Method for improving polarization retentivity of ferroelectric memory

Abstract

The invention relates to a method for improving the polarization retention of a ferroelectric memory, and the method comprises the steps: carrying out the epitaxial growth of a BiFeO thin film on a beveled substrate, regulating and controlling the stress through changing the thickness of the thin film, and achieving the charge injection in combination with a reverse electric field pulse, thereby effectively prolonging the polarization retention time. The beveled substrate is specifically a crystal with the beveled angle of 2-5 degrees along the direction [100], and the thickness of the film is controlled within the range of 50-300 nm; the pulse parameters of the reverse electric field are as follows: the amplitude is-2 V to-5 V, the pulse width is 0.01 ms to 1 s, the frequency is 1-100 kHz, and a periodic application mode is adopted to suppress polarization attenuation. According to the BiFeO film, epitaxial growth is performed on the SrTiO substrate by preferably adopting a pulsed laser deposition (PLD) process, and an excellent single crystal structure and a stress control effect are obtained. The method is suitable for a high-temperature environment or a flexible substrate ferroelectric memory, the polarization maintaining performance is remarkably improved based on a composite regulation and control mechanism of stress engineering and charge injection, and experiments show that the polarization maintaining time can be prolonged by more than 50%.

Description

TECHNICAL FIELD

[0001] The present application relates to the field of ferroelectric memory material and device field, and particularly relates to a method for improving the polarization retention of a ferroelectric memory. BACKGROUND

[0002] With the continuous development of new generation information storage technology, ferroelectric memory (FeRAM) has attracted widespread attention due to its non-volatility, high write speed, low power consumption and excellent repeatable write performance. Among many ferroelectric materials, bismuth ferrite (BiFeO3) is considered an important candidate material for realizing new multifunctional ferroelectric memories due to its good ferroelectricity and antiferromagnetism, and high Curie temperature at room temperature. However, in practical applications, BiFeO3 thin film devices still face many challenges, especially in the aspect of polarization retention.

[0003] The data storage of ferroelectric memory relies on the stable retention of its ferroelectric polarization state, and the decay of polarization retention performance directly affects the service life and data reliability of the device. Current research shows that the polarization retention of ferroelectric materials is affected by many factors, especially lattice stress, interface charge accumulation, leakage current and spontaneous depolarization field. In the actual device structure, due to the lattice mismatch between the ferroelectric thin film and the substrate, this stress effect will be enhanced with the change of the film thickness, resulting in unstable ferroelectric domain structure, and then promoting the rapid relaxation of the polarization state. In experiments, it is often observed that the polarization state decays significantly within a few hours or even shorter time, which seriously restricts the application of ferroelectric memory in long-term data storage scenarios.

[0004] Current technical paths to improve polarization retention mainly include material doping, interface layer regulation, double-layer structure design and other methods. However, these methods generally have the problems of complex process, poor repeatability, easy introduction of additional defects such as oxygen vacancies or stress concentration, etc. In addition, there is a clear nonlinear relationship between thin film thickness and polarization performance. When the film thickness is reduced to below 100 nm, the stress will reduce the formation energy of oxygen vacancy related defect dipoles, thereby forming a built-in electric field. The combined effect of the built-in electric field and the depolarization field will further exacerbate the polarization decay. Therefore, it is difficult to achieve a fundamental breakthrough in device stability by only regulating the retention through the material itself or structure design.

[0005] In recent years, the composite method based on stress regulation and external electric field driving has become a new direction to improve the ferroelectric performance. Studies have found that the oblique cutting of the substrate can induce the formation of specific single crystal orientation of BiFeO3 thin film, so as to realize the ordered arrangement of domain structure in the stress field, which is beneficial to improve the polarization stability. At the same time, by periodically applying reverse electric field pulse, the depolarization field caused by leakage current and interface charge accumulation can be effectively inhibited, and the polarization decay process is delayed. This combined "stress engineering + charge injection" composite regulation mechanism provides a new idea for further improving the polarization retention of ferroelectric memory.

