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Article

High-Performance Ferroelectric Capacitors Based on Pt/BaTiO3/SrRuO3/SrTiO3 Heterostructures for Nonvolatile Memory Applications

by
Zengyuan Fang
,
Yiming Peng
,
Haiou Li
,
Xingpeng Liu
* and
Jianghui Zhai
*
Guangxi Key Laboratory of Precision Navigation Technology and Application, School of Information and Communication, Guilin University of Electronic Technology, Guilin 541004, China
*
Authors to whom correspondence should be addressed.
Crystals 2025, 15(4), 337; https://doi.org/10.3390/cryst15040337
Submission received: 11 March 2025 / Revised: 31 March 2025 / Accepted: 31 March 2025 / Published: 2 April 2025
(This article belongs to the Special Issue Recent Research on Electronic Materials and Packaging Technology)
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Abstract

:
BaTiO3 (BTO), a lead-free chalcogenide ferroelectric material, has emerged as a promising candidate for ferroelectric memories due to its advantageous ferroelectric properties, notable flexibility, and mechanical stability, along with a high dielectric constant and minimal leakage. These attributes lay a crucial foundation for multi-value storage. In this study, high-quality BaTiO3 ferroelectric thin films have been successfully prepared on STO substrates by pulsed laser deposition (PLD), and Pt/BaTiO3/SrRuO3/SrTiO3 ferroelectric heterojunctions were finally prepared by a combination of UV lithography and magnetron sputtering. Characterization and performance tests were carried out by AFM, XRD, and a semiconductor analyzer. The results demonstrate that the ferroelectric heterojunction prepared in this study exhibits excellent ferroelectric properties. Furthermore, the device demonstrates fatigue-free operation after 107 bipolar switching cycle tests, and the polarization value exhibits no significant decrease in the holding characteristic test at 104 s, thereby further substantiating its exceptional reliability and durability. These findings underscore the considerable promise of BTO ferroelectric memories for nonvolatile storage applications and lay the foundation for the development in the fields of both in-memory computing systems and neuromorphic computing.

