An ultrathin monocrystalline ferroelectric PZT thin film and a preparation method thereof

Abstract

The present application belongs to the field of condensed matter physics and material science technology, and particularly relates to an ultrathin single-crystal ferroelectric PZT film and a preparation method thereof. The present application adopts a pulsed laser deposition (PLD) method, and uses pulsed laser to bombard a PZT target material to grow a single-crystal ferroelectric PZT film on a single-crystal substrate. The deposition process includes two stages: a single-crystal layer growth stage and a rapid cooling stage. The method is simple and easy to operate, and the growth of the ultrathin ferroelectric single-crystal film is realized by controlling the laser energy, the substrate deposition growth temperature and the oxygen pressure during the PLD growth. In the present application, a high-energy electron diffraction (RHEED) instrument is used to monitor the growth process in real time, so as to ensure the crystal quality and thickness of the film during the growth. The atomic force microscope is used to observe the morphology of the prepared sample, and the atomic-level flatness of the surface is verified. The PZT film grown on the single-crystal substrate still maintains the ferroelectricity even when it is only 2 lattice layers thick, and the ferroelectric polarization reversal can be realized.

Description

TECHNICAL FIELD

[0001] The present application relates to an ultrathin single-crystal ferroelectric PZT thin film and a preparation method thereof, and belongs to the technical field of condensed matter physics and materials science. BACKGROUND

[0002] Ferroelectric materials are a kind of polar materials, which have a remnant polarization in the absence of an electric field and the polarization direction can be reversed by an applied electric field. Due to the ferroelectric polarization reversal, ferroelectric materials can be applied in many fields such as non-volatile memories, field effect transistors, logic devices, etc. Among the ferroelectric materials, lead zirconate titanate (PZT) stands out due to its strong polarization (~100 μm / cm 2 ) and has been widely used in many fields. With the rapid development of science and technology, the size of devices is constantly shrinking, which requires ferroelectric materials to reduce their thickness while still retaining their ferroelectric properties.

[0003] However, the size effect is found in many ferroelectric materials, as the thickness of the ferroelectric material decreases, the ferroelectric properties gradually weaken, and when the thickness is reduced to a certain thickness, the ferroelectricity will disappear, and this thickness is the critical thickness of the ferroelectric material. This critical thickness is related to many factors, such as stress, film quality, boundary conditions (such as electrodes), etc. In recent years, due to the continuous improvement of thin film preparation technology, the quality of thin films has also been improved, and the critical size of various ferroelectric materials predicted by experiments and theories is also decreasing. Some articles predict that the critical thickness of ferroelectric materials with perovskite structure is three atomic layers thick. Taking the ferroelectric material PZT with strong polarization as an example, its space structure is tetragonal, and its lattice constant is related to the ratio of Zr atoms and Ti atoms in PZT. When the ratio of Zr and Ti is 2:8, the lattice constant of PZT is a = b = 0.395 nm, c = 0.414 nm, i.e. the thickness of each atomic layer is about 0.4 nm. Some articles have observed the polarization arrangement of atoms in 1.5-layer PZT thin films, but this discovery has not demonstrated the reversibility of the polarization strength. SUMMARY

[0004] To address the shortcomings of existing technologies, this invention provides a method for preparing ultrathin single-crystal ferroelectric PZT thin films. Based on the principle of crystal nucleation and growth kinetics, this invention employs pulsed laser deposition (PLD) to achieve the growth of ferroelectric single-crystal PZT thin films on a single-crystal substrate by controlling the laser energy, substrate deposition temperature, and oxygen pressure during PLD growth. The method is simple and easy to operate. This invention utilizes high-energy electron diffraction (RHEED) to monitor the growth process in real time, ensuring the crystal quality and thickness of the film during growth. Atomic force microscopy observation of the prepared samples confirms the atomic-level flatness of their surface. The PZT thin films obtained by this invention maintain ferroelectricity even at a minimum thickness of only 2 unit cells (approximately 0.83 nm), and ferroelectric polarization reversal can be achieved through an electric field.

[0005] In this invention, "ultra-thin" in ultra-thin single-crystal ferroelectric PZT thin film refers to a thickness of less than 10 unit cells. This invention emphasizes "ultra-thin" to illustrate the limit thickness of PZT that can maintain ferroelectricity and achieve reversible ferroelectric polarization.

[0006] The technical solution of the present invention is described in detail below.

