Ferroelectric Stability Research Articles

Physics of Ferroelectric Wurtzite Al1−xScxN Thin Films

AbstractAl1−xScxN emerges as a revolutionary ferroelectric material within the III‐N family. It combines exceptional switchable polarization (80–165 µC cm−2), highly tunable coercive fields (1.5–6.5 MV cm−¹), and a wide bandgap (4.9–5.6 eV). Unlike conventional ferroelectrics, Al1−xScxN exhibits remarkable compatibility with both CMOS and III‐N technologies. It can be fabricated on plastic substrates at low temperatures, demonstrating excellent flexibility and biocompatibility. Remarkably, Al1−xScxN maintains superior performance in harsh environments due to its outstanding thermal stability (up to 1100 °C). These unique characteristics position Al1−xScxN as a highly promising candidate for a wide range of applications, including high‐performance memory, in‐memory computing, neuromorphic computing, and next‐generation wearable and implantable devices, particularly for operation in complex environments. Despite its potential, Al1−xScxN faces challenges such as high coercive fields, significant leakage currents, and limited polarization reversal cycle life. Addressing these challenges require a deeper understanding of the fundamental physics controlling Al1−xScxN films. This review explores the origins of Al1−xScxN's ferroelectricity and phase stability, delves into the fundamental theory of wurtzite ferroelectricity, investigates mechanisms for controlling spontaneous polarization and coercive fields, examines recent research progress in Al1−xScxN ferroelectric devices, and outlines future development directions for this exciting material.

Producing Freestanding Single-Crystal BaTiO3 Films through Full-Solution Deposition.

Strontium aluminate, with suitable lattice parameters and environmentally friendly water solubility, has been strongly sought for use as a sacrificial layer in the preparation of freestanding perovskite oxide thin films in recent years. However, due to this material's inherent water solubility, the methods used for the preparation of epitaxial films have mainly been limited to high-vacuum techniques, which greatly limits these films' development. In this study, we prepared freestanding single-crystal perovskite oxide thin films on strontium aluminate using a simple, easy-to-develop, and low-cost chemical full-solution deposition technique. We demonstrate that a reasonable choice of solvent molecules can effectively reduce the damage to the strontium aluminate layer, allowing successful epitaxy of perovskite oxide thin films, such as 2-methoxyethanol and acetic acid. Molecular dynamics simulations further demonstrated that this is because of their stronger adsorption capacity on the strontium aluminate surface, which enables them to form an effective protective layer to inhibit the hydration reaction of strontium aluminate. Moreover, the freestanding film can still maintain stable ferroelectricity after release from the substrate, which provides an idea for the development of single-crystal perovskite oxide films and creates an opportunity for their development in the field of flexible electronic devices.

Distinguishing the Rhombohedral Phase from Orthorhombic Phases in Epitaxial Doped HfO2 Ferroelectric Films.

Epitaxial strain plays an important role in the stabilization of ferroelectricity in doped hafnia thin films, which are emerging candidates for Si-compatible nanoscale devices. Here, we report on epitaxial ferroelectric thin films of doped HfO2 deposited on La0.7Sr0.3MnO3-buffered SrTiO3 substrates, La0.7Sr0.3MnO3 SrTiO3-buffered Si (100) wafers, and trigonal Al2O3 substrates. The investigated films appear to consist of four domains in a rhombohedral phase for films deposited on La0.7Sr0.3MnO3-buffered SrTiO3 substrates and two domains for those deposited on sapphire. These findings are supported by extensive transmission electron microscopy characterization of the investigated films. The doped hafnia films show ferroelectric behavior with a remanent polarization up to 25 μC/cm2 and they do not require wake-up cycling to reach the polarization, unlike the reported polycrystalline orthorhombic ferroelectric hafnia films.

Ferroelectric stability and fatigue mechanism of BNKT ceramics by Nb doping

Ferroelectric stability and fatigue mechanism of BNKT ceramics by Nb doping

Competing Charge Transfer and Screening Effects in Two-Dimensional Ferroelectric Capacitors.