Graphical Abstract

1. Introduction

In the context of the information explosion and the advent of the big data era, traditional memory devices and computer systems are facing challenges in terms of energy efficiency, speed, and integration [1,2,3]. Although traditional memories such as dynamic random access memory (DRAM) and static random access memory (SRAM) have high-speed read and write characteristics, their volatility leads to data loss after power failure, making it difficult to meet the needs of low-power scenarios such as the Internet of Things (IoT) and edge computing [4,5,6]. Conversely, non-volatile memories encounter limitations due to high write voltage, low endurance, and long latency, which restricts their application in real-time data processing. Consequently, there is an urgent need to research and develop new types of memories that can combine non-volatile, high-speed, low-power consumption and high endurance [7,8,9]. Ferroelectric memory (FeRAM) is based on the spontaneous polarization properties of ferroelectric materials, and data storage is achieved by controlling the orientation of the ferroelectric domains through an electric field. The bistable polarization state of the core material can be maintained for a long time after a power failure, giving FeRAM non-volatile characteristics [10]. Compared with conventional memory, FeRAM has the advantages of ultra-high read/write speed (nanosecond operation speed), low power consumption (polarization reversal energy consumption is only 1/1000 of flash memory), and high durability (>1012 erase/write cycles, which is far more than flash memory) [11,12,13]. These attributes position FeRAM as a frontrunner for future memory architectures [14,15].
In recent years, research on BaTiO3 ferroelectric memory has made important breakthroughs in optimizing the material system and improving device performance [16]. As a new type of lead-free ferroelectric material, BaTiO3 thin films exhibit properties such as high dielectric constant, excellent residual polarization strength, and good thermal stability, which have attracted much attention in the field of non-volatile memory [17,18]. The researchers have optimized the ferroelectric domain structure (domain size < 50 nm) and improved the fatigue resistance (>1010 cycles) of the films by using advanced preparation techniques such as magnetron sputtering and the sol-gel method, combined with interfacial engineering and lattice tuning strategies, thus laying the material foundation for the development of high-density and low-power memory devices [19,20,21].
The development history of BaTiO3 ferroelectric memory materials shows a clear technology iteration. Pioneering studies in the mid-20th century systematically investigated the intrinsic ferroelectric properties of bulk BaTiO3 materials, with particular emphasis on elucidating the Curie phase transition mechanism. These foundational investigations revealed a characteristic tetragonal-to-cubic structural transition occurring at a critical temperature (Tc) of approximately 120 °C, thereby establishing the theoretical framework for subsequent technological applications in ferroelectric devices [22,23]. The increasing demand for microelectronic devices to be miniaturized has led to significant advancements in advanced thin-film fabrication technologies. Key deposition techniques, such as pulsed laser deposition (PLD) and magnetron sputtering, have enabled the creation of epitaxial BaTiO3 thin films with a thickness of less than 500 nm [24,25]. These films demonstrate exceptional structural characteristics, including an in-plane lattice constant of a = 3.992 Å and an epitaxial orientation error of less than 0.5° [26]. This technological advancement is pivotal in facilitating a paradigm shift from bulk ferroelectrics to functional thin-film architectures, particularly driving transformative developments in next-generation ferroelectric memory devices. To address the critical performance limitations, Amey M et al. [27] implemented a strategic doping protocol using rare-earth elements (La/Ce) at controlled concentrations ranging from 0.5% to 2.0% to precisely modulate oxygen vacancy populations. This defect engineering approach led to the attainment of the following breakthrough performance metrics: a remarkable resistive switching ratio (>103) and unprecedented endurance exceeding 108 switching cycles in the ferroelectric diodes, representing a three-order-of-magnitude improvement in operational stability compared to conventional counterparts. The intersection of ferroelectric materials and silicon photonics has given rise to novel advancements in hybrid electronic–photonic systems. Pioneering contributions by Yaacoub, E et al. [28] laid the groundwork for technological progress by integrating BaTiO3-based electro-optic modulators on silicon substrates. Their pioneering heterostructure design achieved exceptional performance metrics, including sub-1 dB optical insertion loss and a record-low Vπ·L product below 2 V·cm, demonstrating the viability of CMOS-compatible ferroelectric–photonic integration for next-generation optoelectronic applications. Complementing these integration breakthroughs, recent advances in compositional engineering have significantly enhanced BaTiO3’s intrinsic properties. Plutenko et al. [29] developed innovative (1 − x)BaTiO3 − xLi0.5Bi0.5TiO3 solid solutions via lithium-bismuth co-doping strategies. This cation substitution approach enabled the simultaneous resolution of two significant material challenges: the attainment of colossal dielectric constants (εr > 104) whilst concurrently reducing sintering temperatures by 140 °C (from 1240 °C to 1100 °C). The dual-ion modification mechanism promoted grain boundary refinement and phase purity optimization, thereby facilitating the fabrication of high-performance devices that exhibit enhanced manufacturability. The emergence of flexible electronics imposes stringent requirements on ferroelectric thin-film systems, particularly regarding interfacial compatibility in heteroepitaxial architectures. Conventional direct deposition of BaTiO3 on mica substrates (lattice mismatch > 8%) is plagued by severe interfacial stress accumulation, resulting in diminished remnant polarization (Pr < 5 μC/cm2), which significantly curtails device performance [30]. To address this limitation, innovative interface engineering strategies have been devised. Liu et al. [31] pioneered a strain-decoupling approach using an NSTO/LSMO/STO multilayer buffer (stepwise lattice mismatch > 98%), enabling epitaxial growth of single-crystal (111)-oriented BaTiO3 films on flexible Mica. This architecture achieved a fourfold enhancement in ferroelectric response (Pr > 20 μC/cm2), thus revolutionizing flexible ferroelectric design paradigms. Meanwhile, Sun et al. [32] demonstrated an alternative pathway through BFO/Mica heterojunction engineering, attaining record-high Pr values (~100 μC/cm2) via interfacial charge redistribution effects. These breakthroughs collectively validate that precision interface modulation can overcome the intrinsic limitations of oxide films in flexible electronics, establishing new design rules for next-generation ferroelectric–photonic hybrids.
Despite significant advances in the development of ferroelectric memory devices, existing research continues to encounter substantial challenges that impede their practical implementation. For instance, although materials such as lead zirconate titanate (PZT) and bismuth ferrite (BFO) have been extensively developed, critical issues including the environmental toxicity of lead in PZT and the volatile nature of bismuth in BFO remain major concerns, hindering their application in sustainable and reliable electronic devices. These limitations have prompted growing interest in developing lead-free alternatives with comparable piezoelectric performance and enhanced environmental compatibility. Furthermore, the inherent instability of conventional perovskite materials and their incompatibility with CMOS technology limit their nanoscale integration, while metastable hafnium oxide (HfO2)-based ferroelectrics suffer from phase transitions and polarization fatigue during cycling. Considerable lattice mismatch between the BTO and STO substrates results in poor crystal quality and electrical properties when BTO films are deposited directly on the substrate, which cannot meet the performance requirements of ferroelectric memories. Conventional ferroelectric thin films suffer from interfacial defects and lattice mismatch, leading to device performance degradation. These defects underscore the imperative for innovative strategies that streamline fabrication processes, ensure underlying layers compatibility, and preserve robust ferroelectric and mechanical properties. Herein, we propose a lattice-engineered heterostructure integrating epitaxial BaTiO3 with a conductive oxide electrode, which synergistically addresses these challenges.
In this work, by characterizing and testing the film quality at different deposition temperatures, we have obtained the optimal process parameters and successfully prepared a high-performance BTO-based ferroelectric heterojunction on the STO substrate.
By introducing the SRO buffer layer, we realize the lattice matching between the BTO films and the STO substrate to reduce the interfacial defects (e.g., oxygen vacancies, lattice stresses), thus suppressing the leakage current and fatigue failure, and analyze the epitaxial growth quality of the devices by using XRD to confirm the high densification and low lattice mismatch rate. Compared with the conventional polycrystalline HfO2 films, the BTO films in this study achieve ultra-flat interfaces by epitaxial growth to enhance device uniformity, and the surface morphology is analyzed by atomic force microscopy (AFM) to verify the heterojunction interfacial flatness and to exclude leakage currents or polarization failures due to interfacial defects. Single-crystal BTO film is used to replace the traditional polycrystalline material, and combined with the optimization of PLD process parameters (e.g., oxygen partial pressure control), higher polarization strength and fatigue resistance are achieved [33,34]. The heterojunction exhibits excellent ferroelectric properties with a saturation polarization (Ps) of about 20 μC/cm2 and a residual polarization (Pr) of about 8 μC/cm2. After 107 bipolar switching cycles, the 2Pr value of the device varies from 6.37 to 6.71 μC/cm2, and the 2Ps value varies from 30.93 to 35.63 μC/cm2, with the polarization value showing only small changes, indicating that the device does not show significant fatigue. In addition, after 104 s, the 2Pr of the device varied from 9.34 to 9.16 µC/cm2 and the 2Ps value varied from 30.93 to 35.63 µC/cm2 with no significant decrease in the polarization value, indicating that the device showed good stability and reliability during long-term use. These superior properties underscore the potential of BTO-based materials for non-volatile memory applications, particularly in scenarios where uncompromised data integrity and long-term storage stability are required. It will also provide important theoretical and technical support for practical ferroelectric memory applications.

