Properties Of Ferroelectric Films Research Articles

Influence of oxygen pressure on the ferroelectricity of pulsed laser deposition fabricated epitaxial Y-doped HfO2

Ferroelectric properties of hafnium-based thin films have gained significant interest, yet the fundamental mechanisms responsible for the emergence of the ferroelectric phase continue to be inadequately investigated. In contrast with polycrystalline films fabricated by atomic layer deposition or sputter methods, which possess uncertainty in polarization orientation, epitaxial ferroelectric HfO2-based materials are less investigated, especially for factors such as electric field and oxygen vacancy, which are proposed and examined for their potential impacts on phase stability. In this study, Y-doped hafnium oxide (HYO) ferroelectric epitaxial films were fabricated using pulsed laser deposition, with variations in oxygen pressure during the deposition process. Structural and electrical analyses of HYO epitaxial ferroelectric films prepared under differing oxygen pressures revealed a correlation between the ferroelectric properties of the films and the oxygen content. An optimal selection of oxygen pressure was found to be conducive to the formation of HYO epitaxial ferroelectric films, presenting a promising avenue for future ferroelectric memory applications.

On the Evolution of Stress and Microstructure in Radio Frequency-Sputtered Lead-Free (Ba,Ca)(Zr,Ti)O3 Thin Films

Thin-film piezoelectrics are widely investigated for actuators and energy harvesters, but there are few alternatives to toxic lead zirconate titanate. Biocompatible Ca- and Zr-modified BaTiO3 (BCZT) is one of the most promising lead-free alternatives due to its high piezoelectric response. However, the dielectric/piezoelectric properties and structural integrity of BCZT films, which are crucial for their applications, are strongly influenced by the substrate upon which the film is grown and the related processing methods. Here, the in-plane stress, microstructure, dielectric, and piezoelectric properties of 100–500 nm thick high-temperature RF-sputtered BCZT films on industrially relevant Si-based substrates were investigated. Obtaining polycrystalline piezoelectric films required deposition temperatures ≥ 700 °C, but this induced tensile stresses of over 1500 MPa, which caused cracking in all films thicker than 200 nm. This degraded the dielectric, piezoelectric, and ferroelectric properties of films with larger electrode areas for applications. Films on SrTiO3, on the other hand, had a compressive residual stress, with fewer defects and no cracks. The grain size and surface roughness increased with increasing deposition temperature. These findings highlight the challenges in processing BCZT films and their crucial role in advancing lead-free piezoelectric technologies for actual device applications.

An Adaptation of the Split-Cylinder Resonator Method for Measuring the Microwave Properties of Thin Ferroelectric Films in a “Thin Film—Substrate” Structure

The split-cylinder resonator method was adapted to measure the microwave properties (dielectric permittivity and loss tangent) of thin ferroelectric films on a dielectric substrate. The mathematical model for calculating the resonance frequency of the split-cylinder resonator was adjusted for the “ferroelectric film—substrate” structure. An approach for correcting the gap effect based on calibrating with a single-layer dielectric was introduced and used to study two-layer dielectrics. The prototype of a split-cylinder resonator designed to measure single-layer dielectric plates at a frequency of 10 GHz was presented. The resonator calibration was performed using dielectric PTFE samples and fused silica, and an example of the correction function was suggested. The measurement error was estimated, and recommendations on the acceptable parameter range for the material under investigation were provided. The method was demonstrated to measure the microwave properties of a ferroelectric film on a fused silica substrate.

Surface Effect of Thickness-Dependent Polarization and Domain Evolution in BiFeO3 Epitaxial Ultrathin Films.