[0007] This invention provides a method for preparing an ultrathin single-crystal ferroelectric PZT thin film. The prepared ultrathin single-crystal ferroelectric PZT film retains ferroelectricity and can achieve ferroelectric polarization reversal even at a thickness as low as 2 unit cells. The preparation method is as follows:

[0008] A pulsed laser deposition method was used to grow single-crystal ferroelectric PZT thin films on a single-crystal substrate by bombarding a PZT target with a pulsed laser. The deposition process consisted of two stages: a single-crystal layer growth stage and a rapid cooling stage.

[0009] During the single-crystal layer growth stage, PZT ultrathin single-crystal films of a specified number of layers were continuously grown on a single-crystal substrate by controlling parameters such as pulsed laser energy and frequency, substrate temperature, and oxygen pressure. The growth conditions were as follows: substrate heating temperature between 600-650℃, laser energy between 20-30mJ, laser frequency between 3-5Hz, and oxygen pressure between 1.0×10⁻⁶. -2 mbar-1.2×10 -1Between mbar; regarding growth parameters, pulsed laser energy and frequency, substrate temperature, and oxygen pressure are interdependent, jointly determining the quality and properties of the grown film. First, excessively high substrate temperatures lead to the volatilization of Pb in PZT, forming pyrochlore-phase PZT, which lacks ferroelectricity; excessively low temperatures affect the material's crystallinity, preventing the formation of a perovskite structure, thus also resulting in PZT lacking ferroelectricity. Second, due to the volatility of Pb, high oxygen pressure growth is used to reduce Pb volatilization and form chemically stoichiometric PZT single-crystal films. However, excessively high oxygen pressure leads to excessive oxygen atom scattering of the plasma, reducing kinetic energy and preventing the formation of a smooth film on the substrate surface. Finally, laser energy is a crucial factor for achieving smooth oxide growth. Too low energy results in insufficient plasma kinetic energy, affecting crystallinity and surface smoothness; too high energy leads to rapid deposition, potentially forming other structural phases. Similarly, too low a laser frequency increases Pb deficiency, while too high a frequency exceeds the target's self-rotation speed, causing target material segregation.

[0010] During the rapid cooling phase, after the PZT ultrathin single crystal film growth is completed, the PZT sample is rapidly cooled from the growth temperature to room temperature by controlling the parameters of the PLD heating device. Due to the volatility of Pb in PZT, the longer the high temperature is maintained after growth, the more Pb volatilizes, which makes PZT more likely to transform into the paraelectric pyrochlore phase. Therefore, the temperature is cooled at a relatively fast rate after growth, with a cooling rate of 45-55℃ / min.

[0011] In this invention, the PZT growth process is monitored in real time by a high-energy electron diffractometer (RHEED) to obtain sample growth process information parameters, providing a feedback monitoring mechanism for in-situ growth and confirming the thickness of the grown film. During the single-crystal layer growth stage, the intensity of the diffraction spots in the RHEED pattern undergoes periodic bright-dark oscillations, with each bright-dark oscillation period corresponding to the growth process of one unit cell.

[0012] In this invention, the single-crystal substrate is Nb:STO. NbSTO is a conductive substrate, which facilitates PFM testing of the ferroelectricity of PZT. In addition, the NbSTO substrate structure is similar to that of PZT, but its lattice constant is slightly smaller than that of PZT, which can provide a certain compressive stress, which is beneficial for obtaining ferroelectricity with out-of-plane polarization.

[0013] In this invention, antiferroelectricity occurs when the proportion of Ti in the PZT target is less than 5%. Therefore, the proportion of Ti should be greater than 5%. In a specific embodiment, the atomic ratio of Zr to Ti is 0.2:0.8.

[0014] In this invention, during the single-crystal growth stage, the grown PZT ultrathin single-crystal film has a cell thickness of 2 to 6 layers.

[0015] The present invention also provides an ultrathin single-crystal ferroelectric PZT thin film prepared by the above-described preparation method.

[0016] In this invention, the PZT thin film has a cell thickness of 2 to 6 layers, a surface roughness of less than 200 μm, and exhibits ferroelectricity and can achieve ferroelectric polarization reversal.