[0006] However, the key to realizing this composite mechanism lies in the precise control of process parameters. On the one hand, the orientation angle of substrate oblique cutting (such as

[100] direction, 2°-5° inclination) has a significant influence on the epitaxial quality of BiFeO3 thin film and the stress field distribution; on the other hand, when the thickness of the thin film changes in the range of 50-300 nm, it will cause different degrees of stress release and ferroelectric domain modulation. In addition, there is a complex coupling relationship between the amplitude (-2 V to -5 V), pulse width (0.01 ms to 1 s) and application frequency (1-100 kHz) of the reverse pulse electric field and the charge injection behavior, polarization shielding effect and device energy consumption.

[0007] Therefore, the polarization retention of ferroelectric memory is easily affected by lattice mismatch stress and depolarization field, which leads to rapid relaxation of reversed polarization and reduces data storage reliability, which is a technical problem in the current research and development of ferroelectric memory. Especially using traditional methods to improve the retention by doping or interface modification, but the process is complex and may introduce additional defects, when the thickness of the thin film is reduced, the lattice mismatch stress is intensified, which leads to the enhancement of depolarization field; pure stress regulation is difficult to completely offset the shielding effect of interface charge accumulation on polarization, which is still a key technical bottleneck in the current research and development of ferroelectric memory. SUMMARY

[0008] In order to improve the technical problem of improving the polarization retention of ferroelectric memory, the application provides a method for improving the polarization retention of ferroelectric memory.

[0009] The method for improving the polarization retention of ferroelectric memory provided by the application adopts the following technical scheme: A method for improving the polarization retention of ferroelectric memory, including epitaxially growing BiFeO3 thin film on an oblique cutting substrate, and controlling stress by adjusting thickness, and combining reverse electric field pulse to inject charge.

[0010] By adopting the technical scheme, the BiFeO3 thin film is epitaxially grown on the bevelled substrate, and the polarization retention of the ferroelectric memory is significantly improved by combining the thin film thickness regulation and the method of injecting charges by reverse electric field pulses. The bevelled substrate induces an anisotropic stress field, which helps to form an oriented and ordered single-crystal BiFeO3 thin film, thereby enhancing the ferroelectric domain stability. By adjusting the thin film thickness, the internal stress distribution caused by the lattice mismatch can be optimized, and the polarization degradation phenomenon caused by the preferential distribution of defect dipoles in the out-of-plane direction due to stress can be inhibited. At the same time, the reverse electric field pulses are periodically applied to effectively inject interface charges to offset the depolarization field and prevent the polarization state from rapidly decaying due to incomplete interface shielding. The above stress regulation and charge compensation mechanisms work together to enable the ferroelectric memory to maintain a long polarization life under complex environments such as different temperatures and flexible substrates, thereby improving its data retention performance and environmental adaptability, and providing an effective technical path for realizing a high-reliability ferroelectric memory.

[0011] Optionally, the specific direction of the bevelled substrate is

[100] , and the angle range is 2°-5°.

[0012] By adopting the technical scheme, the BiFeO3 thin film is epitaxially grown on the bevelled substrate, and the polarization retention of the ferroelectric memory is significantly improved by combining the thin film thickness regulation and the method of injecting charges by reverse electric field pulses. The bevelled substrate induces an anisotropic stress field, which helps to form an oriented and ordered single-crystal BiFeO3 thin film, thereby enhancing the ferroelectric domain stability. By adjusting the thin film thickness, the internal stress distribution caused by the lattice mismatch can be optimized, and the polarization degradation phenomenon caused by the preferential distribution of defect dipoles in the out-of-plane direction due to stress can be inhibited. At the same time, the reverse electric field pulses are periodically applied to effectively inject interface charges to offset the depolarization field and prevent the polarization state from rapidly decaying due to incomplete interface shielding. The above stress regulation and charge compensation mechanisms work together to enable the ferroelectric memory to maintain a long polarization life under complex environments such as different temperatures and flexible substrates, thereby improving its data retention performance and environmental adaptability, and providing an effective technical path for realizing a high-reliability ferroelectric memory.

[0013] Optionally, the thin film thickness is in the range of 50-300 nm.