With the trend of device miniaturization, ultrathin ferroelectric films are gaining more and more attention. However, understanding ferroelectricity in this nanoscale context remains a formidable challenge, primarily due to the heightened relevance of surface effects, which often leads to the loss of net polarization. Here, the influence of surface effects on the polarization as a function of thickness in ultrathin BiFeO3 films is investigated using phase-field simulations. The findings reveal a notable increase in ferroelectric polarization with increasing thickness, with a particularly discernible change occurring below the 10 nm threshold. Upon accounting for surface effects, the polarization is marginally lower than the case without such considerations, with the disparity becoming more pronounced at smaller thicknesses. Moreover, the hysteresis loop and butterfly loop of the ultrathin film were simulated, demonstrating that the ferroelectric properties of films remain robust even down to a thickness of 5 nm. Our investigations provide valuable insights into the significance of ferroelectric thin films in device miniaturization.

Machine Learning-Enabled Superior Energy Storage in Ferroelectric Films with a Slush-Like Polar State.

Heterogeneities in structure and polarization have been employed to enhance the energy storage properties of ferroelectric films. The presence of nonpolar phases, however, weakens the net polarization. Here, we achieve a slush-like polar state with fine domains of different ferroelectric polar phases by narrowing the large combinatorial space of likely candidates using machine learning methods. The formation of the slush-like polar state at the nanoscale in cation-doped BaTiO3 films is simulated by phase field simulation and confirmed by aberration-corrected scanning transmission electron microscopy. The large polarization and the delayed polarization saturation lead to greatly enhanced energy density of 80 J/cm3 and transfer efficiency of 85% over a wide temperature range. Such a data-driven design recipe for a slush-like polar state is generally applicable to quickly optimize functionalities of ferroelectric materials.

The Vertically Heteroepitaxial Structure for Lead-Free Piezoelectric K0.5Na0.5NbO3 Films

The effect of epitaxial strain on the electrical properties of ferroelectric films has been widely investigated. However, this kind of strain is generally attributed to the substrate clamping constraints and is easily relaxed when the thickness of films is over 100 nm. In this work, a vertically epitaxial strain was introduced into lead-free piezoelectric K0.5Na0.5NbO3 films to improve the electrical properties of ferroelectric films. Two-phase, vertically epitaxial composite KNN-ZnO thin films were grown on the (001) STO substrate using a pulsed laser deposition (PLD) method. The highly (001) preferentially oriented KNN phase and (112¯ 0)-oriented ZnO phase were orderly arranged. Two types of morphologies of “square-like” and “stripe-looking” grains were observed in the surface image. An asymmetric “square” out-of-plane phase hysteresis loop and a “butterfly” displacement loop were exhibited in the KNN phase, whereas the ZnO phase showed a closed phase hysteresis loop and a slim displacement-voltage loop. Two different kinds of polarization behaviors for domains were also observed under applied electric fields, in which the domain of the KNN phase is easily switched to the opposite state, whereas the ZnO phase keeps a stable domain state when applying a DC bias of ±50 V. the vertically epitaxial growth of the KNN-ZnO composited films in this work provides a new way to fabricate complex nanoscale materials.

Hierarchical heterostructure design to enhance energy storage properties of ferroelectric films at different electric fields

Polarization and breakdown strength as two sides of one coin are difficult to balance in one dielectric material, and so hierarchical heterostructure would be a good way to combine the advantages of each material enhancing energy density. In this work, high polarization Bi(Mg0.5Zr0.5)O3 (BMZ) and high breakdown strength Ba(Zr0.25Ti0.75)O3 (BZT) are chosen to prepare 1BMZ/4BZT/1BMZ (Type-A), 3BMZ/3BZT (Type-B) and 3(BMZ/BZT) (Type-C) hierarchical heterostructures thin films on Pt/Ti/SiO2/Si substrate via sol-gel method. Microstructure characteristics of thin films indicate that they possess well density, no crack and smooth surface. Thin films with Type-B structure possess a high dielectric constant of ∼500, which exceeds ∼150 of that with Type-A and Type-C structures. For energy storage properties, Type-B and Type-C thin films achieve high recoverable energy density Wrec of 85.6 J/cm3 and 85.7 J/cm3 at low field of 2545 kV/cm and high field of 4980 kV/cm, respectively. The design of hierarchical heterosrtucture would break the limitation of low energy density in single material, and provide a new thinking for the applications of dielectric energy storage materials.

