Ultrathin Ferroelectric Films Research Articles

Switchable Rashba effect by dipole moment switching in an Ag2Te monolayer

Because of the surface depolarization field, there is a critical thickness for ferroelectricity in ultrathin ferroelectric films, hindering miniaturization of high-density nonvolatile memory storage devices. A controllable Rashba effect by external electric field via switchable dipole moment could be a promising way to control and manipulate the spin degrees of freedom in spintronics. Here, based on first principles calculations, we show that non-planar Ag2Te monolayer, which has been recently predicted to be a topological insulator, possess a switchable out-of-plane electric dipole moment. The switching of the dipole can be realized by the penetration of Te atoms through the hexagonal Ag-plane. Additionally, non-planar Ag2Te shows a giant Rashba spin-splitting ( eV Å) due to the out-of-plane electric dipole moment. Our tight binding model indicates that the origin of such large is the large inversion symmetry breaking term ( eV), which is one order of magnitude larger in non-planar Ag2Te monolayer compared with other Rashba materials. Interestingly, the Rashba effect can be turned on/off by the phase transition from non-planar to planar structure via Te displacement. Moreover, the spin-texture can be completely reversed because of switchable electric dipole moment. Our work shows a new way to realize ferroelectric-like dipole moment switching and consequently switchable Rashba spin-splitting, which may facilitate a nonvolatile electrical control of the spin degrees of freedom, down to the monolayer thickness, promising potential applications to electrically controlled spintronic devices.

Elasticity Modulation Due to Polarization Reversal and Ionic Motion in the Ferroelectric Superionic Conductor KTiOPO4.

The coupling between ionic degrees of freedom and ferroelectricity has received renewed attention in recent years, given that surface electrochemical processes have been shown to be intrinsically linked to ferroelectric phase stability in ultrathin ferroelectric films. However, the coupling between bulk ionic transport and local polarization switching has received less attention, as typically the bulk ionic mobilities are low for common ferroelectrics at room temperature. Here, we use the coupled band-excitation method in conjunction with site-correlated time-of-flight secondary ion mass spectrometry, to determine the coupling between ferroelectric switching and ionic motion in single crystal KTiOPO4. The local scanning probe measurements indicate a substantial softening, as determined by resonant frequency changes, during reversal of polarization along one direction. These changes are correlated with the mass spectrometry measurements, showing a polarization-dependent accumulation of K ions at the polar surfaces, thus corroborating their role in the screening process. These studies shed light on the interplay between ionic dynamics and bulk ferroelectric switching and have implications for studies on domain wall conductivity, chemical switching, and bulk and surface-screening phenomena.

Electrodynamics of ferroelectric films with negative capacitance

We construct a comprehensive theory of the electrodynamic response of ferroelectric ultra-thin films containing periodic domain textures (PDT) with 180{\deg} polarization-oriented domains. The focal point of the theory is the negative-capacitance phenomenon which naturally arises from the depolarization field induced by PDT. We derive frequency-dependent dielectric permittivity related to the PDT dynamics across the entire frequency range. We find the resonance mode of domain oscillations in the THz spectral band and the singular points in the phase of the reflected THz beam that are intimately related to the negative capacitance. Our findings provide a platform for the THz negative capacitance-based optics of ferroelectric films and for engineering the epsilon-near-zero plasmonic THz metamaterials.

Model of dielectric breakdown in hafnia-based ferroelectric capacitors

Ultra-thin ferroelectric hafnia-based thin films are very promising candidates for nanoscale ferroelectric random access memories. However, dielectric breakdown is a main failure mechanism during repeated polarization switching. Generalizing Lou et al.'s local phase decomposition model, originally for ferroelectric fatigue, we propose a dielectric breakdown model for ferroelectric hafnia. While charging injection during the polarization reversal is regarded as a key step, eventual phase separation of the Hf cluster accounts for the dielectric breakdown. Using this model, we explain why TaN/HfO2/TaN ferroelectric capacitors are more prone to dielectric breakdown than TiN/HfO2/TiN, and conclude that the lower Schottky barrier for the TaN/Pca21-HfO2 interface stabilizes neutral oxygen vacancies within the dielectric. On the other hand, when TiN electrodes are employed, oxygen vacancies tend to be positively charged. They can further pin the domain walls, resulting in ferroelectric fatigue. The relationship between the conductive filament formation, dielectric breakdown, wake up, and fatigue in ferroelectric HfO2 is discussed within the framework of our model.