2. Materials and Methods

2.1. Experimental Section and Materials

2.1.1. Epitaxial Growth

It should be noted that the STO substrate needs to be sonicated in acetone, anhydrous ethanol, and deionized water sequentially for 10 min each to remove organic matter and impurities on the surface before the experiment starts. SRO and BTO films were epitaxially grown on STO substrates by using PLD technology (PLD-S40-L, Beijing Perfect Technology Co., Ltd., Beijing, China) with a KrF excimer laser (Complex 102, American Coherent, Inc., Fremont, CA, USA) at a wavelength of 248 nm. The laser uses a mixture of krypton chloride (KrCl) and fluorine gas (F2) to excite krypton atoms by applying a high voltage, releasing UV laser pulses at a wavelength of 248 nm. KrF lasers have an extremely high energy density and a short pulse width, which produces a strong ablation effect on the surface of the target material. The laser light is reflected through the optical path to the target inside the growth cavity. The laser pulses have a repetition rate of 1–20 Hz and a maximum output energy of 400 mJ. During the deposition of the STO films, the substrate temperature was maintained at 750 °C and the oxygen pressure was set at 10 Pa. SRO films were then deposited at 680 °C and 10 Pa oxygen pressure, serving as the bottom electrodes of the devices. A ferroelectric functional layer, BTO, was deposited on top of the SRO film at 750 °C and 10 Pa.

2.1.2. UV Lithography

To pattern the Pt top electrode, the sample was first spin-coated with the AZ5214 photoresist. The spin-coating was divided into two stages, with the first stage setting the spin coater speed at 500 rpm for 10 s, and the second stage setting the speed at 5000 rpm for 40 s. The sample was then placed on a heating plate for pre-baking with the plate’s temperature set at 85 °C and the baking time at 120 s. The sample was then exposed for the first time in direct contact mode using model H94-25C 4-side lithography from Sichuan Nanguang Vacuum Co. The baking time was 120 s. Subsequently, the samples were exposed for the first time using the H94-25C 4-side photolithography machine of Sichuan Nanguang Vacuum Technology Co. Ltd., which used the direct contact mode to expose the samples for 7 s. The exposed samples were placed on the heating plate, the temperature of the hot plate was set to 115 °C, and the baking time was set to 90 s, to make the nature of the photoresist turn over. The heated sample was placed directly on the sample holder without the photolithography plate, and the pan exposure time was set to 90 s. The pan-exposed sample was put into the developing solution (AZ-238) and gently shaken for 45 s, and then put into the deionized water and shaken for 30 s to remove the residual developing solution on the surface, and then, when the development was completed, the sample was taken out and blown dry with a nitrogen gun to make it ready. Lastly, photolithography of 70 μm × 70 μm square array electrodes was carried out on BTO films by UV lithography, and subsequently 70 nm Pt was grown as the upper electrode of the device using magnetron sputtering.

2.1.3. Magnetron Sputtering

The deposition of the top electrode Pt metal film was accomplished by means of the magnetron sputtering process, utilizing a TL-500 magnetron sputtering system (produced by Beijing Talon Electronic Technology Co., Ltd. in Beijing China, and under the technical supervision of the Institute of Microelectronics, Chinese Academy of Sciences). The key process parameters were meticulously regulated: the vacuum chamber was pre-evacuated to a background vacuum of 2.0 × 10−4 Pa by a molecular pumping unit, high-purity argon (99.999%) was introduced to maintain the working air pressure of 0.5 Pa (flow rate of 100 sccm), and the sputtering was carried out under the condition of 100 W of DC power. To control the film stress, the substrate temperature was stabilized at 30 °C by a water-cooling system, and the thickness of the Pt metal was 70 nm after 1000 s of continuous deposition. The samples were then removed from the chamber and placed in acetone. The ultrasonic instrument was switched on, and the ultrasonic power was set to 300 W. After 30 s of ultrasonication, the metal detachment was evident, and the samples were subsequently placed into deionized water for 30 s. The samples were then taken out and the surfaces were dried by blowing them with a nitrogen gas gun.

