Ferroelectric Polarization State Research Articles

True ferroelectric switching and trap characterization in BaTiO3/Nb:STO heterostructures

This paper reports the true ferroelectric characteristics of BTO as a function of temperature and interface charge density at the BTO/Nb:STO interface. True ferroelectric switching characterization of BTO at 100 kHz shows a reduction in average remanent polarization from 4.79 ± 0.14 to 1.81 ± 0.13 μC/cm2 as the temperature is increased from 200 to 350 K. An activation energy of 0.082 ± 0.026 eV is reported for the BTO/Nb:STO interface. Trapping at the interface under positive and negative ferroelectric polarization states has been characterized, with polarization specific trap densities up to 1.75 × 1013 cm−2 eV−1 and interfacial trap densities up to 3.5 × 1012 cm−2 eV−1 reported.

Monolithic In2Se3–In2O3 heterojunction for multibit non-volatile memory and logic operations using optoelectronic inputs

Stable ferroelectricity at room-temperature down to the monolayer limit, harnessed with strong sensitivity towards visible-to-near-infrared illumination in α-In2Se3, facilitates its potential as versatile building block for developing ultrathin multifunctional photonic integrated networks. Herein, we demonstrated a planar ferroelectric-semiconductor heterojunction (FeS-HJ) field-effect transistor (FET) fabricated out of α-In2Se3 and In2O3, where the ferroelectric-polarization state in α-In2Se3 is utilized to control the device characteristics. The robust in-plane (IP) polarization flipping triggered by out-of-plane (OOP) electrostatic field along with clear anticlockwise hysteresis loop were readily revealed by scanning Kelvin-probe force microscopy (KPFM) and electrical probing. The orthogonally tangled ferroelectric switching was used to manipulate the HJ channel conductance and thereby to realize non-volatile memory (NVM) states. Moreover, gate-tuneable diode-like characteristics and superior photoresponse in HJ compared to its individual constitutes were observed. Utilizing the concurrent ferro-photonic coupling, high bandwidth optical inputs further tailored the outputs into four distinguished current states induced by different polarization directions. Our results pave the way for developing advanced (opto) electronic devices with diverse signal modulation capability to realize next generation low-power neurocomputing, brain-inspired visionary systems, and on-chip optical communications.

Multiferroic van der Waals heterostructure FeCl2/Sc2CO2 : Nonvolatile electrically switchable electronic and spintronic properties

Multiferroic van der Waals (vdW) heterostrucutres offers an exciting route towards novel nanoelectronics and spintronics device technology. Here we investigate the electronic and transport properties of multiferroic vdW heterostructure composed of ferromagnetic FeCl$_2$ monolayer and ferroelectric Sc$_2$CO$_2$ monolayer using first-principles density functional theory and quantum transport simulations. We show that FeCl$_2$/Sc$_2$CO$_2$ heterostructure can be reversibly switched from semiconducting to half-metallic behavior by electrically modulating the ferroelectric polarization states of Sc$_2$CO$_2$. Intriguingly, the half-metallic phase exhibits a Type-III broken gap band alignment, which can be beneficial for tunnelling field-effect transistor application. We perform a quantum transport simulation, based on a \emph{proof-of-concept} two-terminal nanodevice, to demonstrate all-electric-controlled valving effects uniquely enabled by the nonvolatile ferroelectric switching of the heterostructure. These findings unravels the potential of FeCl$_2$/Sc$_2$CO$_2$ vdW heterostructures as a building block for designing a next generation of ultimately compact information processing, data storage and spintronics devices.

Tuning Negative Capacitance in PbZr0.2Ti0.8O3/SrTiO3 Heterostructures via Layer Thickness Ratio

In this work, we exploit the ferroelectric-dielectric layer thickness ratio r as an effective tuning parameter to control the ferroelectric polarization and transient negative capacitance (NC) state in epitaxial ${\mathrm{Pb}\mathrm{Zr}}_{0.2}{\mathrm{Ti}}_{0.8}{\mathrm{O}}_{3}/{\mathrm{Sr}\mathrm{Ti}\mathrm{O}}_{3}$ bilayer heterostructures. The remnant polarization decreases monotonically with decreasing r, with the system exhibiting an abrupt transition from ferroelectric to dielectric dominated behavior at a critical ratio ${r}_{c}$ of 8 to 7, which is consistent with the evolution of the free-energy profile modeled via Landau theory. For samples with large r, the polarization switching dynamics during the transient NC regime can be well described by the nucleation and growth model, with the narrow distribution of the characteristic switching time (${t}_{0}$) pointing to a domain-wall-motion-limited behavior. Right below ${r}_{c}$, we observe a significantly broadened distribution of ${t}_{0}$, which can be attributed to the emergence of a multidomain state. The transient NC mode is quenched in samples with r 6, confirming that the ferroelectric order is suppressed. Our study provides critical information for optimizing the materials design for complex oxide-based NC transistors, paving the path for their application in low-power nanoelectronics.