Accumulative Polarization Reversal in Nanoscale Ferroelectric Transistors.

The electric-field-driven and reversible polarization switching in ferroelectric materials provides a promising approach for nonvolatile information storage. With the advent of ferroelectricity in hafnium oxide, it has become possible to fabricate ultrathin ferroelectric films suitable for nanoscale electronic devices. Among them, ferroelectric field-effect transistors (FeFETs) emerge as attractive memory elements. While the binary switching between the two logic states, accomplished through a single voltage pulse, is mainly being investigated in FeFETs, additional and unusual switching mechanisms remain largely unexplored. In this work, we report the natural property of ferroelectric hafnium oxide, embedded within a nanoscale FeFET, to accumulate electrical excitation, followed by a sudden and complete switching. The accumulation is attributed to the progressive polarization reversal through localized ferroelectric nucleation. The electrical experiments reveal a strong field and time dependence of the phenomenon. These results not only offer novel insights that could prove critical for memory applications but also might inspire to exploit FeFETs for unconventional computing.

Polarization switching behavior of Hf–Zr–O ferroelectric ultrathin films studied through coercive field characteristics

The electrical properties of ferroelectric Hf–Zr–O ultrathin films, particularly the dependences of remnant polarization, leakage current, coercive field, and breakdown field on the metal composition and film thickness, are systematically examined. Physical analyses show that the Hf–Zr–O films in this experiment consist of polycrystalline grains and contain both ferroelectric and dielectric phases. It is found that changes in metal composition and thickness strongly influence the remnant polarization and the leakage current simultaneously. In contrast, the coercive field was relatively unaffected by these parameters. This particular behavior of the coercive field suggests that the polarization switching in Hf–Zr–O films is predominantly determined by the nature of nanometer-scale ferroelectric domains dispersed in the films.

Giant Polarization Sustainability in Ultrathin Ferroelectric Films Stabilized by Charge Transfer.

Ferroelectricity is generally deteriorated or even vanishes when the ferroelectric films are downsized to unit cell scale. To maintain and enhance the polarization in nanoscale ferroelectrics are of scientific and technological importance. Here, giant polarization sustainability is reported in a series of ultrathin PbTiO3 films scaled down to three unit cells grown on NdGaO3 (110) substrates with La0.7 Sr0.3 MnO3 as bottom electrodes. Atomic mappings via aberration-corrected scanning transmission electron microscopy demonstrate the robust ferroelectricity for the sub-10 nm thick film. For the 1.2 nm thick film, the polarization reaches ≈50 µC cm-2 . The 2 nm thick film possesses a polarization as high as the bulk value. The films ranging from 10 to 35 nm display a giant elongation of out-of-plane lattice parameter, which corresponds to a polarization of 100 µC cm-2 , 20% larger than that of the bulk PbTiO3 . The giant enhancement of polarization in the present films is proposed to result from the charge transfer at the La0.7 Sr0.3 MnO3 /PbTiO3 interface, as supported by the anomalous decrease of Mn valence measured from X-ray photoelectron spectroscopy. These results reveal the significant role of charge transfer at interfaces in improving large polarizations in ultrathin ferroelectrics and are meaningful for the development of future electronic devices.

Nanoscale Bubble Domains and Topological Transitions in Ultrathin Ferroelectric Films.

Observation of a new type of nanoscale ferroelectric domains, termed as "bubble domains"-laterally confined spheroids of sub-10 nm size with local dipoles self-aligned in a direction opposite to the macroscopic polarization of a surrounding ferroelectric matrix-is reported. The bubble domains appear in ultrathin epitaxial PbZr0.2 Ti0.8 O3 /SrTiO3 /PbZr0.2 Ti0.8 O3 ferroelectric sandwich structures due to the interplay between charge and lattice degrees of freedom. The existence of the bubble domains is revealed by high-resolution piezoresponse force microscopy (PFM), and is corroborated by aberration-corrected atomic-resolution scanning transmission electron microscopy mapping of the polarization displacements. An incommensurate phase and symmetry breaking is found within these domains resulting in local polarization rotation and hence impart a mixed Néel-Bloch-like character to the bubble domain walls. PFM hysteresis loops for the bubble domains reveal that they undergo an irreversible phase transition to cylindrical domains under the electric field, accompanied by a transient rise in the electromechanical response. The observations are in agreement with ab-initio-based calculations, which reveal a very narrow window of electrical and elastic parameters that allow the existence of bubble domains. The findings highlight the richness of polar topologies possible in ultrathin ferroelectric structures and bring forward the prospect of emergent functionalities due to topological transitions.