2.1.4. Materials

The target materials were from Tim New Material Co., Ltd. (Beijing, China), with a purity of 99.9% and a size specification of 25 × 6 mm. The atomic ratio of Sr, Ti, and O in the SrTiO3 target was 1:1:3. Similarly, the atomic ratio of Sr, Ru, and O in the SrRuO3 target was 1:1:3, and the atomic ratio of Ba, Ti, and O in the BaTiO3 target was also 1:1:3.
Figure 1 illustrates the simulation of the BaTiO3 crystal cell. The location of the Ba ions is indicated at the diagonal of the octahedron, the location of the O atoms is at the face center of the hexahedron, and the location of the Ti ions is at the center region of the diagonal of the top corners of the octahedron [35]. The material exhibits tetragonal, paraelectric, orthorhombic, and rhombic phases. The tetragonal phase belongs to the tetragonal (Tetragonal) crystalline system, which exhibits ferroelectricity, and the space group is P4mm (a = 3.994 Å, b = 3.994 Å, c = 4.038 Å, α = 90.0°, β = 90.0°, ɣ = 90.0°) [36]. The c/a ratio of the tetragonal system is 1.01, which implies that the BaTiO3 thin films have high Curie temperatures and ferroelectric polarization strength [37,38]. The paraelectric phase belongs to the cubic crystal system (Cubic) with the space group Pm3m (a = b = c = 4.01 Å, α = 90.0°, β = 90.0°, ɣ = 90.0°).

2.2. Test Methods

The surface morphology of SRO and BTO thin films at different temperatures was investigated using AFM (AFM, Cypher S/Oxford Instruments Sheltered Research). Table 1 below shows the detailed parameters of the cantilevers used. The AFM measurement mode was conducted in tapping mode in this test. The setpoint amplitude was 10 nm, the scanning rate was 1.50 Hz, and the resolution was 256 × 256. The calibration grating we chose in this experiment was a two-dimensional projection grating with a specification of 65 mm × 40 mm, a thickness of 1.21 mm, and a material of glass. For different ranges of measurements, we chose different scale lines for calibration. In this study, a grating with 600 lines (period 1 mm/600 = 1666.7 nm) was used, while for smaller measurements, a grating with 1800 lines (period 1 mm/1800 = 555.6 nm) can be chosen; the more lines in the grating, the more accurate it will be. In addition, the specific environmental parameters at the time of the AFM measurements were as follows: (1) air pressure: at standard atmospheric pressure; (2) temperature at the time was approximately 21 °C; (3) humidity was approximately 45%. Furthermore, the grain distribution and size were further analyzed in depth based on the AFM test maps combined with Nano Measurer 1.2 software. Specific statistical analyses can be found in the Section “Grain Size Statistics Measure” of the Supplementary Material.
X-ray diffraction (XRD, PANalytical’s Empyrean Sharp Imaging) with a Cu target Kα radiation (λ = 1.5406 Å) was used to analyze the phase structure and orientation of the Pt/BTO/SRO/STO heterostructure. The specific test conditions were set to an operating voltage of 40 kV, operating current of 40 mA, scanning range of 10–60°, and scanning step of 0.01°. Electrical properties were measured using a ferroelectric analyzer (HUACE FE-2000) and a semiconductor parameter analyzer (Keysight B1500).

