Ultrathin monocrystal ferroelectric PZT film and preparation method thereof

Abstract

The invention belongs to the technical field of condensed physics and material science, and particularly relates to an ultrathin single crystal ferroelectric PZT film and a preparation method thereof. The method comprises the following steps of: bombarding a PZT target material by using pulse laser to grow a single crystal ferroelectric PZT film on a single crystal substrate by using a pulse laser deposition PLD method; the deposition process comprises 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 PLD growth. According to the method, the growth process is monitored in real time by using a high-energy electron diffractometer RHEED, the quality and thickness of a thin film crystal during growth are ensured, and the atomic-scale flatness of the surface of a prepared sample is proved by observing the surface morphology of the prepared sample through an atomic force microscope; the PZT thin film grown on the single crystal substrate still keeps ferroelectricity when only two layers of crystal lattices are thinnest, and ferroelectric polarization overturning can be achieved.

Description

Technical Field

[0001] The invention relates to an ultra-thin single crystal ferroelectric PZT film and a preparation method thereof, belonging to the technical field of condensed matter physics and material science. Background Art

[0002] Ferroelectric materials are polar materials that have residual polarization in the absence of an electric field and whose polarization direction can be reversed by an external electric field. Due to the characteristic of ferroelectric polarization reversal, ferroelectric materials can be used in many fields such as non-volatile memory, field effect transistors, and logic devices. Among ferroelectric materials, lead zirconate titanate (PZT) has a strong polarization strength (~100μm / cm 2 ) stands out among many ferroelectric materials. With the rapid development of science and technology, the requirements for device size are constantly shrinking, which requires ferroelectric materials to reduce their thickness while still retaining their ferroelectric properties.

[0003] However, the size effect found in many ferroelectric materials means that as the thickness of the ferroelectric material decreases, the ferroelectric properties gradually weaken. When the thickness decreases 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 film preparation technology, the quality of films has also been improved, and the critical size predicted by experiments and theories of various ferroelectric materials has also been continuously reduced. Some articles predict that the critical thickness of ferroelectric materials with perovskite structure is three unit cell layers thick. Taking the ferroelectric material PZT with strong polarization strength as an example, the spatial structure is tetragonal phase, and its lattice constant is related to the ratio of Zr atoms to Ti atoms in PZT. When the ratio of Zr to Ti is 2:8, the lattice constant of PZT is a=b=0.395nm, c=0.414nm, that is, the thickness of each atomic layer is about 0.4nm. An article observed the existence of polarized atomic arrangements in 1.5-layer PZT films, but this finding did not prove the reversibility of its polarization strength. Summary of the invention

[0004] In view of the shortcomings of the prior art, the present invention provides a method for preparing an ultra-thin single-crystal ferroelectric PZT film. According to the crystal nucleation growth dynamics principle, the present invention adopts a pulsed laser deposition PLD method, and realizes the growth of a ferroelectric single-crystal PZT film on a single-crystal substrate by controlling the laser energy during PLD growth, the substrate deposition growth temperature and the oxygen pressure. The method is simple and easy to operate. The present invention uses a high-energy electron diffractometer RHEED to monitor the growth process in real time, ensuring the quality and thickness of the thin film crystal during growth. The surface morphology of the prepared sample is observed by an atomic force microscope, confirming the atomic-level flatness of its surface. The PZT film obtained by the present invention still maintains ferroelectricity when the thinnest layer is only 2 unit cell thicknesses (about 0.83nm), and the ferroelectric polarization reversal can be achieved through an electric field.

[0005] In the present invention, the "ultra-thin" in the ultra-thin single-crystal ferroelectric PZT film refers to a thickness of less than 10 unit cells. The present invention emphasizes "ultra-thin" to illustrate the maximum thickness of PZT at which ferroelectricity can be maintained and reversible ferroelectric polarization can be achieved.

[0006] The technical solution of the present invention is specifically described as follows.