Excellent low-voltage operating flexible ferroelectric organic transistor nonvolatile memory with a sandwiching ultrathin ferroelectric film

The high operating voltage is a primary issue preventing the commercial application of the ferroelectric organic field-effect transistor (Fe-OFET) nonvolatile memory (NVM). In this work, we propose a novel route to resolve this issue by employing two ultrathin AlOX interfacial layers sandwiching an ultrathin ferroelectric polymer film with a low coercive field, in the fabricated flexible Fe-OFET NVM. The operation voltage of Fe-OFET NVMs decreases with the downscaling thickness of the ferroelectric film. By inserting two ultrathin AlOX interfacial layers at both sides of the ultrathin ferroelectric film, not only the gate leakage is prominently depressed but also the mobility is greatly improved. Excellent memory performances, with large mobility of 1.7 ~ 3.3 cm2 V−1 s−1, high reliable memory switching endurance over 2700 cycles, high stable data storage retention capability over 8 × 104 s with memory on-off ratio larger than 102, are achieved at the low operating voltage of 4 V, which is the lowest value reported to data for all Fe-OFET NVMs. Simultaneously, outstanding mechanical fatigue property with the memory performances maintaining well over 7500 bending cycles at a bending radius of 5.5 mm is also achieved in our flexible FE-OFET NVM.

Giant electrocaloric effect in ferroelectric ultrathin films at room temperature mediated by flexoelectric effect and work function

In ferroelectric ultrathin films, built-in electric fields are often present due to the flexoelectric effect and the difference of work functions at asymmetric electrodes, which may change the properties of the materials. In this paper, the influence of build-in electric fields induced by flexoelectric effect and/or work function difference on the misfit strain-temperature phase diagrams, and the electrocaloric properties of epitaxial BaTiO3 ultrathin films are investigated by using an extended nonlinear thermodynamic theory. It is found that the flexoelectric effect, i.e., the coupling of polarization and strain gradient, changes the misfit strain-temperature phase diagrams notably, in which the phases with out-of-plane polarizations increase due to the presence of a built-in field. The electrocaloric properties are remarkably enhanced when the built-in fields induced by both the flexoelectric effect and work function difference are considered. In particular, a giant adiabatic temperature change of 7.89 K in ultrathin Pt/BaTiO3/SrRuO3 capacitors at 460 K is predicted. Moreover, it is demonstrated that the peak of adiabatic temperature change versus working temperature is shifted from a high temperature to room temperature, suggesting that ferroelectric ultrathin films with asymmetric electrodes and strain gradient are promising candidates for room temperature refrigeration.

Possible absence of critical thickness and size effect in ultrathin perovskite ferroelectric films

Although the size effect in ferroelectric thin films has been known for long time, the underlying mechanism is not yet fully understood and whether or not there is a critical thickness below which the ferroelectricity vanishes is still under debate. Here, we directly measure the thickness-dependent polarization in ultrathin PbZr0.2Ti0.8O3 films via quantitative annular bright field imaging. We find that the polarization is significantly suppressed for films <10-unit cells thick (∼4 nm). However, approximately the polarization never vanishes. The residual polarization is ∼16 μCcm−2 (∼17%) at 1.5-unit cells (∼0.6 nm) thick film on bare SrTiO3 and ∼22 μCcm−2 at 2-unit cells thick film on SrTiO3 with SrRuO3 electrode. The residual polarization in these ultrathin films is mainly attributed to the robust covalent Pb–O bond. Our atomic study provides new insights into mechanistic understanding of nanoscale ferroelectricity and the size effects.

Giant Ferroelectric Polarization in Ultrathin Ferroelectrics via Boundary-Condition Engineering.

Tailoring and enhancing the functional properties of materials at reduced dimension is critical for continuous advancement of modern electronic devices. Here, the discovery of local surface induced giant spontaneous polarization in ultrathin BiFeO3 ferroelectric films is reported. Using aberration-corrected scanning transmission electron microscopy, it is found that the spontaneous polarization in a 2 nm-thick ultrathin BiFeO3 film is abnormally increased up to ≈90-100 µC cm-2 in the out-of-plane direction and a peculiar rumpled nanodomain structure with very large variation in c/a ratios, which is analogous to morphotropic phase boundaries (MPBs), is formed. By a combination of density functional theory and phase-field calculations, it is shown that it is the unique single atomic Bi2 O3-x layer at the surface that leads to the enhanced polarization and appearance of the MPB-like nanodomain structure. This finding clearly demonstrates a novel route to the enhanced functional properties in the material system with reduced dimension via engineering the surface boundary conditions.