3. Results

3.1. Scanning Probe Microscopy Analysis

Surface morphology and ferroelectric domain distribution were analyzed by atomic force microscopy (AFM) to verify the heterojunction interfacial flatness (and uniformity of ferroelectric domains) and to exclude leakage current or polarization failure due to interfacial defects. Figure S1 shows the original unprocessed AFM topography images of SRO films. Figure 2 demonstrates the preparation of SRO films on STO substrates at different temperatures. Figure 2a shows the SRO films prepared at 620 °C temperature with a surface roughness (Ra) of 1.8 nm, which is relatively rough and has some obvious defects or particles. Figure 2b presents the SRO thin film deposited at 680 °C, demonstrating significantly improved surface characteristics compared with the counterpart in Figure 2a. Quantitative analysis reveals a reduction in Ra to 0.9 nm (28% lower than that of the reference sample), accompanied by smoother surface morphology and reduced defect density. These observations confirm that elevated deposition temperatures effectively promote surface planarization through enhanced atomic mobility during the crystallization process. Figure 2c shows the SRO films prepared at a higher temperature with Ra of 1.4 nm, indicating that too high a temperature may lead to grain enlargement or the formation of certain surface structures, which in turn affects the surface quality of the films. For further surface roughness analysis, height profiles of SRO films deposited at different temperatures were plotted. As shown in Figure S2, the profiles show the average line on the same cross-section as well as the magnitude of the deviation from the average, which gives a more intuitive picture of the roughness of the films, corroborating that 680 °C is the optimal temperature for the growth of SRO films.
Figure 3 presents the particle size distribution of SRO films prepared at different deposition temperatures. As illustrated in Figure S3, the presence of specific grain size markings is evident. Figure 3a demonstrates the particle size distribution of SRO films prepared at a temperature of 620 °C, and the particle size is mainly concentrated between 20 nm and 80 nm. Figure 3b illustrates the particle size distribution of SRO films deposited at a temperature of 680 °C. The particle sizes are primarily concentrated between 40 nm and 60 nm, showing a narrower distribution and a more uniform particle size with a higher proportion. This suggests that SRO films deposited at 680 °C achieve optimal particle homogeneity. In contrast, Figure 3c depicts the particle size distribution of SRO films deposited at elevated temperatures, where the particle sizes range from 20 nm to 100 nm, with a significantly higher proportion of larger particles. This indicates that the particle sizes increase under high-temperature deposition conditions, resulting in a broader size distribution, which may lead to larger particles on the film’s surface, adversely impacting film uniformity. Analysis confirms that the SRO films prepared at 680 °C exhibit superior surface flatness.
Figure S4 shows the original unprocessed AFM topography images of BTO films. Figure 4 demonstrates the surface morphology of BTO films prepared at different temperatures. The Ra values of the BTO films deposited at 680 °C and 800 °C were 3.5 nm and 2.2 nm, respectively. While the BTO films deposited at a temperature of 750 °C, with Ra of 0.8 nm, have a smoother surface and a more uniform distribution of particles, as can be seen in Figure 4a,b, lower temperature films form larger particles or defects on the film surface during growth, and too high a temperature results in the formation of an inhomogeneous structure on the film surface. It has been demonstrated that a deposition temperature of 750 °C can enhance the flatness of the film surface and markedly reduce surface defects and particles. As illustrated in Figure S5, the height profiles of BTO films at varying growth temperatures are presented, along with the mean value and the standard deviation from the mean value at different locations on the BTO films. In conjunction with the AFM images, these data offer a more comprehensive visualization of the enhanced surface flatness of the BTO films at 750 °C in comparison to films prepared at other temperatures. The experimental results are in general agreement with the grain growth kinetics and the recrystallization threshold of chalcogenides [39].
Figure 5 presents the grain size plots of BTO films deposited at different temperatures. From the analysis of the data in the plots, it can be seen that the grain size distribution of the BTO films deposited at 750 °C is more desirable, with the concentration of the grain size in a smaller range and a more homogeneous grain size distribution. The deposition temperature demonstrates a significant influence on the microstructure evolution of BTO films. Comparative analysis revealed that films deposited at 750 °C exhibited optimal structural characteristics compared with those prepared at 680 °C and 800 °C. Specifically, the 750 °C-processed films displayed two distinctive microstructural advantages: (1) a remarkable reduction in average grain size to the sub-100 nm regime (50–80 nm), representing an approximately 30% size reduction compared with other temperatures; (2) a narrow, symmetric grain size distribution indicating superior microstructural homogeneity. These optimized morphological features directly correlate with enhanced film quality. The combined effects of refined grain dimensions and uniform size distribution contribute to exceptionally smooth surfaces, making these films particularly suitable for high-precision applications in microelectromechanical systems and advanced photonic devices where surface topography critically determines performance.

3.2. X-Ray Diffraction Analysis

X-ray diffraction (XRD) is used to confirm the crystal orientation and lattice matching of BTO films to avoid stress accumulation and ferroelectric property degradation caused by lattice mismatch. As can be seen from Figure 6, there are no stray peaks except for the substrate peaks of STO (100) and (200) and the diffraction peaks of the SRO and BTO films, indicating that SRO and BTO are well crystallized on the STO substrate. In addition, we also found that the diffraction peaks of BTO and SRO films almost overlap, which is because the diffraction peak corresponding to the standard card of (100) SRO films is 22.356° and that of (100) BTO films is 22.206°. The difference in the standard diffraction peaks corresponding to their standard cards is only 0.15°. The BTO/SRO/STO ferroelectric heterojunction’s XRD patterns show that the BTO and SRO films have good crystallinity.
To further characterize the quality of the films, we performed ω-scans of each layer and substrate to test the film’s half-height width (FWHW). In general, a smaller film half-height width means a better crystalline quality of the film. As determined by the XRD swing curve (Figure 6), the diffraction peak FWHW of the substrate (200) STO is 0.018°, and the lower FWHW provides a good basis for the subsequent epitaxial film on the substrate, and the FWHW of the (200) SRO and BTO films are 0.52° and 0.74°, respectively, which indicates that the crystals in the film are better arranged in an orderly fashion, with fewer defects, and with good crystallinity.