Two-dimensional (2D) ferroelectrics promise ultrathin flexible nanoelectronics, typically utilizing a metal-ferroelectric-metal sandwich structure. Electrodes can either contribute free carriers to screen the depolarization field, enhancing nanoscale ferroelectricity, or induce charge doping, disrupting the long-range crystalline order. We explore electrodes' dual roles in 2D ferroelectric capacitors, supported by first-principles calculations covering a range of electrode work functions. Our results reveal volcano-type relationships between ferroelectric-electrode binding affinity and work function, which are further unified by a quadratic scaling between the binding energy and the transferred interfacial charge. At the monolayer limit, charge transfer dictates the ferroelectric stability and switching properties. This general characteristic is confirmed in various 2D ferroelectrics including α-In2Se3, CuInP2S6, and SnTe. As the ferroelectric layer's thickness increases, the capacitor stability evolves from a charge-transfer-dominated state to a screening-dominated state. The delicate interplay between these two effects has important implications for 2D ferroelectric capacitor applications.

Three-Dimensional Visualization of Oxygen-Vacancy Migration and Redistribution in Ca-Substituted BiFeO3.

Ionic movement has received renewed attention in recent years, particularly in the field of ferroelectric oxides, since it is intrinsically linked to chemical reaction kinetics and ferroelectric phase stability. The associated surface electrochemical processes coupled local ionic transport with an applied electric bias, exhibiting very high ionic mobility at room temperature based on a simple electrostatics scenario. However, few studies have focused on the applied-polarity dependence of ionic migration with directly visualized maps. Here, we use incorporated experiments of conductive scanning probe microscopy and time-of-flight secondary ion mass spectrometry to investigate oxygen ionic migration and cation redistribution in ionic oxides. The local concentrations of oxygen vacancies and other cation species are visualized by three-dimensional mappings, indicating that oxygen vacancies tend to be ejected toward the surface. An accumulation of oxygen vacancies and ionic redistribution strongly depend on tip polarity, thus corroborating their role in the electrochemical process. This work illustrates the interplay between ionic kinetics and electric switching.

Kinetical phase transition paths and phase stability in ferroelectric HfO2

Novel ferroelectric HfO2 films have obtained great attention owing to their super advantages in electronic applications. However, the phase transition paths and ferroelectric phase stability in HfO2 still remain elusive, represented by several contradictions in theoretical and experimental results. Based on kinetical calculations on the premise of crystal axis mapping relationship rules, we systematically study the transition paths and illustrate the kinetical process in HfO2 film during the cooling process. We find two paths with the same low energy barriers from tetragonal phase transforming to orthorhombic phase. Their polarization axes are exactly orthogonal, predicting 90° domain formation at the beginning of stage of new nucleus formation. This might explain the existence of high-proportioned 90° domains in HfO2 films. Furthermore, the impact of Zr doping concentration on the stabilization of ferroelectric phase is investigated, revealing the mechanism for 50 % doping concentration as the best condition to induce ferroelectricity in HfO2.

Infrared Signatures for Phase Identification in Hafnium Oxide Thin Films.

Phase identification in HfO2-based thin films is a prerequisite to understanding the mechanisms stabilizing the ferroelectric phase in these materials, which hold great promise in next-generation nonvolatile memory and computing technology. While grazing-incidence X-ray diffraction is commonly employed for this purpose, it has difficulty unambiguously differentiating between the ferroelectric phase and other metastable phases that may exist due to similarities in the d-spacings, their low intensities, and the overlapping of reflections. Infrared signatures provide an alternative route. However, their use in phase identification remains limited because phase control has overwhelmingly been accomplished via substituents, thereby convoluting infrared signatures between the substituents and the phase changes that they induce. Herein, we report the infrared optical responses of three undoped hafnium oxide films where annealing conditions have been used to create films consisting primarily of the ferroelectric polar orthorhombic Pca21, antipolar orthorhombic Pbca, and monoclinic P21/c phases, as was confirmed via transmission electron microscopy (TEM), UV-visible optical properties, and electrical property measurements. Vibrational signatures acquired from synchrotron nano-Fourier transform infrared spectroscopy (nano-FTIR) are shown to be capable of differentiating between the phases in a nondestructive, rapid, and nanoscale manner. The utility of nano-FTIR is illustrated for a film exhibiting an antiferroelectric polarization response. In this sample, it is proven that this behavior results from the Pbca phase rather than the often-cited tetragonal phase. By demonstrating that IR spectroscopy can unambiguously distinguish phases in this material, this work establishes a tool needed to isolate the factors dictating the ferroelectric phase stability in HfO2-based materials.