BaTiO3/SrTiO3 heterostructures for ferroelectric field effect transistors

Integration of ultrathin ferroelectric thin films with semiconductors is of interest for negative capacitance transistors that exhibit internal voltage gain, which may allow for scaling the supply voltage of low power circuits. In this study, BaTiO3 thin films were grown on doped SrTiO3 channels using molecular beam epitaxy. The BaTiO3 films are ferroelectric despite their low thickness (∼10 nm). Parallel plate capacitor devices exhibit anti-clockwise hysteresis, and a comparison with reference structures without BaTiO3 shows that the polarization in the BaTiO3 thin films is switchable and controls the charge density in the channel. Field effect transistors were fabricated to study the effect of ferroelectricity on the transistor characteristics. Anti-clockwise hysteresis and a shift in threshold-voltage are observed in the output characteristics of the transistors. These properties make these heterostructures a suitable system for studying negative capacitance effects.

Local Enhancement of Polarization at PbTiO3/BiFeO3 Interfaces Mediated by Charge Transfer.

Ferroelectrics hold promise for sensors, transducers, and telecommunications. With the demand of electronic devices scaling down, they take the form of nanoscale films. However, the polarizations in ultrathin ferroelectric films are usually reduced dramatically due to the depolarization field caused by incomplete charge screening at interfaces, hampering the integrations of ferroelectrics into electric devices. Here, we design and fabricate a ferroelectric/multiferroic PbTiO3/BiFeO3 system, which exhibits discontinuities in both chemical valence and ferroelectric polarization across the interface. Aberration-corrected scanning transmission electron microscopic study reveals an 8% elongation of out-of-plane lattice spacing associated with 104%, 107%, and 39% increments of δTi, δO1, and δO2 in the PbTiO3 layer near the head-to-tail polarized interface, suggesting an over ∼70% enhancement of polarization compared with that of bulk PbTiO3. Besides that in PbTiO3, polarization in the BiFeO3 is also remarkably enhanced. Electron energy loss spectrum and X-ray photoelectron spectroscopy investigations demonstrate the oxygen vacancy accumulation as well as the transfer of Fe3+ to Fe2+ at the interface. On the basis of the polar catastrophe model, FeO2/PbO interface is determined. First-principles calculation manifests that the oxygen vacancy at the interface plays a predominate role in inducing the local polarization enhancement. We propose a charge transfer mechanism that leads to the remarkable polarization increment at the PbTiO3/BiFeO3 interface. This study may facilitate the development of nanoscale ferroelectric devices by tailing the coupling of charge and lattice in oxide heteroepitaxy.

Ferroelectricity in ultrathin yttrium-doped hafnium oxide films prepared by chemical solution deposition based on metal chlorides and alcohol

Abstract We suggest a facile way to prepare precursor solutions with metal salts and alcohol and fabricate ultrathin ferroelectric yttrium-doped hafnium oxide films on Pt(111)/TiO 2 /SiO 2 /Si substrates by chemical solution deposition and post-annealing treatment. The samples were prepared with 5.2 mol% yttrium-doping and had a thickness ranging from 3 nm to 9 nm. We also varied the post-annealing temperature from 600 °C to 800 °C. The ultrathin films were characterized by transmission electron microscopy (TEM) and Raman spectroscopy. Their local ferroelectric properties were investigated by piezoresponse force microscopy (PFM) for domain imaging and polarization switching at nanoscale.

Mixed electrochemical–ferroelectric states in nanoscale ferroelectrics

Ferroelectricity on the nanoscale has been the subject of much fascination in condensed-matter physics for over half a century. In recent years, multiple reports claiming ferroelectricity in ultrathin ferroelectric films based on the formation of remnant polarization states, local electromechanical hysteresis loops, and pressure-induced switching were made. However, similar phenomena were reported for traditionally non-ferroelectric materials, creating a significant level of uncertainty in the field. Here we show that in nanoscale systems the ferroelectric state is fundamentally inseparable from the electrochemical state of the surface, leading to the emergence of a mixed electrochemical–ferroelectric state. We explore the nature, thermodynamics, and thickness evolution of such states, and demonstrate the experimental pathway to establish its presence. This analysis reconciles multiple prior studies, provides guidelines for studies of ferroelectric materials on the nanoscale, and establishes the design paradigm for new generations of ferroelectric-based devices. Nanoscale ferroelectricity is hard to characterize. Studies of BaTiO3 thin films now reveal a close coupling between the ferroelectric and the surface electrochemical states — a notion important for future applications of ferroelectric nanomaterials.