3.3. Study on Electrical Properties

Figure 7a shows the structure of our prepared device. The typical current-voltage (I-V) characteristic curves of Pt/BTO/SRO/STO ferroelectric heterojunctions are shown in Figure 7b. For the test using the two-probe method, the top electrode Pt is applied with forward bias, the bottom electrode SRO is kept grounded, and a clockwise voltage hysteresis loop is constructed through a closed-loop scanning path of 0 V → −10 V → 0 V → +10 V → 0 V, following the same direction indicated by the red arrows. Significant asymmetric hysteresis characteristics are observed with no overlap of the I-V curves, which can be attributed to the resistance change due to the internal polarization characteristics of the BTO ferroelectric film [40,41]. Ferroelectric materials have reversible polarization properties, where changes in the electric field result in the reversal or orientation of their internal electric dipole moments. When the applied voltage is increased, the polarization intensity in the BTO film gradually increases and is accompanied by a change in current. However, during the increase in current, the hysteresis effect of polarization reversal leads to a change in resistance, which is manifested as a hysteresis phenomenon in the I-V characteristic curve.
Figure 8a illustrates the polarization-voltage (P-V) hysteresis curves of ferroelectric materials at different voltages. A ferroelectric material undergoes an orientation change in its internal electric dipole moment in response to an applied electric field, resulting in a polarization effect. The P-V curve in Figure 8a shows a typical hysteresis phenomenon, where the polarization value increases gradually with the increase in the applied voltage and a polarization reversal occurs under the action of a reversed electric field, forming two symmetrical peaks. As shown in Figure 9, the residual polarization value (2Pr) of the Pt/BTO/SRO/STO ferroelectric heterojunction is about 17.47 µC/cm2 and the saturation polarization value (2Ps) is about 36.94 µC/cm2 at a test voltage of 25 V, indicating that the ferroelectric material is capable of polarity reversal under the action of an electric field, thus possessing the property of non-volatile storage. The polarization of the Pt/BTO/SRO/STO ferroelectric heterojunction shows a typical hysteresis phenomenon. An STO ferroelectric heterojunction polarization flip current diagram is demonstrated in Figure 8b. With the increase in voltage, two obvious current peaks appear at positive and negative coercive fields; the reason for the formation of the current peaks is due to the drastic reversal of the polarization of the ferroelectric material at the coercive field, which leads to a sudden increase in the polarization current. When the electric field strength reaches or exceeds the coercive field, the reorientation of the electric dipole moment requires overcoming the intrinsic resistance of the material such as lattice defects, dipole inertia, etc. This process is usually accompanied by a rapid movement of charge, resulting in a peak in current. The formation of two current peaks due to the rapid charge migration during the polarization reversal process also further confirms the ferroelectric properties of Pt/BTO/SRO/STO ferroelectric heterojunctions.
Figure 10a shows the results of the fatigue test; after 107 bipolar switching cycles, the 2Pr value of the device varies from 6.37 to 6.71 µC/cm2. In addition, the 2Ps value varies from 30.93 down to 35.63 µC/cm2, with only minor changes in the polarization value. The small change in the polarization value of the device indicates that it has good fatigue resistance.
The retention characteristics directly affect the reliability of ferroelectric memory data. Ferroelectric memories rely on storing information in the direction of polarization, and good retention characteristics indicate that the stored information can be stored for a long period of time without loss or degradation. To fully evaluate the reliability of flexible Pt/BTO/SRO/STO ferroelectric devices, we have analyzed the retention characteristic curves at a bias voltage of 5 V and a pulse width of 100 ms (Figure 10b). The results show that the BTO ferroelectric memory exhibits good stability and reliability during long-term use. The polarization values did not show any significant decrease after 104 s. The 2Pr of the device varied from 9.34 to 9.16 µC/cm2, and the 2Ps value fluctuated between 30.93 and 35.63 µC/cm2. This emphasizes the potential of this material for non-volatile memory applications, especially where data integrity and long-term storage are vital. These properties ensure that BTO memory devices meet the needs of a wide range of high-performance and high-reliability electronic applications.

4. Discussion

The BTO-based ferroelectric non-volatile memory we have developed exhibits excellent ferroelectric properties and stability, which provide new insights into in-memory computing architectures and neuromorphic computing. Firstly, the ferroelectric memory can achieve multi-value storage through the process of adjusting the polarization state. This, in conjunction with its low operating voltage characteristics, has the potential to facilitate the design of high-density and low-power storage-computing systems. Consequently, this could result in the overcoming of the data transfer bottleneck that is characteristic of conventional von Neumann architectures. Secondly, based on the observed reversible resistive properties, the developed heterojunction structure can be designed as an artificial synaptic device whose conductivity can be continuously regulated by pulsed stimulation to achieve synaptic-like weights. This mimetic function of long-time range potentiation (LTP) and inhibition (LTD) is of the utmost importance for the construction of artificial neural networks to advance neuromorphic computing [42].
Based on the above discussion, our future work will focus on the following: (1) Evaluating the device LTP, LTD, STDP, etc., through the utilization of semiconductor analyzers. (2) Enhancing device homogeneity through interface engineering, facilitating standardized device array preparation. (3) Establishing a multi-parameter optimization model linking “material-composition-microstructure-synaptic/memory performance” via high-throughput experiments and machine learning, accelerating the next-generation neuromorphic chips’ design iteration. (4) Developing micro- and nanofabrication processes compatible with CMOS processes to facilitate the conversion of laboratory prototypes to industrial applications.