[0014] By adopting the technical scheme, the BiFeO3 thin film thickness is controlled in the range of 50-300 nm, which can effectively regulate the internal stress distribution caused by the lattice mismatch while maintaining good crystalline quality. In this thickness range, the thin film can form a stable ferroelectric domain structure, and can also avoid the problems of domain instability or polarization degradation caused by excessive stress. In particular, in the range of 100-200 nm, the stress regulation and polarization retention improvement are most significant.

[0015] Optionally, the amplitude of the reverse electric field pulse parameter is -2 V to -5 V, the pulse width is 0.01 ms-1 s, and the frequency is 1-100 kHz.

[0016] By adopting the technical scheme, the reverse electric field pulse with the amplitude of -2 V to -5 V, the pulse width of 0.01 ms to 1 s and the frequency of 1-100 kHz can be applied to realize effective injection and regulation of the interface charge in the BiFeO3 thin film, so as to compensate for the potential difference caused by the depolarization field, inhibit the natural attenuation of the polarization state, and periodically inject the pulse to inject the charge into the interface between the thin film and the electrode, which not only can shield the depolarization field and the built-in field to enhance the stability of the polarization direction, but also can reduce the influence of the leakage current on the polarization retention, and realize the improvement of the data retention ability of the ferroelectric memory in long-term operation.

[0017] Optionally, the method is applied in a high-temperature environment greater than 100 DEG C or a flexible substrate memory.

[0018] By adopting the technical scheme, the method can be applied in a ferroelectric memory in a high-temperature environment greater than 100 DEG C or a flexible substrate, and can effectively solve the polarization instability problem caused by the temperature rise or the bending of the substrate. The stress regulation mechanism induced by the bevel substrate and the reverse electric field pulse injection strategy cooperate with each other, so that the polarization domain still has good stability under the conditions of thermal disturbance or mechanical strain, thereby significantly improving the working reliability and data retention ability of the device in a harsh environment.

[0019] Optionally, the stress engineering and the charge injection are combined to regulate the mechanism, and the polarization retention time is cooperatively improved.

[0020] By adopting the technical scheme, the stress engineering and the charge injection are combined to regulate the mechanism, and the polarization retention time is cooperatively improved.

[0021] Optionally, the BiFeO3 thin film is epitaxially grown on a bevel SrTiO3 substrate by a pulse laser deposition (PLD) process to obtain a single crystal orientation and a stress regulation effect.

[0022] By adopting the technical scheme, the BiFeO3 thin film is epitaxially grown on a bevel SrTiO3 substrate by a pulse laser deposition (PLD) process to obtain a single crystal orientation and a stress regulation effect.

[0023] Optionally, the reverse electric field pulse is applied periodically, with a pulse width of 10 ms to 1 s, to suppress the polarization decay caused by incomplete polarization shielding.

[0024] By adopting the above technical solution, the reverse electric field pulse is applied periodically, with a pulse width of 10 ms to 1 s, to continuously inject compensation charges without destroying the ferroelectric domain structure, effectively suppress the polarization decay caused by depolarization field, maintain the electric field environment stable through periodic charge adjustment, delay the spontaneous relaxation process of the polarization state, thereby significantly improve the data retention time and operation reliability of the ferroelectric memory, especially suitable for non-volatile memory devices in long-time operation and high-frequency read-write environment.

[0025] Optionally, when the thickness of the thin film is 100 nm to 200 nm, the stress effect and the improvement of polarization stability are most significant.

[0026] By adopting the above technical solution, when the thickness of the BiFeO3 thin film is controlled within the range of 100 nm to 200 nm, an optimal balance between thin film stress and polarization performance can be achieved, which is conducive to the formation of a uniform and continuous crystal structure, enhances the stress regulation effect, reduces the piezoelectric energy and built-in field, and promotes the stability of the reversed polarization, effectively suppressing the polarization relaxation phenomenon caused by excessive or insufficient stress.

[0027] Optionally, the stress engineering is achieved by adjusting the lattice mismatch between the epitaxial thin film and the off-cut substrate, and the charge injection is completed by periodically applying reverse pulses to the electrodes, which synergistically improves the polarization retention time by more than 50%.