Synergistic effect of Si concentration and distribution on ferroelectric properties optimization of Si:HfO2 ferroelectric thin films

In this paper, a phase-field model of Si-doped hafnium oxide-based ferroelectric thin films is established. And then, the synergistic effect of Si concentration and distribution on ferroelectric properties optimization of Si:HfO2 ferroelectric thin films is studied with the proposed model. It is found that no matter how Si dopant is distributed in the film, the volume fraction of the ferroelectric phase in the film increases first and then decreases with the increase of Si concentration. However, compared with the uniform distribution, the layered distribution is more likely to great improve ferrelectric properties. When Si dopant is uniformly distributed in the film, the highest remanent polarization value that the film can obtain via Si concentration modulation is 38.7 μC cm−2, and the corresponding Si concentration is 3.8 cat%, which is consistent with the experimental results. When Si dopant is layered in the film, and the concentration difference between the Si-rich and Si-poor layers is 7.6%, in the Si concentration range of 3.6 cat%–3.8 cat%, the residual polarization of the film reaches 46.4–46.8 μC cm−2, which is 20% higher than that when Si dopant are evenly distributed in the film. The above results show that selecting the Si layered distribution mode and controlling the concentration difference between Si-rich and Si-poor layers in an appropriate range can greatly improve the films’ ferroelectric properties and broaden the Si concentration optimization range of the ferroelectric properties of the films. The result provides further theoretical guidance on using Si doping to adjust the ferroelectric properties of hafnium oxide-based films.

Top Electrode Engineering for Freedom in Design and Implementation of Ferroelectric Tunnel Junctions Based on Hf1–xZrxO2

Ferroelectric tunnel junctions (FTJs) based on ultrathin HfO2 have great potential as a fast and energy-efficient memory technology compatible with complementary metal oxide semiconductors. FTJs consist of a ferroelectric film sandwiched between two distinct electrodes, the properties of which are intricately linked to the electrical properties of the FTJs. Here we utilize a W crystallization electrode (CE) to achieve a high and reproducible remanent polarization, combined with a metal replacement process in which the W is carefully removed and replaced by another top electrode (TE). In this way we separate the ferroelectric film properties from the device design and can thereby evaluate the effect of the TE work function (WF) and conduction band electron density (ne) on the tunneling electroresistance (TER) and device reliability. We compare FTJs designed with a TiN bottom electrode and W, Cr, or Ni TE and find that the use of high electron density metals such as Ni or Cr as TE allows for an improved TER, albeit at the cost of reliability due to a large built-in electric field. To bypass this effect, a bilayer Cr/Ni TE is implemented, which allows for a high TER and minimal built-in field, leading to excellent retention and endurance beyond 108 cycles. The results presented here thus highlight a process flow for reliable design and implementation of FTJs.

Enhanced Tunability of BaTixSn1−xO3 Films on Dielectric Substrate

The structural properties of ferroelectric films of barium titanate-stannate on alumina substrates and the microwave characteristics of planar capacitive elements based on them are studied. It is established that the composition of the gas medium and the temperature of the substrate during the deposition of the film has a significant effect on the crystal structure, phase composition of the films and their electrical characteristics. Planar capacitors based on films subjected to high-temperature annealing after deposition exhibit 85% tunability at a frequency of 2 GHz, which is the best result for today.

Deposition-rate-dependent ferroelectric properties of epitaxially grown BaTiO3 thin films

We report the deposition-rate-dependent ferroelectric properties of BaTiO3 thin films fabricated through pulsed laser deposition. BaTiO3 thin films were epitaxially grown on single-crystal (001) Rh substrates. The BaTiO3 thin films exhibited strained tetragonal structures with c/a ratios of 1.05 (0.02 nm/pulse) and 1.03 (0.3 nm/pulse), depending on the film deposition rate. The increased tetragonality enhances the ferroelectric properties of the films. In addition, their deposition rate determines their leakage current and bandgap energy. Using scanning probe microscopy, we observed the surface morphologies and ferroelectric domain structures of the films. Furthermore, a ferroelectric domain analysis based on the Landau–Lifshitz–Kittel scaling law indicated that a reduction in the growth rate leads to an increase in the domain size and domain wall energy of BaTiO3 thin films. Finally, it was demonstrated that the optical bandgap of BaTiO3 thin films is related to their tetragonality.