[0007] The present invention provides a method for preparing an ultra-thin single-crystal ferroelectric PZT film. The prepared ultra-thin single-crystal ferroelectric PZT film still has ferroelectricity and can achieve ferroelectric polarization reversal when the thickness is as low as 2 layers of unit cells. The preparation method is as follows: The pulsed laser deposition method is used to grow a single crystal ferroelectric PZT thin film on a single crystal substrate by bombarding a PZT target with a pulsed laser. The deposition process includes two stages: a single crystal layer growth stage and a rapid cooling stage. Among them: In the single crystal layer growth stage, the PZT ultra-thin single crystal film with a specified number of layers is continuously grown on the single crystal substrate by controlling the pulse laser energy and frequency, substrate temperature and oxygen pressure. The growth condition parameters are: substrate heating temperature in the range of 600-650℃, laser energy in the range of 20-30mJ, laser frequency in the range of 3-5Hz, oxygen pressure in the range of 1.0×10 -2 mbar-1.2×10 -1mbar; Regarding growth parameters, pulse laser energy and frequency, substrate temperature and oxygen pressure and other parameters are coupled to each other and jointly determine the quality and properties of the grown film; First, if the substrate temperature is too high, Pb in PZT will volatilize and form pyrochlore phase PZT, which does not have ferroelectricity; if the temperature is too low, it will affect the crystallinity of the material and cannot form a perovskite structure, resulting in PZT not having ferroelectricity. Secondly, due to the volatility of Pb, in order to reduce the volatilization of Pb and form a chemically matched PZT single crystal film, high oxygen pressure is used during the growth process. However, if the oxygen pressure is too high, the plasma will be scattered too strongly by oxygen atoms, the kinetic energy will decrease, and a flat film cannot be formed on the substrate surface. Finally, laser energy is a very critical factor in achieving flat growth of oxides. If the energy is too low, the plasma kinetic energy will be insufficient, affecting the crystallinity and surface flatness; if the energy is too high, the deposition will be too fast and other structural phases will be easily formed. Similarly, if the laser frequency is too low, the lack of Pb elements will increase, and if the frequency is too high, it will exceed the self-propagation speed of the target platform, which will cause the segregation of the target material composition.

[0008] In the rapid cooling stage, after the growth of the PZT ultra-thin single crystal film is completed, the PZT sample is quickly cooled from the growth temperature to room temperature by controlling the parameters of the PLD heating device; due to the volatility of the Pb element in PZT, the longer the high temperature is kept after the growth, the more Pb evaporates, causing PZT to easily transform into the paraelectric pyrochlore phase. Therefore, the temperature is cooled at a faster speed after the growth is completed, and the cooling rate is 45-55℃ / min.

[0009] In the present invention, during the deposition process, the PZT growth process is monitored in real time by a high energy electron diffractometer RHEED spectrum to obtain information parameters of the sample growth process in real time, provide an in-situ growth feedback monitoring mechanism, and confirm the thickness of the growing film at the same time. During the single crystal layer growth stage, the diffraction spot intensity of the RHEED spectrum undergoes a light and dark periodic oscillation, and each light and dark oscillation period corresponds to the growth process of a layer of unit cells.

[0010] In the present invention, the single crystal substrate is a Nb:STO substrate. NbSTO is a conductive substrate, which is convenient for PFM testing the ferroelectricity of PZT. In addition, the NbSTO substrate structure is similar to that of PZT, and the lattice constant is slightly smaller than that of PZT, which can provide a certain compressive stress, which is conducive to obtaining ferroelectricity with out-of-plane polarization.

[0011] In the present invention, antiferroelectricity will appear when the proportion of Ti in the PZT target is less than 5%, so the proportion of Ti should be greater than 5%; in a specific embodiment, the atomic number ratio of Zr and Ti is 0.2:0.8.

[0012] In the present invention, during the single crystal layer growth stage, the grown PZT ultra-thin single crystal film has a unit cell thickness of 2 to 6 layers.

[0013] The present invention also provides an ultra-thin single crystal ferroelectric PZT film prepared by the above preparation method.

[0014] In the present invention, the PZT film has a unit cell thickness of 2 to 6 layers, the surface roughness of the PZT film is less than 200 pm, and the PZT film has ferroelectricity and can realize ferroelectric polarization reversal.