[0028] By adopting the above technical solution, the lattice mismatch between the epitaxial BiFeO3 thin film and the off-cut substrate realizes the stable regulation of the ferroelectric domain structure by stress engineering, and combined with the periodic application of reverse pulses to the electrodes for charge injection, the depolarization field caused by the interface charge and the built-in field in the thin film are effectively compensated. The synergistic effect not only enhances the retention ability of the polarization direction, but also significantly prolongs the polarization life. Experiments show that the polarization retention time can be improved by more than 50%, significantly enhancing the data stability and long-term reliability of the ferroelectric memory, meeting the application requirements of high-performance non-volatile memory.

[0029] In summary, the present application includes at least one of the following beneficial technical effects: 1. Simple technical difficulty. By controlling the stress in the thin film or unidirectional charge injection, the polarization retention of the thin film can be controlled, and the combination of the two can greatly improve the retention of the reversed polarization.

[0030] 2. Low cost. By controlling the thickness of the thin film or annealing, the stress can be adjusted without complex doping or interface layer process.

[0031] 3. Multi-scenario universality. Applicable to various device structures (capacitive type, planar type) and material systems (such as BiFeO3, PZT, and other perovskite ferroelectrics), and strong technology scalability.

[0032] 4. Flexible device adaptability. Polarization stability is maintained under bending strain (> 3%) through stress regulation to adapt to flexible substrates (such as PI, PET), providing a high-reliability storage solution for flexible electronics BRIEF DESCRIPTION OF DRAWINGS

[0033] Figure 1 is a BiFeO3 thin film thickness x-ray reflection fitting graph atlas of an embodiment of the present application.

[0034] Figure 2 is a graph atlas of the measurement of the θ-2θ of thin films of different thicknesses and the in-plane and out-of-plane lattice constants of thin films of different thicknesses by an X-ray diffractometer used in an embodiment of the present application.

[0035] Figure 3 is an inverse space diffraction graph atlas of thin films of different thicknesses measured by an X-ray diffractometer used in an embodiment of the present application.

[0036] Figure 4 is a hysteresis loop of thin films of different thicknesses in a thin film tested by a ferroelectric instrument used in a capacitive type device of an embodiment of the present application.

[0037] Figure 5 is a polarization switching time test and a polarization retention time test atlas of capacitive type devices of thin films of different thicknesses of an embodiment of the present application.

[0038] Figure 6 is a schematic diagram of the observation of the switching polarization retention of stress distribution in different directions by a piezoelectric force microscope used in the preparation of a planar structure device of an embodiment of the present application.

[0039] Figure 7 is a schematic diagram of the stress clamping direction and the polarization rapid relaxation of an embodiment of the present application.

[0040] Figure 8 is a schematic diagram of the use of opposite direction charge injection to improve the retention of the switching polarization of a 150 nm thin film that originally does not retain the switching polarization of an embodiment of the present application. DETAILED DESCRIPTION

[0041] The present application will be further described in detail below with reference to the accompanying drawings.

[0042] The embodiment of the present application discloses a method for improving the polarization retention of a ferroelectric memory. Referring to Figure 1 , The application aims to solve the problem of rapid relaxation of polarization caused by lattice mismatch stress and depolarization field in ferroelectric memory, and provides a composite optimization method for synergistic regulation by stress engineering and charge injection. By precisely controlling the thickness of BiFeO3 thin film to reduce the negative impact of built-in field generated by oxygen vacancy related defect dipole on polarization, and introducing a reverse electric field pulse to induce charge injection and form a reverse shielding electric field to offset the depolarization field, the polarization retention time is significantly prolonged (e.g. from several hours to hundreds of hours). This method has process compatibility and high reliability, and can be applied to high temperature, flexible and high density storage scenarios, providing core technical support for the next generation of non-volatile memory.

[0043] Reference Figure 1 The thickness of the film grown per minute was calibrated using an X-ray diffractometer to achieve precise control of the thickness. During the growth of the film, the control was 2 nm / min.

[0044] Reference Figure 2 , Figure 3 Different thicknesses of BFO thin films (50 nm, 100 nm, 150 nm, 200 nm, 300 nm) were deposited on (001) direction STO substrates with 2° tilt in

[100] direction using a pulsed laser deposition system. The reciprocal space diffraction pattern and the θ-2θ spectrum of the thin films of different thicknesses were measured using an X-ray diffractometer to obtain the lattice constant of the thin film, and to determine the crystal quality of the thin film and the stress distribution in the thin film.