[0017] In summary, this invention obtains ferroelectric single-crystal PZT that retains its ferroelectric properties even at a thickness of only 2 unit cells by growing ultrathin PZT single crystals on a single-crystal conductive substrate Nb:STO and then rapidly cooling them. Compared with existing technologies, the advantages of this invention are as follows:

[0018] The growth method of this invention utilizes a high-energy electron diffractometer (RHEED) for real-time in-situ monitoring. RHEED provides reciprocal diffraction information of the sample, thus providing feedback on the film condition and facilitating real-time control of growth conditions such as laser energy, substrate growth temperature, and oxygen pressure. The high quality of the sample is reflected in two aspects: the RHEED diffraction pattern is clear and sharp, the surface morphology smoothness of the sample scanned by atomic force microscopy is less than 200 μm, and ferroelectric signals are still detected in the PZT film at a thickness of only two unit cells using piezoelectric microscopy.

[0019] The growth method of this invention is simple and enables the growth of ultrathin ferroelectric material films on conductive substrates Nb:STO. Even in samples as thin as 2 layers, obvious ferroelectric signals can still be observed and ferroelectric polarization reversal can be achieved, which further reduces the thickness limit of PZT films to exhibit ferroelectricity. Attached Figure Description

[0020] When considered in conjunction with the accompanying drawings, the invention will be more fully and better understood, and its many accompanying advantages will become readily apparent, by referring to the following detailed description. However, the accompanying drawings, which are provided to further illustrate the invention and form part of this invention, are used to explain the invention and do not constitute an undue limitation thereof, as shown in the figures:

[0021] Figure 1 This is the basic structural composition of the thin film material of the present invention.

[0022] Figure 2 This is a diagram illustrating the equipment used for growing ultrathin single-crystal ferroelectric PZT thin films.

[0023] Figure 3 The thin film sample growth process steps of the present invention are as follows: The main processes of growing ultrathin single crystal ferroelectric PZT thin films are: the first stage is single crystal layer growth, which is the growth process of PZT thin films; the second stage is a rapid cooling process, in which the sample is cooled from the growth temperature to room temperature at a relatively fast rate.

[0024] Figure 4 These are RHEED data graphs of PZT thin films with different thicknesses (2, 3, 4, 5, and 6 cell thicknesses). The curves represent RHEED intensity oscillations, and the number of film layers can be determined by the number of oscillations. The inset to the right of each curve shows the corresponding RHEED diffraction pattern.

[0025] Figure 5 This is an XRD data image of an ultrathin single-crystal ferroelectric PZT thin film sample, which shows that the prepared PZT thin film has good single-crystal lines.

[0026] Figure 6 The images show surface morphology images of PZT thin films with different thicknesses (2, 3, 4, 5, and 6 cell thicknesses) obtained by AFM scanning and ferroelectric signals obtained by PFM scanning. The first row shows the surface morphology images obtained by AFM scanning, the second row shows the piezoelectric amplitude data obtained by FFM scanning, and the third row shows the piezoelectric phase data obtained by PFM scanning. Detailed Implementation

[0027] The present invention will be further described below with reference to the accompanying drawings and embodiments.

[0028] Obviously, many modifications and variations made by those skilled in the art based on the spirit of this invention fall within the scope of protection of this invention.

[0029] Those skilled in the art can understand the specific meaning of the above terms in this invention in more detail.

[0030] Those skilled in the art will understand that, unless otherwise defined, all terms used herein, including technical and scientific terms, have the same meaning as commonly understood by one of ordinary skill in the art.

[0031] This invention provides a method for preparing ultrathin single-crystal ferroelectric PZT thin films. The sample generated by this method includes two important structural components, such as... Figure 1 The bottom layer is a conductive single-crystal substrate with a specific crystal orientation, using an Nb:STO substrate; the top layer is a PZT single-crystal ferroelectric layer.