[0015] As described above, the present invention grows an ultra-thin PZT single crystal on a single crystal conductive substrate Nb:STO, and then rapidly cools down the substrate to obtain a single crystal PZT that still has ferroelectricity when the thickness is as thin as two unit cells. Compared with the prior art, the present invention has the following beneficial effects: The growth method of the present invention uses a high-energy electron diffractometer RHEED for real-time in-situ monitoring. RHEED can provide sample reciprocal space diffraction information, thereby feeding back the film condition, which is beneficial for us to control the growth conditions such as laser energy, substrate growth temperature, oxygen pressure, etc. in real time. The high quality of the sample is reflected in two aspects: the RHEED diffraction pattern is clear and sharp, the surface morphology flatness of the sample scanned by atomic force microscope is less than 200pm, and the PZT film still has ferroelectric signals when it is as thin as two layers of unit cells using piezoelectric force microscopy.

[0016] The growth method of the present invention is simple and can achieve the growth of an ultra-thin ferroelectric material film on a conductive substrate Nb:STO. Obvious ferroelectric signals can still be observed in samples as thin as 2 layers and ferroelectric polarization reversal can be achieved, which further reduces the thickness limit of the ferroelectricity of the PZT film. BRIEF DESCRIPTION OF THE DRAWINGS

[0017] When considered in conjunction with the accompanying drawings, the present invention can be more completely and better understood and many of the accompanying advantages can be easily known by referring to the detailed description below, but the drawings described herein are used to provide a further understanding of the present invention and constitute a part of the present invention. The exemplary embodiments of the present invention and their descriptions are used to explain the present invention and do not constitute an improper limitation of the present invention, as shown in the figure: Figure 1 It is the basic composition structure of the film material of the present invention.

[0018] Figure 2 It is a diagram describing the equipment for growing ultra-thin single crystal ferroelectric PZT thin film samples.

[0019] Figure 3 The present invention provides a thin film sample growth process; the main growth process of the ultra-thin single crystal ferroelectric PZT film includes: the first stage is the growth of the single crystal layer, which is the growth process of the PZT film; the second stage is the rapid cooling process, which cools the sample from the growth temperature to the room temperature at a relatively fast speed.

[0020] Figure 4This is the RHEED data graph of PZT films with different thicknesses (2 layers, 3 layers, 4 layers, 5 layers, and 6 layers of unit cell thickness). The curve is the RHEED intensity oscillation. The number of layers grown in the film can be determined by the number of oscillations. The illustration on the right side of each curve is the corresponding RHEED diffraction pattern.

[0021] Figure 5 This is the XRD data diagram of the ultra-thin single-crystal ferroelectric PZT film sample, which shows that the prepared PZT film has good single crystal lines.

[0022] Figure 6 These are the surface morphology images of PZT films with different thicknesses (2-layer, 3-layer, 4-layer, 5-layer, and 6-layer cell thickness) scanned by AFM and the ferroelectric signals scanned by PFM; the first row is the surface morphology image scanned by AFM, the second row is the piezoelectric amplitude data scanned by FFM, and the third row is the piezoelectric phase data scanned by PFM. DETAILED DESCRIPTION

[0023] The present invention is further described below in conjunction with the accompanying drawings and embodiments.

[0024] Obviously, many modifications and changes made by those skilled in the art based on the purpose of the present invention belong to the protection scope of the present invention. For those skilled in the art, the specific meanings of the above terms in the present invention can be understood in more specific situations.

[0025] It should be understood by those skilled in the art that unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by those skilled in the art.

[0026] The present invention provides a method for preparing an ultra-thin single crystal ferroelectric PZT thin film. The sample generated by the method includes two important components, such as Figure 1 The bottom layer is a conductive single crystal substrate with a specific crystal orientation, and a Nb:STO substrate is adopted; the top layer is a PZT single crystal ferroelectric layer.