Irradiation behavior and recovery effect of ferroelectric properties of PZT thin films* *Project supported by the National Natural Science Foundation of China (Grant No. 51802056), the Science Foundation of the National Key Laboratory of Science and Technology on Advanced Composites in Special Environments, and the Key Laboratory of Micro-systems and Micro-structures Manufacturing (Harbin Institute of Technology), Ministry of Education, China.

Lead zirconate titanate piezoelectric ceramics have important applications in space and aerospace technology, but the effect and physical mechanism of charged particle radiation on their performance yet to be clarified. In this study, we characterized PbZr0.52Ti0.48O3 (PZT) thin films, and changes in the ferroelectric properties of the films before and after electron and proton irradiation were investigated. The natural and heat treatment recoverability of the ferroelectric properties were studied, and the damages and mechanisms of different types of radiation in PZT films were also investigated. The results show that, in addition to ionization damages, electron irradiation causes certain structural damage on the PZT film, and the large structural damage caused by proton irradiation reduces drastically the ferroelectricity of the PZT film.

Enhanced photovoltaic-pyroelectric coupled effect of BiFeO3/Au/ZnO heterostructures

The photovoltaic properties of ferroelectric films and ferroelectric-semiconductor heterostructures have been extensively studied due to their application prospects in photodetection, energy conversion and nonvolatile storage. However, light-induced pyroelectric effect accompanied by the photovoltaic effect of ferroelectric-semiconductor heterostructures lacks sufficient attention. Herein, BiFeO 3 (BFO)/Au/ZnO heterostructures are constructed by controlling sputtering time of Au nanoparticles and the regulation mechanism of integrating pyroelectric and photovoltaic effects is explored. The short-circuit photocurrent density of the optimal BFO/Au/ZnO heterostructures is nearly 1.4 times that of BFO/ZnO heterostructures under 405 nm light illumination. Moreover, BFO/Au/ZnO heterostructures broaden the response wavelength from 360 nm to 1060 nm light beyond the bandgap limitation. Compared with merely the photovoltaic effect found in BFO films and BFO/ZnO heterostructures, BFO/Au/ZnO heterostructures display the photovoltaic-pyroelectric coupled effect that is associated with light-induced temperature variation rate and band alignment by introducing Au nanoparticles into BFO and ZnO layers. The implementation of the photovoltaic-pyroelectric coupled effect endows BFO/Au/ZnO heterostructures with outstanding photoelectric performances modulated by self-polarization phenomenon and band bending. This work not only provides in-depth understanding about the photovoltaic-pyroelectric coupled effect of ferroelectric-metal-semiconductor heterostructures, but also provides an effective and facile route to design high-performance photoelectric related devices with wideband response. The improvement in photovoltaic performances of ferroelectric-semiconductor heterojunctions is ascribed to the construction of energy band structure tuning, whereas ferroelectric-semiconductor heterojunctions have negligible light-induced pyroelectric effect. In this work, the introduction of Au nanoparticles into BFO and ZnO layers not only endows the BFO/Au/ZnO heterostructures with broadened response wavelength from 360 nm to 1060 nm light beyond the bandgap limitation, but also integrates light-induced pyroelectric effect and photovoltaic effect. BFO/20 s Au/ZnO heterostructures display the optimal photoelectric performances and appreciable pyroelectric effect governed by self-polarization phenomenon and band bending. • Au nanoparticles modified BiFeO 3 /Au/ZnO heterostructures display improved photoelectric properties and broadband response. • The integration of light-induced pyroelectric and photovoltaic effects of BiFeO 3 /Au/ZnO heterostructures is manipulated. • The photoelectric properties of BiFeO 3 /Au/ZnO heterostructures are governed by self-polarization phenomenon and band bending.