[0032] This invention employs a pulsed laser deposition (PLD) system for sample growth, which includes five subsystems (such as...). Figure 2The first subsystem is an ultra-high vacuum chamber, in which the substrate and target are placed. This chamber provides a stable and efficient vacuum environment for the nucleation and growth of single-crystal thin films. The second subsystem is an infrared heating system, used to heat the substrate to a specific temperature, thus ensuring the crystallinity of the film. The third subsystem is a pulsed laser system. The pulsed laser, through a total reflection mirror and a focusing lens, is focused onto the target within the vacuum chamber, sputtering plasma to form a plume, which is then deposited on a specifically oriented single-crystal substrate to form a thin film. The fourth subsystem is a gas circulation system. During the growth process, a certain flow rate of oxygen is introduced into the vacuum chamber to maintain a stable oxygen pressure within the chamber. The energy and frequency of the pulsed laser, the temperature of the substrate, and the oxygen pressure within the vacuum chamber are all key factors affecting the quality and properties of the thin film. The fifth subsystem is the real-time diffraction pattern feedback system of the in-situ monitoring system RHEED. According to the principle of diffraction, the RHEED diffraction pattern can provide feedback on the real-time status of the surface structure of PZT single crystal thin films, thereby enabling timely and accurate control of growth conditions. In addition, the change in the intensity of the RHEED diffraction pattern can also reflect the film growth thickness, thus accurately growing films of the required thickness.

[0033] In this invention, the growth process of single-crystal ferroelectric PZT thin films is mainly divided into two stages (e.g., Figure 3 The process consists of two stages: a single-crystal layer growth stage and a rapid cooling stage. Before the first stage, the laser energy intensity of the PLD is adjusted, and the target surface is struck with the laser until the surface roughness is uniform. Then, the cleaned conductive Nb:STO substrate is transferred to the heating stage using an operating rod. The Nb:STO substrate is heated to a specified temperature using an infrared laser. The substrate temperature can be measured using an infrared thermometer. Before heating, oxygen is introduced into the vacuum chamber to a specific pressure range, and then the PZT ferroelectric layer can begin to grow. The second stage involves rapid cooling to reduce the PZT from the growth temperature to room temperature. Since Pb in PZT has high volatility, rapid cooling reduces Pb loss during the cooling process, thus ensuring the ferroelectricity of the PZT film.

[0034] In this invention, during the single-crystal layer growth stage, the growth status is monitored in situ in real time using a reflective high-energy electron diffractometer (RHEED), thus providing a feedback mechanism for the surface structure of the ferroelectric PZT single-crystal thin film. Based on the diffraction principle, the RHEED, which possesses both surface sensitivity and atomic-level resolution, is used to monitor the surface growth of the ferroelectric PZT single-crystal thin film in situ. By observing the shape and brightness of the spatial imaging pattern (diffraction spot) on the RHEED fluorescent screen, the growth dynamics of the ferroelectric PZT single-crystal thin film can be monitored in real time, thereby guiding the control of growth parameters such as laser energy and frequency, substrate temperature, and oxygen pressure. During the growth process, the surface roughness of the thin film sample undergoes periodic changes. When the unit cell is just fully covered, the surface is smoothest, and the RHEED diffraction spot is brightest. Conversely, when the unit cell is just half-covered, the surface is roughest, and the RHEED diffraction spot is darkest. Therefore, as growth progresses, the intensity of the RHEED diffraction spot oscillates periodically, and the growth thickness of the single-crystal thin film can be determined by these oscillations. Figure 4 Therefore, the thickness of the grown film can be precisely controlled by RHEED.

[0035] To facilitate understanding of the technical solution, further explanations will be provided below with examples, and these examples do not constitute a limitation on the scope of protection of the technical solution.

[0036] Example 1

[0037] First, place the PZT target and the 5mm*5mm Nb:STO substrate into a vacuum chamber, and then introduce oxygen into the vacuum chamber until the oxygen pressure reaches 1×10⁻⁶. -2 -1×10 -1 The laser energy is adjusted to 20-30 mJ and the frequency to 3-5 Hz. The PZT target is ablated with the laser until the generated plasma plume brightness is uniform (target cleaning process). The Nb:STO substrate is placed on the heating platform and heated to 600-650℃ using an infrared laser, with temperature measured using an infrared thermometer. The first stage of PZT growth begins at this point. According to the RHEED diffraction principle, during growth, the sample surface is smoothest when it is just fully covered, resulting in the brightest RHEED diffraction spot. When half the sample surface is covered, it is roughest, resulting in the darkest RHEED diffraction spot. During growth, as the film grows, the sample surface continuously exhibits periodic changes in smoothness, causing periodic oscillations in the RHEED diffraction spot intensity. The PZT thickness is determined based on the number of oscillations. Once the desired thickness is reached (2, 3, 4, 5, 6 layers), the pulsed laser is turned off, and the substrate is rapidly cooled to room temperature at a rate of 50℃ / min. The grown PZT film was characterized using XRD, such as... Figure 5 , Figure 6As shown, only the diffraction peaks corresponding to the PZT(001) crystal orientation are visible, indicating the single-crystal nature of the prepared PZT film. Atomic force microscopy (AFM) scanning of the PZT surface morphology reveals a surface roughness of only 100-200 μm, indicating its smoothness. Piezoelectric microscopy (PFM) testing of the PZT's ferroelectricity shows that amplitude represents the ferroelectric test amplitude, and phase represents the ferroelectric test phase. The phase contrast indicates that the polarization direction of PZT reverses under opposite voltages, thus demonstrating its ferroelectricity. The 6-layer PZT exhibits a very clear phase contrast, indicating strong ferroelectricity. The phase contrast of samples from the 5th to 2nd layers is relatively blurred, indicating weak ferroelectricity in thinner PZT layers, consistent with the trend of PZT ferroelectricity variation. While the phase contrast of the 2nd layer PZT in the phase diagram is weaker than that of the 6th layer, it is still significant, indicating that ferroelectricity still exists in the 2nd layer PZT.

