / min, and finally cooled to room temperature.

8. Ferroelectric functional thin films prepared by the preparation method according to any one of claims 1 to 7.

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