Wake-Up Free Ultrathin Ferroelectric Hf0.5Zr0.5O2 Films.

The development of the new generation of non-volatile high-density ferroelectric memory requires the utilization of ultrathin ferroelectric films. The most promising candidates are polycrystalline-doped HfO2 films because of their perfect compatibility with silicon technology and excellent ferroelectric properties. However, the remanent polarization of HfO2 films is known to degrade when their thickness is reduced to a few nanometers. One of the reasons for this phenomenon is the wake-up effect, which is more pronounced in the thinner the film. For the ultrathin HfO2 films, it can be so long-lasting that degradation occurs even before the wake-up procedure is completed. In this work, an approach to suppress the wake-up in ultrathin Hf0.5Zr0.5O2 films is elucidated. By engineering internal built-in fields in an as-prepared structure, a stable ferroelectricity without a wake-up effect is induced in 4.5 nm thick Hf0.5Zr0.5O2 film. By analysis of the functional characteristics of ferroelectric structures with a different pattern of internal built-in fields and their comparison with the results of in situ piezoresponse force microscopy and synchrotron X-ray micro-diffraction, the important role of built-in fields in ferroelectricity of ultrathin Hf0.5Zr0.5O2 films as well as the origin of stable ferroelectric properties is revealed.

Layer control of Sr1.8Bi0.2Nan-3NbnO3n+1 (n = 3–5) perovskite nanosheets: dielectric to ferroelectric transition of film deposited by Langmuir Blodgett method

Solution-based processable high-k 2-dimensional (2D) ferroelectrics have attracted significant interest for use in next-generation nanoelectronics. Although few studies on potential 2D ferroelectric nanosheets in local areas have been conducted, reports on the thin-film characteristics applicable to the device are insufficient. In this study, we successfully synthesize high-k 2D Sr1.8Bi0.2Nan-3NbnO3n+1 (octahedral units, n = 3–5) nanosheets by the engineering of the n of NbO6 octahedral layers with A-site modification, and realized ferroelectric characteristics in ultrathin films (below 10 nm). The nanosheets are synthesized by a solution-based cation exchange process and deposited using the Langmuir-Blodgett (LB) method. As increasing the NbO6 octahedral layer, the thickness of the nanosheets increased and the band gaps are tuned to 3.80 eV (n = 3), 3.76 eV (n = 4), and 3.70 eV (n = 5). In addition, the dielectric permittivity of the 5-layer stacked nanofilm increase to 26 (n = 3), 33 (n = 4), and 62 (n = 5). In particular, the increased perovskite layer exhibits large distortions due to the size mismatch of Sr/Bi/Na ions at the A-site and promotes local ferroelectric instability due to its spontaneous polarization along the c-axis caused by an odd n number. We investigate the stable ferroelectricity in Pt/ 5-layer Sr1.8Bi0.2Na2Nb5O16 / Nb:STO capacitor by polarization-electric field (P-E) hysteresis; the coercive electric field (Ec) was 338 kV cm−1 and the remnant polarization (Pr) 2.36 μC cm−2. The ferroelectric properties of ultrathin 2D materials could drive interesting innovations in next-generation electronics.

High-Temperature Piezoelectric Response and Thermal Stability of BiGaO3 Modified BiFeO3–BaTiO3 Lead-Free Piezoelectric Ceramics