Anisotropic in-plane dielectric and ferroelectric properties of tensile-strained BaTiO3 films with three different crystallographic orientations

Ferroelectric properties of films can be tailored by strain engineering, but a wider space for property engineering can be opened by including crystal anisotropy. Here, we demonstrate a huge anisotropy in the dielectric and ferroelectric properties of BaTiO3 films. Epitaxial BaTiO3 films deposited on (100), (110), and (111) SrTiO3 substrates were fabricated by chemical solution deposition. The films were tensile-strained due to thermal strain confirmed by the enhanced Curie temperature. A massive anisotropy in the dielectric constant, dielectric tunability, and ferroelectric hysteresis loops was observed depending on the in-plane direction probed and the orientation of the films. The anisotropy was low for (111) BaTiO3, while the anisotropy was particularly strong for (110) BaTiO3, reflecting the low in-plane rotational symmetry. The anisotropy also manifested at the level of the ferroelectric domain patterns in the films, providing a microscopic explanation for the macroscopic response. This study demonstrates that the properties of ferroelectric films can be tailored not only by strain but also by crystal orientation. This is particularly interesting for multilayer stacks where the strain state is defined by the boundary conditions. We propose that other materials can be engineered in a similar manner by utilizing crystal anisotropy.

Optimizing Annealing Process for Ferroelectric Y‐Doped HfO2 Thin Films by All‐Inorganic Aqueous Precursor Solution

Abstract10 nm thick yttrium doped HfO2 (Y:HfO2) thin films are prepared on Si (100) substrates by the chemical solution deposition method using all‐inorganic aqueous salt precursors. The influence of the annealing process, consisting of annealing temperature, holding time, and heating rate, on the crystalline structure and ferroelectric properties of thin films is investigated. Results show that the crystalline structure and ferroelectric properties of films exhibit a strong annealing process dependence. The monoclinic phase and asymmetric orthorhombic phase coexist in the films. The annealing process of the best ferroelectric behavior is obtained by using annealing temperature at 700 °C for 30 s with a heating rate of 30 °C s−1 in N2 atmosphere. The film exhibits lowest m‐phase fraction of 17.9%, accompanied with the highest remanent polarization of 21.4 µC cm−2.

Significant improvement of ferroelectricity and reliability in Hf0.5Zr0.5O2 films by inserting an ultrathin Al2O3 buffer layer

Due to the full compatibility with modern complementary-metal-oxidesemiconductor (CMOS) technology and scalable capability, HfO2-based ferroelectric films have been considered as the most potential materials in micro-nano non-volatile memories. However, despite great achievements, the existence of poor interface still poses a great threat to the improve of ferroelectricity and polarization stability. Here, the excellent ferroelectricity and reliability in the 21.6 nm-thick Hf0.5Zr0.5O2 (HZO) films are reported through interface modification by inserting an ultrathin Al2O3 (AO) layer using atomic layer deposition method. Improved polarization and stability are achieved in HZO/AO bilayers with the AO thickness below 2.4 nm. When the thickness of AO dielectric layer is 1.6 nm, the maximum remnant polarization reaches 30.19 μC/cm2 and the leakage current is decreased by 2 ~ 3 orders of magnitude. The strong interface polarization resulted from the electrostatic coupling between HZO and AO films contributes to the improved ferroelectricity of HZO films. Furthermore, the AO layer, as a dielectric capacitor after switching or during non-switching, can block the electron injection from bottom electrode and therefore improve the cycling reliability. This work demonstrates that an inlaid dielectric layer with much lower dielectric constant can significantly enhance the electrical properties of ferroelectric films.