[0038] In summary, this invention provides a method for preparing ultrathin single-crystal ferroelectric PZT thin films. This method utilizes mature pulsed laser deposition equipment and in-situ growth monitoring equipment (RHEED spectra) to monitor the growth of each film layer in situ. Through two processes—single-crystal layer growth and rapid cooling—we ultimately obtained PZT films with atomic-level flatness and ferroelectricity achieved in thicknesses as thin as two unit cells. The above embodiments are merely specific implementations of this invention, but the scope of protection of this invention is not limited thereto. Any variations and substitutions that can be easily conceived by those skilled in the art within the scope of this invention should be included within the scope of protection of this invention. Where there is no conflict, the embodiments and features described in the embodiments of this invention can be combined with each other.

Claims

1. A method for preparing an ultrathin single-crystal ferroelectric PZT thin film, characterized in that, The prepared ultrathin single-crystal ferroelectric PZT film retains ferroelectricity and can achieve ferroelectric polarization reversal even at a thickness as low as 2 unit cells; its preparation method is as follows: A pulsed laser deposition method was used to grow single-crystal ferroelectric PZT thin films on a single-crystal substrate by bombarding a PZT target with a pulsed laser. The deposition process consisted of two stages: a single-crystal layer growth stage and a rapid cooling stage. In the single-crystal layer growth stage, a specified number of PZT ultrathin single-crystal films are continuously grown on a single-crystal substrate by controlling the pulsed laser energy, substrate growth temperature, and oxygen pressure parameters. The growth conditions are as follows: substrate heating temperature between 600-650℃, laser energy between 20-30mJ, laser frequency between 3-5Hz, and oxygen pressure between 1.0×10⁻⁶. -2 mbar-1.2×10 - 1 Between mbars; During the rapid cooling phase, after the PZT ultrathin single crystal film growth is completed, the PZT sample is rapidly cooled from the growth temperature to room temperature by controlling the parameters of the PLD heating device; the cooling rate is 45-55℃ / min.

2. The method for preparing ultrathin single-crystal ferroelectric PZT thin films according to claim 1, characterized in that, During the deposition process, the PZT growth process is monitored in real time by the RHEED pattern of a high-energy electron diffractometer. The intensity of the diffraction spots in the RHEED pattern undergoes periodic oscillations between light and dark, and each period of light and dark oscillation corresponds to the growth process of one unit cell.

3. The method for preparing ultrathin single-crystal ferroelectric PZT thin films according to claim 1, characterized in that, The single-crystal substrate is Nb:STO.

4. The method for preparing ultrathin single-crystal ferroelectric PZT thin films according to claim 1, characterized in that, In the PZT target material, the atomic ratio of Zr to Ti is 0.2:0.

8.

5. The method for preparing ultrathin single-crystal ferroelectric PZT thin films according to claim 1, characterized in that, During the single-crystal growth stage, the grown PZT ultrathin single-crystal film has a cell thickness of 2 to 6 layers.

6. An ultrathin single-crystal ferroelectric PZT thin film prepared by the preparation method according to any one of claims 1-5.

7. The ultrathin single-crystal ferroelectric PZT thin film according to claim 6, characterized in that, The PZT thin film has a cell thickness of 2 to 6 layers, a surface roughness of less than 200 μm, and exhibits ferroelectricity and can achieve ferroelectric polarization reversal.

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