[0045] Reference Figure 4 The hysteresis loop of the thin film with different thicknesses in the thin film was tested using a ferroelectric instrument, and the retention of the reversed polarization was determined by the opening size of the hysteresis loop. The thicker the thin film, the smaller the opening of the hysteresis loop, and the stronger the retention. Therefore, the retention of the reversed polarization in the thin film can be regulated by manipulating the stress in the thin film.

[0046] Reference Figure 5 The polarization reversal of the capacitive device structure was monitored by a -4.5V voltage, and it was found that the polarization of all thicknesses of the thin film was reversed within 0.05ms. After using a -4.5V / 0.05ms pulse to reverse all the polarization, the relaxation of the reversed polarization was monitored, and it was found that the reversed polarization was maintained in the stress-relaxed thin film.

[0047] Reference Figure 6 A planar structure device was prepared, and a piezoelectric force microscope was used to observe the retention of the reversed polarization in different stress distribution directions. In the stress-relaxed direction, the reversed polarization can still be maintained after 50h.

[0048] Reference Figure 7, the figure shows the stress clamping direction of the fast relaxation of the polarization, further reveals the influence mechanism of the stress direction on the polarization retention: in the stress clamping area, the ferroelectric performance and the influence of the depolarization field at the domain wall lead to the instability of the polarization state, so that the fast relaxation occurs; while in the stress relaxation direction, the defects including dislocations release the ferroelectric energy, and the reversed polarization can be stably maintained.

[0049] Referring to Figure 8 For the 150nm thin film which originally does not maintain the reversed polarization, the use of charge injection in the opposite direction can improve the retention of the reversed polarization. The injection voltage is-4.5v, the injection time is 0.01ms-1s, and the frequency is 1-100 kHz. The longer the injection time, the better the retention. The figure shows the positive effect of reverse charge injection on polarization retention. By periodically applying-4.5V reverse pulses, the frequency is controlled between 1-100 kHz, and the pulse time varies from 0.01 ms to 1 s, verifying the trend that the longer the charge injection time, the stronger the polarization retention. This shows that charge injection compensates for the depolarization field, significantly enhancing the overall structure of the domain stability.

[0050] The implementation principle of a method for improving the polarization retention of a ferroelectric memory in an embodiment of the present application is as follows: based on the coordinated regulation mechanism of stress engineering and reverse charge injection, the present application comprehensively regulates the material properties from multiple dimensions such as crystal growth process, structural characterization, and electrical performance testing to prolong the retention time of the ferroelectric polarization state and improve the long-term data stability and practical performance of the ferroelectric memory; firstly, targeting the root causes of the limited polarization retention in the ferroelectric memory - lattice mismatch stress and depolarization field effect, starting from substrate design and film epitaxial growth, a technology of beveling 2° along the