[0027] The present invention adopts a pulsed laser deposition PLD system for sample growth, which accordingly includes five subsystems (such as Figure 2): The first subsystem is an ultra-high vacuum chamber, in which the substrate and target are placed. The ultra-high vacuum chamber provides a stable and fast vacuum environment for the nucleation and growth of single crystal thin films. The second subsystem is an infrared heating system, which is used to heat the substrate to a specific temperature to ensure the crystallinity of the film. The third subsystem is a pulsed laser system. The pulsed laser is focused on the target in the vacuum chamber through a total reflection mirror and a focusing lens, thereby sputtering out plasma to form a plume, which is deposited on a single crystal substrate with a specific orientation to form a thin film. The fourth subsystem is a gas circulation system. During the growth process, we introduce a certain flow of oxygen into the vacuum chamber so that the chamber is stabilized at a specific oxygen pressure. The energy and frequency of the pulsed laser, the temperature of the substrate, and the oxygen pressure in the vacuum chamber are all key factors affecting the quality and properties of the film. The fifth subsystem is the real-time diffraction pattern feedback system of the in-situ monitoring system RHEED. According to the diffraction principle, the RHEED diffraction pattern can feedback the real-time situation of the surface structure of the PZT single crystal film, so that the growth conditions can be accurately controlled in a timely manner. In addition, the change in the intensity of the RHEED diffraction pattern can also reflect the thickness of the film growth, so as to accurately grow a film of the required thickness.

[0028] In the present invention, the single crystal ferroelectric PZT thin film growth process is mainly divided into two stages (such as Figure 3 ): single crystal layer growth stage, rapid cooling stage. Before the first stage, first adjust the laser energy intensity of PLD, and use the laser to hit the target surface until the surface roughness is uniform; then transfer the cleaned conductive Nb:STO substrate to the heating table with an operating rod, and use an infrared laser to heat the Nb:STO substrate to a specified temperature. The substrate temperature can be measured by an infrared thermometer. Before heating, introduce oxygen into the vacuum chamber to a specific pressure range, and then the PZT ferroelectric layer can begin to grow. In the second stage, PZT is cooled from the growth temperature to room temperature by rapid cooling. Since Pb in PZT is highly volatile, the rapid cooling process can reduce the loss of Pb during the cooling process, thereby ensuring the ferroelectricity of the PZT film.

[0029] In the present invention, during the growth stage of the single crystal layer, a reflection high energy electron diffractometer (RHEED) is used to monitor the growth status in real time in situ, thereby providing a feedback mechanism for the surface structure of the ferroelectric PZT single crystal film. According to the diffraction principle, the RHEED with both surface sensitivity and atomic-level resolution is used to monitor the surface growth status of the ferroelectric PZT single crystal film in situ. By observing the shape and brightness of the spatial imaging pattern (diffraction spot) of the RHEED fluorescent screen, the growth dynamics of the ferroelectric PZT single crystal film can be grasped in real time, thereby guiding the regulation of growth parameters such as laser energy and frequency, substrate temperature, and oxygen pressure. During the growth process, as the thin film sample grows, the surface roughness of the sample will change periodically. When the unit cell is just covered with one layer, the surface is the flattest, and the RHEED diffraction spot is the brightest. When the unit cell is just covered with half a layer, the surface is the roughest and the RHEED diffraction spot is the darkest. Therefore, as the growth proceeds, the intensity of the RHEED diffraction spot will oscillate periodically, so that the growth thickness of the single crystal film can be judged by the oscillation, such as Figure 4 Therefore, the thickness of the grown film can be precisely controlled by RHEED.

[0030] In order to facilitate the understanding of the technical solution, further explanation will be given below in conjunction with examples, and the examples do not constitute a limitation on the protection scope of the technical solution.