5. Conclusions

In this study, high-quality Pt/BTO/SRO/STO ferroelectric heterostructure memory devices have been successfully fabricated and systematically characterized. The SRO and BTO films at different deposition temperatures were characterized by AFM to find the optimum deposition temperature. The results show that the SRO films have the flattest surface and more uniform grain size distribution at 680 °C deposition temperature, while the BTO ferroelectric films have the best quality at 750 °C deposition. The test results of XRD demonstrated that all the films exhibited high levels of crystallinity on the STO substrate. The high quality of the growth of the prepared films on STO substrates was further substantiated by the analysis of the rocking curves of each layer of film against the substrate. By the way, the BTO-based ferroelectric memory exhibited excellent ferroelectric properties and stable mechanical flexibility, with a 2Pr of 15.63 μC/cm2 and a 2Ps of 36.61 μC/cm2. Remarkably, the device showed negligible polarization degradation even after 107 bipolar switching cycles, highlighting its superior fatigue resistance compared to existing studies. In addition, the retention characteristics exceeded 104 s, further confirming its exceptional reliability and durability [43]. The combination of high polarization stability, robust retention, and fatigue resistance ensures that BTO memory devices meet the stringent requirements of high-performance electronics, including mission-critical systems, aerospace electronics, and industrial automation, where reliability and long-term data retention are paramount. These characteristics not only address the current limitations of conventional memory technologies but also establish BTO as a frontrunner for next-generation memory solutions capable of maintaining signal fidelity over extended operating lifetimes. The systematic study of Pt/BTO/SRO/STO ferroelectric devices also provides a good basis for the subsequent fabrication and study of flexible BTO ferroelectric devices.

Supplementary Materials

The following supporting information can be downloaded at https://www.mdpi.com/article/10.3390/cryst15040337/s1, Figure S1: AFM topography images of SRO films prepared on STO substrate at different growth temperatures at 5 × 5 μm region (original unprocessed image); Figure S2: Height profiles of SRO films prepared on STO substrates at different deposition temperatures; Figure S3: Partial enlargement of grain size markers; Figure S4: AFM topography images of BTO films at different growth temperatures at 5 × 5 μm region (original unprocessed image); Figure S5: Height profiles of BTO films at different growth temperatures; Grain Size Statistics Measure; Table S1: Grain Size Statistics Raw Data Sheet.

Author Contributions

Conceptualization, Z.F. and Y.P.; methodology, Z.F.; software, Z.F. and Y.P.; validation, Z.F. and Y.P.; formal analysis, Z.F.; investigation, Z.F. and Y.P.; resources, Z.F. and Y.P.; data curation, Z.F.; writing—original draft preparation, Z.F.; writing—review and editing, Z.F., Y.P. and X.L.; visualization, Z.F. and H.L.; supervision, Y.P., X.L. and J.Z.; project administration, Z.F. and Y.P.; funding acquisition, H.L., Z.F. and X.L. All authors have read and agreed to the published version of the manuscript.

Funding

This work was funded by the Guangxi Key Laboratory of Information Materials and Guangxi Collaborative Innovation Center of Structure and Property for New Energy Materials (Grant No. 231015-Z), the Guangxi Science and Technology Project (No. 2023GXNSFBA026216), the Guilin Science and Technology Project (No. 20220101), and the Innovation Project of GUET Graduate Education (NO. 2025YCXS039).

Data Availability Statement

The original contributions presented in the study are included in the article; further inquiries can be directed to the corresponding author.

Conflicts of Interest

The authors declare no conflicts of interest.