Emulation of synaptic behavior by organic ferroelectric tunnel junctions

The synapses are essential to the brain neuromorphic system, and especially the plasticity of synapse contributes significantly in cognitive functions. Ferroelectric tunnel junction (FTJ) can be utilized to emulate the synaptic functions by the effective control of resistance state, which is related with the ferroelectric polarization state of barrier layer. Here, we study the emulation of a synaptic response in the fabricated organic ferroelectric tunnel junctions (OFTJs) with Polyvinylidene fluoride (PVDF) polymer barriers. The functions of long-term potentiation and long-term depression are demonstrated by exploring the relationship between resistance and polarizing pulse. In addition, our device can be also utilized to realize multi-memory function, a device with six-resistance states is demonstrated. Our results may provide a new degree of freedom for the realization of synaptic functions in OFTJs.

Low-energy complementary ferroelectric-nanocrack logic

Abstract Ferroelectric-based electronic devices have excellent low-energy characteristics due to the highly insulating property, in which the Joule heating can be neglected. The recent discovery of electrically switchable cracks in ferroelectric/alloy film heterostructure gave rise to a new way to construct transistor, where the opening and closing of crack switched the channel current off and on. Here, we observed the complementary switching of cracks for the first time, and demonstrated a complementary inverter without any additional process to set different types of transistors, which was implemented by forming the n- and p-type transistor in conventional complementary metal oxide semiconductor (CMOS) technology. The complementary states were generated spontaneously once the cracks were induced and varied with the change of applied input voltage polarity. The low ON resistance and near-zero OFF state leakage current result in the high current on/off ratio (~107) and allow for the low dynamic and static power dissipation. Further, the switching of cracks is coupled to the surrounding ferroelectric polarization states, offering the non-destructive readout operation. We believe that our work provides a very simple route to build complementary logic gates and paves a way for the energy-efficient electronic devices, while promoting the “crack nanoelectronics”.

Temperature dependence of transport mechanisms in organic multiferroic tunnel junctions

Organic multiferroic tunnel junctions (OMFTJs) with multi-resistance states have been proposed and have drawn intensive interests due to their potential applications, for example in memristor and spintronics based synapse devices. The ferroelectric control of spin polarization at a ferromagnet/ferroelectric organic (FE-Org) interface by electrically switching the ferroelectric polarization of the FE-Org has been recently realized. However, there is still a lack of understanding of the transport properties in OMFTJs, especially the interplay between the ferroelectric domain structure in the organic barrier and the spin polarized electron tunneling through the barrier. Here, we report on a systematic study of the temperature dependent transport behavior in La0.6Sr0.4MnO3/PVDF/Co OMFTJs. It is found that thermal fluctuation of the ferroelectric domains play an important role on the transport properties. When T > 120 K, the opposite temperature dependence of resistance for up and down ferroelectric polarization states results in a rapid diminishing of the tunneling electroresistance. These results contribute to the understanding of the transport properties for designing high performance OMFTJs for memristor and spintronics applications.

Enhanced and adjustable photoresponse performance in Bi4Ti3O12/TiO2 bilayer films

Bi4Ti3O12/TiO2 bilayer films were synthesized on general fluorine-doped tin oxide glass (FTO) substrates adopting a simple solution coating processing. The Bi4Ti3O12 film with different thickness was first prepared on FTO substrates, and then TiO2 layer with different thickness was spin-coated on the Bi4Ti3O12 layer, to obtain different Bi4Ti3O12/TiO2 bilayer films with almost the same total thickness. The morphology and crystal structure of Bi4Ti3O12/TiO2 bilayer films were characterized and analyzed by scanning electron microscopy and X-ray diffraction. The enhanced photoresponse was observed in bilayer films compared with the single-phase films of Bi4Ti3O12 or TiO2. Besides, it was also observed that the photoresponse of the bilayer films could drastically be adjusted by applied bias voltage and ferroelectric polarization states. The reasons for the above phenomena were also investigated.

Probing the local piezoelectric behavior in stretched barium titanate/poly(vinylidene fluoride) nanocomposites

The nanoscale piezoelectric and ferroelectric behavior of barium titanate/poly(vinylidene fluoride) (BT/PVDF) nanocomposite films has been investigated by means of atomic force microscopy (AFM). An uniaxial stretching step was first carried out to promote the polar β crystal phase of the PVDF matrix, as confirmed by infrared spectroscopy and piezoelectric force microscopy (PFM) analysis. Fragmentation of the original polymer crystalline structure upon drawing, as evidenced by the presence of nanometric crystalline blocks did not damage the composite film, thanks to the strong interfacial cohesion between ceramic and polymer brought by nitrodopamine functionalization of the BT inclusions. By scanning the composite surface using PFM, highly piezo-active regions were evidenced and attributed to the BT nanoparticles that could not be identified on the AFM topography images. The precise manipulation of the ferroelectric polarization states in these individual BT grains embedded into the PVDF matrix has been successfully achieved, confirmed by the local electromechanical deformation simultaneously detected. Reversible switching of the out-of-plane polarization orientation spatially confined into the particles was evidenced. The ability of the contact PFM tool to both directly visualize individual piezoelectric nanofillers dispersed into a polymer matrix and monitor the polarization states is demonstrated, thus highlighting the versatility of PFM for the advanced characterization of electroactive nanocomposites.