BiGaO3 doped BiFeO3–BaTiO3 ceramics were prepared by the traditional solid-phase synthesis process. The phase analysis, microstructure, piezoelectric, ferroelectric, dielectric properties, and thermal stability of 0.7BiFeO3-(0.3 − x)BaTiO3-xBiGaO3 (Abbreviated as BF–BT-xBG) were investigated. The results show that the ceramics have rhombohedral (R) and tetragonal (T) structures. Particle dimensions gradually get bigger with the increase of BiGaO3 concentration, and dense ceramic grains were observed through SEM. Electrical properties of BF–BT-xBG are improved after adding a small amount of BiGaO3: piezoelectric constants d33 = 141 pC/N, electromechanical coupling coefficient kp = 0.314, mechanical Quality Factor Qm = 56.813, dielectric loss tanδ = 0.048, residual polarization intensity Pr = 18.3 µC/cm2, Curie temperature Tc = 485.2 °C, depolarization temperature Td = 465 °C for x = 0.003. The “temperature-piezoelectric performance” curve under in situ d33 indicates that piezoelectric properties d33 increase rapidly with increasing temperature. Remarkably, the piezoelectric response d33 reaches a maximum of 466 pC/N at a temperature T = 340 °C, and afterward, reduces gradually to zero with increasing temperature until 450 °C.

Overcoming Size Effects in Ferroelectric Thin Films

AbstractFerroelectric thin films have recently received unprecedented attention due to the need to miniaturize electronic circuit devices. Synthesis and deposition processes along with theoretical calculations are improved remarkably to realize stable ferroelectric thin films up to nanometer thickness. However, even with technological advances, it is still difficult to overcome the size effect of ferroelectrics, so research is being conducted to achieve stable ferroelectricity in unit‐cell thicknesses thinner than the typical critical thicknesses. In this review, the size effects in ferroelectric thin films are described, and their importance and fundamental limitations are discussed. First, intrinsic and extrinsic factors affecting ferroelectricity are introduced based on the theoretical background of the size effects in ferroelectricity. Then, on understanding the size effects by considering complex interacting factors, the recent works showing ferroelectricity below the commonly known critical thicknesses in perovskite, fluorite oxides, and two‐dimensional (2D) ferroelectrics are introduced. Finally, the results of research efforts in scaling ferroelectric thin films with a future perspective are summarized.

The Potential of ferroelectric HfO2 for Next-Generation Memory Device: Ferroelectric Properties and Applications

The discovery of the ferroelectricity of HfO<sub>2</sub> in 2011 has opened up new avenues for the application of ferroelectric technology. With the stability of ferroelectricity in a few nm scales, HfO<sub>2</sub> has become a valuable material for the development of next-generation electronic memory devices. The unique structure of HfO<sub>2</sub> gives rise to various ferroelectric properties and behaviors that can be utilized in different types of devices such as ferroelectric field effect transistors (FeFETs), negative capacitance field effect transistors (NCFETs), ferroelectric tunnel junctions (FTJs), and ferroelectric capacitors (FeCAPs). In this review, we explore the potential of HfO<sub>2</sub> for the high-density storage of data and low-energy consumption in next-generation devices. We demonstrate the operating principles and strengths of HfO<sub>2</sub> in various device applications, shedding light on how this material can help address the electronics industry's current challenges.

Ferroelectricity in Low-Permittivity SrZrO3 Epitaxial Films

Bulk SrZrO3 has an orthorhombic perovskite structure (Pbnm) with a central symmetry but exhibits a low dielectric constant. In this study, we reported a room-temperature ferroelectric SrZrO3 thin film with a low dielectric constant, induced by compressive strain from the SrTiO3 substrate. The presence of an out-of-phase boundary structure allows SrZrO3 with a large lattice mismatch to grow epitaxially on SrTiO3 substrates. The apparent atomic displacements in the lattice are revealed by scanning transmission electron microscopy imaging. Compressive strain induces an orthorhombic-to-monoclinic phase transition, evidenced by both the atomic structure obtained from the annular bright-field image and the structure optimized with first-principles calculation. Further first-principles calculations demonstrated that compressive strain causes a polar displacement of the Zr atom inside the ZrO6 octahedron, which is the origin of the ferroelectricity. Our work shows a way to design novel ferroelectrics from conventional non-ferroelectric materials, which is of significant importance to robust, stable, and lead-free ferroelectrics applications.

Prominent Size Effects without a Depolarization Field Observed in Ultrathin Ferroelectric Oxide Membranes.