[0031] Example 1

[0032] First, the PZT () target and the 5 mm*5 mm Nb:STO substrate were placed in a vacuum chamber, and oxygen was introduced into the vacuum chamber until the oxygen pressure reached 1×10 -2 -1×10 -1 mbar, adjust the laser energy to 20-30mJ, the frequency to 3-5Hz, and use the laser to hit the PZT target until the brightness of the plasma plume is uniform (target cleaning process); put the Nb:STO substrate into the heating platform, use the infrared laser to heat the substrate to 600-650℃, and use the infrared thermometer to measure the temperature; at this time, the first stage of PZT growth begins. According to the RHEED diffraction principle, during the growth process, when the sample surface is just covered, the sample surface is the flattest, and the corresponding RHEED diffraction spot is the brightest. When the sample surface is half covered, the sample surface is the roughest, and the corresponding RHEED diffraction spot is the darkest. During the growth process, as the film grows, the sample surface will continue to show flat and smooth periodic changes, so the RHEED diffraction spot intensity will also oscillate periodically. Determine the PZT thickness according to the number of oscillations. After reaching the required thickness (2 layers, 3 layers, 4 layers, 5 layers, 6 layers), turn off the pulsed laser and cool down quickly to room temperature at a rate of 50℃ / min. The grown PZT film was characterized by XRD, such as Figure 5 , Figure 6As shown, it can be seen that only the diffraction peak corresponding to the PZT (001) crystal direction is present, indicating the single crystallinity of the prepared PZT film; the surface morphology of the PZT is scanned using an atomic force microscope (AFM), and it can be seen that its surface roughness is only 100-200pm, indicating its surface flatness; the ferroelectricity of the PZT is tested using a piezoelectric force microscope (PFM), where Amplitude represents the ferroelectric test amplitude, and Phase represents the phase of the ferroelectric test. The contrast of Phase indicates that the polarization direction of the PZT is reversed under the opposite voltage, thus indicating the existence of its ferroelectricity. The phase contrast of the 6-layer PZT is very obvious, indicating that it has strong ferroelectricity, and the phase contrast of the 5-2 layer sample is relatively fuzzy, indicating that the thin layer of PZT has weak ferroelectricity, which is consistent with the change trend of the PZT ferroelectricity. Although the phase contrast of the 2-layer PZT in the Phase diagram is weaker than that of the 6-layer, it is still very obvious, indicating that ferroelectricity still exists in PZT as thin as 2 layers.

[0033] In summary, the present invention provides a method for preparing an ultra-thin single-crystal ferroelectric PZT film. With the help of mature pulsed laser deposition equipment and in-situ growth detection equipment RHEED spectrum, the method can monitor the growth of each layer of film in-situ. Through the two processes of single crystal layer growth and rapid cooling, we finally obtain a PZT film with atomic-level flatness and ferroelectricity in a thickness as thin as two unit cells. The above embodiments are only specific implementation methods of the present invention, but the protection scope of the present invention is not limited thereto. Any technician familiar with any field can easily think of changes and substitutions within the technical scope of the present invention, which should be covered within the protection scope of the present invention; the embodiments of the present invention and the features in the embodiments can be combined with each other without conflict.

Claims

1. A method for preparing an ultra-thin single-crystal ferroelectric PZT thin film, characterized in that: The prepared ultra-thin single crystal ferroelectric PZT film still has ferroelectricity and can achieve ferroelectric polarization reversal when the thickness is as low as 2 unit cells; the preparation method is as follows: The pulsed laser deposition method is used to grow a single crystal ferroelectric PZT thin film on a single crystal substrate by bombarding the PZT target with a pulsed laser. The deposition process includes two stages: a single crystal layer growth stage and a rapid cooling stage. Among them: In the single crystal layer growth stage, the PZT ultra-thin single crystal film with a specified number of layers is continuously grown on the single crystal substrate by controlling the pulse laser energy, substrate growth temperature and oxygen pressure condition parameters; the growth condition parameters are: substrate heating temperature in the range of 600-650℃, laser energy in the range of 20-30mJ, laser frequency in the range of 3-5Hz, oxygen pressure in the range of 1.0×10 -2 mbar-1.2×10 - 1 mbar; In the rapid cooling stage, after the growth of the PZT ultra-thin single crystal film is completed, the PZT sample is quickly 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 the ultra-thin single crystal ferroelectric PZT thin film according to claim 1, characterized in that: During the deposition process, the PZT growth process is monitored in real time by the high-energy electron diffractometer RHEED spectrum. The intensity of the diffraction spot in the RHEED spectrum oscillates periodically, and each bright and dark oscillation cycle corresponds to the growth process of a layer of unit cells.

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

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

8.

5. The method for preparing an ultra-thin single crystal ferroelectric PZT thin film according to claim 1, characterized in that: In the single crystal layer growth stage, the grown PZT ultra-thin single crystal film has a unit cell thickness of 2 to 6 layers.

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

7. The ultra-thin single crystal ferroelectric PZT film according to claim 6, characterized in that: The PZT film has a unit cell thickness of 2 to 6 layers, and the surface roughness of the PZT film is less than 200 pm. The PZT film has ferroelectricity and can realize ferroelectric polarization reversal.

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