Polarization switching in ultrathin polyvinylidene fluoride homopolymer ferroelectric films

ABSTRACTPolarization switching in PVDF/P(VDF-TrFE) Langmuir-Blodgett films using Molecular Dynamic simulation method was studied. Simulations showed that the polarization switching for ultra-thin films proceeded according to intrinsic homogeneous switching mechanism Landau-Ginzburg-Devonshire. Values of coercive field are within 0.5–2.5 GV/m, which is consistent with experimental data. At high values of applied electric field and for thick samples the polarization switching corresponds to the Kolmogorov-Avrami-Ishibashi domain mechanism. Critical width begins from (3–6) nm being in line with experimental 10 nm. Obtained data describe the polarization switching in the transition from homogeneous to inhomogeneous region for ultrathin polymer ferroelectrics.

Ferroelectric-Driven Performance Enhancement of Graphene Field-Effect Transistors Based on Vertical Tunneling Heterostructures.

A vertical graphene heterostructure field-effect transistor (VGHFET) using an ultrathin ferroelectric film as a tunnel barrier is developed. The heterostructure is capable of providing new degrees of tunability and functionality via coupling between the ferroelectricity and the tunnel current of the VGHFET, which results in a high-performance device. The results pave the way for developing novel atomic-scale 2D heterostructures and devices.

(Invited) Emerging Ferroelectric Devices

Ferroelectrics comprise a class of polar materials with an electrically switchable spontaneous polarization allowing their use as binary data storage media in nonvolatile ferroelectric random access memories (FeRAMs). Currently available FeRAMs utilize a destructive read process of stored information, which requires the memory cells to be re-written by a relatively high voltage, increases the read time and limits the scalability. An alternative approach to the read operation is based on the polarization-driven resistive switching, known as the tunneling electroresistance (TER) effect. If the ferroelectric film is sufficiently thin (of the order of several nanometers), conduction electrons can quantum-mechanically tunnel through the ferroelectric barrier. By flipping the polarization of the ferroelectric barrier it is possible to change an internal electronic potential profile and, hence, alter the transmission probability and produce the TER effect. Using this effect, reading of the polarization state can be performed in a non-destructive manner by measuring the resistance of the memory cell to a relatively low voltage. Since TER does not depend on the amount of the stored charge it allows a much better scalability. In addition to being highly relevant to technological applications, the TER effect concerns the fundamental issues of the critical behavior and switching dynamics in ultrathin ferroelectric films, role of structural defects and interfacial properties in the transport behavior. Maintaining a stable polarization in ultrathin ferroelectric films is essential for exploiting the functionality of these materials in nonvolatile memory applications. This talk will focus on investigation of the polarization-controlled tunneling conductance in the FTJ devices that incorporate both ferroelectric films and 2D transition metal dichalcogenides (TMD). In addition, the in-plane transport of the TMD conducting channel in the ferroelectric field effect transistor (FE-FET) devices modulated by polarization reversal will be discussed. We show that interface engineering in these hybrid TMD-FE systems provides a possibility of successfully addressing the most serious challenges relevant to ferroelectric device performance, such as ON/OFF ratio, lifetime, operation endurance and reliability.

Ferroelectricity and Self-Polarization in Ultrathin Relaxor Ferroelectric Films.

We report ferroelectricity and self-polarization in the (001) oriented ultrathin relaxor ferroelectric PMN-PT films grown on Nb-SrTiO3, SrRuO3 and La0.7Sr0.3MnO3, respectively. Resistance-voltage measurements and AC impedance analysis suggest that at high temperatures Schottky depletion width in a 4 nm thick PMN-PT film deposited on Nb-SrTiO3 is smaller than the film thickness. We propose that Schottky interfacial dipoles make the dipoles of the nanometer-sized polar nanoregions (PNRs) in PMN-PT films grown on Nb-SrTiO3 point downward at high temperatures and lead to the self-polarization at room temperature with the assistance of in-plane compressive strain. This work sheds light on the understanding of epitaxial strain effects on relaxor ferroelectric films and self-polarization mechanism.