The increasing miniaturization of electronics requires a better understanding of material properties at the nanoscale. Many studies have shown that there is a ferroelectric size limit in oxides, below which the ferroelectricity will be strongly suppressed due to the depolarization field, and whether such a limit still exists in the absence of the depolarization field remains unclear. Here, by applying uniaxial strain, we obtain pure in-plane polarized ferroelectricity in ultrathin SrTiO_{3} membranes, providing a clean system with high tunability to explore ferroelectric size effects especially the thickness-dependent ferroelectric instability with no depolarization field. Surprisingly, the domain size, ferroelectric transition temperature, and critical strain for room-temperature ferroelectricity all exhibit significant thickness dependence. These results indicate that the stability of ferroelectricity is suppressed (enhanced) by increasing the surface or bulk ratio (strain), which can be explained by considering the thickness-dependent dipole-dipole interactions within the transverse Ising model. Our study provides new insights into ferroelectric size effects and sheds light on the applications of ferroelectric thin films in nanoelectronics.

Evaluation of the structure and electrical properties of (1−x)Bi0.5(Na0.80K0.20)0.5TiO3–xLiNbO3 ceramic composite for piezoelectric sensor applications

Lead-free piezoceramics ([Formula: see text])Bi[Formula: see text](Na[Formula: see text]K[Formula: see text]TiO3–xLiNbO3 (BNKT–LN) (where [Formula: see text], 0.025, 0.050 and 0.075[Formula: see text]mol fraction) were examined for their ferroelectric stability and subsequent modifications. Traditional solid-state reaction techniques were used to create the BNKT–LN piezoceramics, which were then sintered at a temperature of 1200°C. We concentrated on the structural, dielectric, piezoelectric and ferroelectric properties of the BNKT–LN ceramics. The powder X-ray diffraction pattern revealed a single perovskite structure with no secondary phases, as was observed. The temperature-dependent dielectric curves demonstrated that the maximum dielectric constant ([Formula: see text]) is present in the pure BNKT sample and diffuses to a low value when the LN concentration rises. SEM confirms the material’s surface shape, and decreasing grain sizes are seen as the LN content rises. The BNKT–LN ceramics’ P–E hysteresis loops demonstrate good ferroelectric characteristics, with a maximum at 20 kV/cm. With an increase in LN concentration, remnant polarization ([Formula: see text]) and coercive field ([Formula: see text]) show a trend of increasing, reducing and then increasing. The maximum value of the piezoelectric coefficient ([Formula: see text]) for the combined samples was 147[Formula: see text]pC/N. As a result, the lead-free ceramic BNKT (BNKT–LN) doped with LN is an auspicious choice for piezoelectric sensor applications.

Sm-Doped PIN-PMN-PT Transparent Ceramics with High Curie Temperature, Good Piezoelectricity, and Excellent Electro-Optical Properties.

Transparent piezoelectric materials are capable of coupling several physical effects such as optics, acoustics, electricity, and mechanical deformation together, which expands applications for mechanical-electro-optical multifunctional devices. However, piezoelectricity, transparency, and Curie temperature restrict each other, so it is difficult to achieve high piezoelectricity with both good transparency and a high Curie point. In this paper, Sm-doped 24Pb(In1/2Nb1/2)O3-42Pb(Mg1/3Nb2/3)O3-34PbTiO3 (PIN-PMN-PT) transparent ceramic with a high piezoelectric coefficient of 905 pC/N, excellent electro-optical coefficient of 814 pm/V, and high Curie-point of 179 °C is fabricated. Sm doping effect on the phase structures, piezoelectricity, ferroelectricity, optical transparency, electro-optical properties, and thermal stability is systematically investigated. Compared with PMN-PT transparent ceramics, PIN-PMN-PT transparent ceramics exhibit better temperature stability. Electro-optical modulation and energy conversion are achieved using PIN-PMN-PT transparent piezoelectric ceramic, which indicates that it has great potential to develop mechanical-electrical-optical multifunctional coupling devices for optical communication, energy harvesting, photoacoustic imaging, and so on.