Thickness-dependent evolution of piezoresponses and a/c domains in [101]-oriented PbTiO3 ferroelectric films

It is known that high-index perovskite ferroelectric thin films exhibit giant permittivity, piezoelectric response, and a particular switching behavior. However, the fine domain configuration in high-index ferroelectric films is not yet clarified, which triggers difficulties to further modulate their electric properties. In this work, we focus on the tetragonal PbTiO3 thin films with thicknesses of 20, 40, 55, and 70 nm deposited on a [101]-oriented KTaO3 substrate. By using piezoresponse force microscopy and state-of-the-art scanning transmission electron microscopy, the stripe a/c domains with alternately wide c and narrow a domains are observed in all these PbTiO3 films. The periodic stripe a/c domains with {101} domain walls extend along the in-plane [111¯] or [11¯1¯] direction, which almost completely relaxes the misfit strain between the PbTiO3 films and the KTaO3 substrate. The domain width decreases as the film thickness is reduced following the square root dependence. This results in an increase of ferroelastic a/c domain walls and promotes the enhancement of the piezoresponse amplitude for the thinner PbTiO3 films. In addition, the piezoresponse amplitude of a 20 nm PbTiO3 film is comparable to that of a 40 nm PbTiO3 film, which indicates that the piezoelectric response of ferroelectric films may saturate at a certain film thickness and scarcely increase even after the film thickness reduces further. These results clarify the domain configurations of [101]-oriented PbTiO3 thin films and provide useful information for understanding the relationship between microstructures and piezoelectric properties in ferroelectric films.

Optics of epitaxial strained strontium titanate films

Diverse effects of lattice strain on the optical properties in the near-infrared to vacuum ultraviolet spectral range are experimentally revealed in cube-on-cube-type epitaxial perovskite SrTiO3 films grown on compressive substrate. Compared to the reference crystal, the tetragonal antiferrodistortive film exhibits spectral blueshifts, which are consistent with the theoretically predicted bandgap widening. In addition to this strain-induced interband effect, the peculiar near-edge absorption and smearing of interband transitions are found. It is shown that Fröhlich-type electron–phonon coupling can be enhanced by strain and lead to this behavior. It is suggested that electron–phonon interactions can play an important role in optical properties of ferroelectric films and deserve further studies.

Strain-induced room-temperature ferroelectricity in SrTiO3 membranes

Advances in complex oxide heteroepitaxy have highlighted the enormous potential of utilizing strain engineering via lattice mismatch to control ferroelectricity in thin-film heterostructures. This approach, however, lacks the ability to produce large and continuously variable strain states, thus limiting the potential for designing and tuning the desired properties of ferroelectric films. Here, we observe and explore dynamic strain-induced ferroelectricity in SrTiO3 by laminating freestanding oxide films onto a stretchable polymer substrate. Using a combination of scanning probe microscopy, optical second harmonic generation measurements, and atomistic modeling, we demonstrate robust room-temperature ferroelectricity in SrTiO3 with 2.0% uniaxial tensile strain, corroborated by the notable features of 180° ferroelectric domains and an extrapolated transition temperature of 400 K. Our work reveals the enormous potential of employing oxide membranes to create and enhance ferroelectricity in environmentally benign lead-free oxides, which hold great promise for applications ranging from non-volatile memories and microwave electronics.

Effect of annealing protection atmosphere on the ferroelectric yttrium doped hafnium oxide thin films

The 10 nm thick yttrium doped hafnium oxide (Y:HfO2) thin films, prepared by chemical solution deposition which using all-inorganic aqueous salt reagents, were fabricated on Si (100) substrates. The crystalline structure, chemical composition and ferroelectric properties of thin films, annealed in protection atmosphere of Air, Ar and N2, were examined. Result showed that the crystalline structure and ferroelectric properties of films exhibited a strong annealing protection atmosphere dependence. When compared to annealing protection atmosphere of Air and Ar, the films with the N2 exhibited lowest m-phase fraction of 19.4%, and the highest oxygen vacancy percentage content of 3.06%, accompanied with the highest relative permittivity of 50.9 and the remanent polarization of 14.6 μC/cm2. These excellent ferroelectric properties were correlated with asymmetric orthorhombic phase and the concentration of oxygen vacancy introduced from the nitrogen doping concentration.