[100] direction on a SrTiO3 (STO) substrate in the (001) direction is adopted to precisely regulate the growth direction and stress distribution of the BiFeO3 (BFO) film. The beveled substrate can effectively guide the preferred orientation of the BFO film, improve the epitaxial quality, and regulate the distribution of defect dipole pairs related to oxygen defects in the film through the stress induced by lattice mismatch, thereby enhancing the stability of the polarization; in the film deposition process, a pulsed laser deposition (PLD) system is used to deposit 50 nm, 100 nm, and 150 nm at a rate of 2 nm / min, respectively. BFO films with thicknesses of 100 nm, 200 nm, and 300 nm were prepared. The deposition rate and film thickness were calibrated in real time by X-ray diffractometer (XRD) to achieve precise control of the growth thickness. Films of different thicknesses have different stress states, which affect their lattice constants and polarization behaviors. The θ-2θ spectra and reciprocal space diffraction patterns obtained by XRD tests reveal the accuracy of the film crystal orientation and the distribution law of stress. The hysteresis loops of films of different thicknesses were obtained by ferroelectric tester. It was found that with the increase of film thickness, the opening of the hysteresis loop gradually decreased, indicating that thicker films have stronger polarization retention ability. This is because as the film thickness increases, the stress caused by lattice mismatch is relatively weakened, the defect dipole pairs related to oxygen defects are no longer preferentially distributed, the built-in field is reduced, and the stability of the flipped polarization state is enhanced. Especially between 100 nm and 200 nm, the improvement of polarization retention ability is particularly significant, indicating that the thickness within this range can achieve the best match between stress and polarization retention. In addition, this technical solution also introduces reverse electric field pulse technology. By applying -4.5V and a pulse width of 0.05 A 10-ms voltage pulse causes all polarization regions to flip within a short period of time. This pulse induces charge injection, forming a reverse shielding electric field to offset the internal depolarization field and prevent rapid relaxation of the polarization state. Experimental results show that after reverse charge injection, even films with poor retention, such as 150 nm thick samples, can achieve a significantly prolonged polarization state, even maintaining it for more than 50 hours. Further, piezoelectric force microscopy (PFM) is used to image the polarization regions under different stress states, verifying the influence of stress distribution direction on polarization retention. In the stress relaxation direction, dislocations and other defects release the ferroelectric energy, and the flipped domain structure is more stable, and remains clear and distinguishable after 50 hours. In the stress clamping direction, the polarization rapidly degrades, showing a significant difference in polarization retention.

[0051] The technical route provided by the embodiment has clear physical mechanism support. Stress is guided and controlled by beveling the substrate, the internal built-in field of the thin film is adjusted by adjusting the thickness of the thin film, charge injection and depolarization field compensation are realized by reverse electric field pulses, and finally the synergistic improvement of polarization retention is realized. The method has the advantages of simple process, strong adaptability, good compatibility and the like, is suitable for advanced application scenarios such as high-temperature environment, flexible electronics, ultra-thin structure, and is an important technical scheme for the next generation of non-volatile ferroelectric memories The above are preferred embodiments of the present application, and are not intended to limit the protection scope of the present application, therefore: any equivalent changes made according to the structure, shape, principle of the present application should be covered within the protection scope of the present application.

Claims

1. A method for improving polarization retention of a ferroelectric memory, characterized by: The method includes epitaxially growing a BiFeO3 film on a beveled substrate, controlling stress by adjusting the thickness, and injecting charges in combination with a reverse electric field pulse.

2. The method for improving polarization retention of a ferroelectric memory according to claim 1, wherein: The specific direction of substrate beveling is [100] and the angle range is 2°-5°.

3. The method for improving polarization retention of a ferroelectric memory according to claim 1, wherein: The film thickness ranges from 50 to 300 nm.

4. The method for improving polarization retention of a ferroelectric memory according to claim 1, wherein: The reverse electric field pulse parameters have an amplitude of -2 V to -5 V, a pulse width of 0.01 ms to 1 s, and a frequency of 1 to 100 kHz.

5. The method for improving polarization retention of a ferroelectric memory according to claim 1, wherein: This method is applicable to high-temperature (>100°C) or flexible substrate memories.

6. The method for improving polarization retention of a ferroelectric memory according to claim 1, wherein: The composite regulation mechanism of stress engineering and charge injection, and its synergistic improvement of polarization retention time.

7. The method for improving polarization retention of a ferroelectric memory according to claim 1, wherein: The BiFeO3 film is epitaxially grown on an oblique-cut SrTiO3 substrate by a pulsed laser deposition (PLD) process to obtain single crystal orientation and stress regulation effects.

8. The method for improving polarization retention of a ferroelectric memory according to claim 4, wherein: The reverse electric field pulse is applied periodically with a pulse width of 10ms to 1s to suppress polarization attenuation caused by the depolarization field.

9. The method for improving polarization retention of a ferroelectric memory according to claim 3, wherein: When the film thickness is 100 nm to 200 nm, the stress effect and polarization stability are most significantly improved.

10. The method for improving polarization retention of a ferroelectric memory according to claim 6, characterized in that: The stress engineering is achieved by adjusting the lattice mismatch between the epitaxial film and the beveled substrate, and the charge injection is completed by periodically applying reverse pulses to the electrodes. The synergistic effect of the two increases the polarization retention time by more than 50%.

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