Parameters for ferroelectric phase stabilization of sputtered undoped hafnium oxide thin films

In this work, various stabilization factors for the ferroelectric phase of undoped hafnium oxide prepared by physical vapour deposition were investigated. The capping of the thin films with platinum top electrodes before annealing, as well as the amount of oxygen during sputter deposition and subsequent annealing was shown to have a significant influence on the resulting ferroelectric properties and phase of the HfO x layer. When the prepared films were not capped, only one specific set of process parameters was found to lead to the formation of the ferroelectric phase. We conclude that capping is a crucial condition for stabilizing the ferroelectric phase. Furthermore, it is shown that the amount of oxygen supply during all fabrication processes determines the resulting ferroelectric phase fraction for capped samples. Increasing the oxygen flow during sputtering and annealing results in a larger monoclinic phase fraction and thus a decrease of the remanent polarization.

Effects of (Bi0.5Li0.5)TiO3 addition on microstructures, electrical properties and thermal stability of BiFeO3-BaTiO3 piezoelectric ceramics

Herein, lead-free 0.7BiFeO3-(0.297-x)BaTiO3-x(Bi0.5Li0.5)TiO3+0.003BiGaO3+0.01MnO2 (abbreviated as BF-BT-xBLT, 0 ≤ x ≤ 0.0096) ceramics have been prepared by conventional solid-state sintering. The phase structures, microstructures, piezoelectric, ferroelectric, dielectric, and thermal stability of these materials were investigated. The XRD analysis show that rhombohedral and tetragonal phases coexist in the phase structure of BF-BT-xBLT ceramic. As the BLT concentration rises, the grain size first increases to 8.6 μm and then decreases to 6.14 μm. The grain size decreased from 7.68 μm to 5.02 μm with the decrease of sintering temperature. The piezoelectric, ferroelectric and dielectric properties of the ceramic increase first and then decrease as BLT concentration increases. The optimal electrical properties are obtained by adding 0.0012 BLT, piezoelectric constant d33 = 152 pC/N, electromechanical coupling coefficient kp = 0.321, mechanical quality factor Qm = 58.184, remnant polarization Pr = 25.07 μC/cm2, Curie temperature Tc = 485 °C and depolarization temperature Td = 465 °C. The BLT addition concentration may modify the aforementioned BF-BT ceramic structures and properties, while the decrease of sintering temperature from 960 °C to 880 °C causes modest decreases of relative density of BF-BT-0.0072BLT ceramics from 94.73% to 93.56% and d33 from 146 pC/N to 135 pC/N. Enhanced piezoelectric properties of d33 = 135 pC/N, kp = 0.279, and Qm = 63.713 are obtained for BF-BT-0.0072BLT ceramic even at an extremely low sintering temperature of 880 °C. In addition, the d33 of BF-BT-0.0012BLT ceramic reached 392 pC/N at 352 °C by in-situ dynamic d33 testing. This indicates the piezoelectric properties of the piezoelectric ceramic in its actual operating state. The main novelty of this study is the relatively high piezoelectric properties obtained at low sintering temperature and the d33 reaching 392 pC/N at 352 °C. The findings suggest that the ceramic is a viable material for the use of lead-free piezoelectric ceramics.

A Perspective on ferroelectricity in hafnium oxide: Mechanisms and considerations regarding its stability and performance

Ferroelectric hafnium oxides are poised to impact a wide range of microelectronic applications owing to their superior thickness scaling of ferroelectric stability and compatibility with mainstream semiconductors and fabrication processes. For broad-scale impact, long-term performance and reliability of devices using hafnia will require knowledge of the phases present and how they vary with time and use. In this Perspective article, the importance of phases present on device performance is discussed, including the extent to which specific classes of devices can tolerate phase impurities. Following, the factors and mechanisms that are known to influence phase stability, including substituents, crystallite size, oxygen point defects, electrode chemistry, biaxial stress, and electrode capping layers, are highlighted. Discussions will focus on the importance of considering both neutral and charged oxygen vacancies as stabilizing agents, the limited biaxial strain imparted to a hafnia layer by adjacent electrodes, and the strong correlation of biaxial stress with resulting polarization response. Areas needing additional research, such as the necessity for a more quantitative means to distinguish the metastable tetragonal and orthorhombic phases, quantification of oxygen vacancies, and calculation of band structures, including defect energy levels for pure hafnia and stabilized with substituents, are emphasized.