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Crystals 15 00337 g001
Figure 1. Cellular analogue of BaTiO3. (a) Ball-and-stick model structure of BaTiO3. (b) Polyhedral model structure of BaTiO3.
Figure 1. Cellular analogue of BaTiO3. (a) Ball-and-stick model structure of BaTiO3. (b) Polyhedral model structure of BaTiO3.
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Figure 2. AFM topography images of SRO films prepared on STO substrate at different growth temperatures in a 5 × 5 μm region (post−processed image). (a) 620 °C, Ra = 1.8 nm. (b) 680 °C, Ra = 0.9 nm. (c) 750 °C, Ra = 1.4 nm.
Figure 2. AFM topography images of SRO films prepared on STO substrate at different growth temperatures in a 5 × 5 μm region (post−processed image). (a) 620 °C, Ra = 1.8 nm. (b) 680 °C, Ra = 0.9 nm. (c) 750 °C, Ra = 1.4 nm.
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Crystals 15 00337 g003
Figure 3. Grain size distributions of SRO films prepared on STO substrates at different deposition temperatures (based on the measurement data of 100 random particles, analyzed by Gaussian fitting). (a) 620 °C. (b) 680 °C. (c) 750 °C.
Figure 3. Grain size distributions of SRO films prepared on STO substrates at different deposition temperatures (based on the measurement data of 100 random particles, analyzed by Gaussian fitting). (a) 620 °C. (b) 680 °C. (c) 750 °C.
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Figure 4. AFM topography images of BTO films at different growth temperatures in a 5 × 5 μm region (post−processed image). (a) 680 °C, Ra = 3.5 nm. (b) 750 °C, Ra = 0.8 nm. (c) 800 °C, Ra = 2.2 nm.
Figure 4. AFM topography images of BTO films at different growth temperatures in a 5 × 5 μm region (post−processed image). (a) 680 °C, Ra = 3.5 nm. (b) 750 °C, Ra = 0.8 nm. (c) 800 °C, Ra = 2.2 nm.
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Figure 5. Grain size distributions of BTO films at different deposition temperatures (based on the measurement data of 100 random particles, analyzed by Gaussian fitting). (a) 680 °C. (b) 750 °C. (c) 800 °C.
Figure 5. Grain size distributions of BTO films at different deposition temperatures (based on the measurement data of 100 random particles, analyzed by Gaussian fitting). (a) 680 °C. (b) 750 °C. (c) 800 °C.
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Figure 6. (a) XRD pattern of BTO/SRO/STO ferroelectric heterojunction. (b) ω-scan of diffraction peaks of STO film (200). (c) ω-scan of diffraction peaks of SRO film (200). (d) ω-scan of diffraction peaks of SRO film (200).
Figure 6. (a) XRD pattern of BTO/SRO/STO ferroelectric heterojunction. (b) ω-scan of diffraction peaks of STO film (200). (c) ω-scan of diffraction peaks of SRO film (200). (d) ω-scan of diffraction peaks of SRO film (200).
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Figure 7. (a) Device structure schematic. (b) I-V characteristic curves of Pt/BTO/SRO/STO ferroelectric heterojunctions.
Figure 7. (a) Device structure schematic. (b) I-V characteristic curves of Pt/BTO/SRO/STO ferroelectric heterojunctions.
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Figure 8. Pt/BTO/SRO/STO ferroelectric memory under various test conditions. (a) P-V hysteresis loops of different voltages. (b) Polarization-switching current curves of different voltages.
Figure 8. Pt/BTO/SRO/STO ferroelectric memory under various test conditions. (a) P-V hysteresis loops of different voltages. (b) Polarization-switching current curves of different voltages.
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Figure 9. P-V hysteresis loops and polarization-switching current curve of BTO thin film.
Figure 9. P-V hysteresis loops and polarization-switching current curve of BTO thin film.
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Figure 10. Polarization−switching current curves under various conditions: (a) Fatigue characteristic curve of the device. (b) Polarization retention force characteristic curve.
Figure 10. Polarization−switching current curves under various conditions: (a) Fatigue characteristic curve of the device. (b) Polarization retention force characteristic curve.
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Table 1. Parameters of the cantilevers.
Table 1. Parameters of the cantilevers.
ModelStiffness *Tip RadiusManufacturerCountry of Origin
Multi75Al-G3 N/m<10 nmBudget SensorsBulgaria
* Factory calibration range: 1–7 N/m.
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MDPI and ACS Style

Fang, Z.; Peng, Y.; Li, H.; Liu, X.; Zhai, J. High-Performance Ferroelectric Capacitors Based on Pt/BaTiO3/SrRuO3/SrTiO3 Heterostructures for Nonvolatile Memory Applications. Crystals 2025, 15, 337. https://doi.org/10.3390/cryst15040337

AMA Style

Fang Z, Peng Y, Li H, Liu X, Zhai J. High-Performance Ferroelectric Capacitors Based on Pt/BaTiO3/SrRuO3/SrTiO3 Heterostructures for Nonvolatile Memory Applications. Crystals. 2025; 15(4):337. https://doi.org/10.3390/cryst15040337

Chicago/Turabian Style

Fang, Zengyuan, Yiming Peng, Haiou Li, Xingpeng Liu, and Jianghui Zhai. 2025. "High-Performance Ferroelectric Capacitors Based on Pt/BaTiO3/SrRuO3/SrTiO3 Heterostructures for Nonvolatile Memory Applications" Crystals 15, no. 4: 337. https://doi.org/10.3390/cryst15040337

APA Style

Fang, Z., Peng, Y., Li, H., Liu, X., & Zhai, J. (2025). High-Performance Ferroelectric Capacitors Based on Pt/BaTiO3/SrRuO3/SrTiO3 Heterostructures for Nonvolatile Memory Applications. Crystals, 15(4), 337. https://doi.org/10.3390/cryst15040337

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