Stable Subloop Behavior in Ferroelectric Si-Doped HfO2.

The recent demand for analogue devices for neuromorphic applications requires modulation of multiple nonvolatile states. Ferroelectricity with multiple polarization states enables neuromorphic applications with various architectures. However, deterministic control of ferroelectric polarization states with conventional ferroelectric materials has been met with accessibility issues. Here, we report unprecedented stable accessibility with robust stability of multiple polarization states in ferroelectric HfO2. Through the combination of conventional voltage measurements, hysteresis temperature dependence analysis, piezoelectric force microscopy, first-principles calculations, and Monte Carlo simulations, we suggest that the unprecedented stability of intermediate states in ferroelectric HfO2 is due to the small critical volume size for nucleation and the large activation energy for ferroelectric dipole flipping. This work demonstrates the potential of ferroelectric HfO2 for analogue device applications enabling neuromorphic computing.

Electric field control of the semiconductor-metal transition in two dimensional CuInP2S6/germanene van der Waals heterostructure

We propose a van der Waals heterostructure CuInP2S6/germanene by combining two dimensional ferroelectric semiconductor CuInP2S6 with germanene. By density functional theory calculations, we find that the metal-semiconductor transition can be realized in the CuInP2S6/germanene heterostructure by controlling the ferroelectric polarization direction. CuInP2S6 induces the sublattice imbalance of germanene by interface interaction and thus makes it become a normal semiconductor. Then, two opposite ferroelectric polarization states in CuInP2S6/germanene lead to a different band alignment and finally determine its metallic or semiconductor properties. Large transition barriers from ferroelectric to antiferroelectric phases ensure its stability at room temperature. This is a pure electric field controlled metal-semiconductor transition, which has great application potential for exploring nonvolatile ferroelectric switches and memory devices.

Back-scattered electron visualization of ferroelectric domains in a BiFeO3 epitaxial film

Three-dimensional orientation of the ferroelectric (FE) domain structure of a BiFeO3 epitaxial film was investigated by scanning electron microscopy (SEM) using back-scattered electrons and piezoresponse-force microscopy (PFM). By changing the crystallographic orientation of the sample and the electron collection angle relative to the detector, we establish a link between the orientation of polarization vectors (out-of-plane and in-plane) in the BiFeO3 film and the back-scattered electron image contrast in agreement with PFM investigations. The different FE polarization states in the domains correspond to altered crystalline environments for the impingent primary beam electrons. We postulate that the resultant back-scattered electron domain contrast arises as a result of either differential absorption (through a channelling effect) or through back-diffraction from the sample, which leads to a projected diffraction pattern super-imposed with the diffuse conventional back-scattered electron intensity. We demonstrate that SEM can be sensitive for both out-of-plane and in-plane polarization directions using the back-scattered electron detection mode and can be used as a non-destructive and fast method to determine 3D FE polarization orientation of domains.

Physical properties in polydomain c/a/c/a phase PbTiO3 ferroelectric thick films: effect of thermal stresses

The thermal stress dependence of the physical properties of polydomain PbTiO3 films on different substrates are investigated using a nonlinear Ginzburg–Landau–Devonshire thermodynamic model as a function of deposition temperature T G and thermal expansion coefficients. It is found that the thermal strain has a large impact on the ferroelectric polarization states and other physical properties for the thicker ferroelectric thin films. Extrinsic contributions from 90° domain wall displacements are found to dramatically impact the dielectric, pyroelectric, and piezoelectric responses. Most importantly, the dielectric and piezoelectric constants in the polydomain c/a/c/a phase is much larger than that of the monodomain c phase, while the results are opposite for the ferroelectric polarization and pyroelectric coefficient. Careful choice of thermal stress and domain states allows one to harness the intrinsic and extrinsic contributions to obtain large physical responses. Our work is in good agreement with experimental results and phase–field simulation.

Disordered ferroelectricity in the PbTiO3/SrTiO3 superlattice thin film

The PbTiO3/SrTiO3 superlattice thin films with a low volume fraction of PbTiO3 have not attracted much interest because they are thought to exhibit only a paraelectric state. In this study, we focus on a superlattice thin film with thin PbTiO3 (PTO) and thick SrTiO3 (STO) layers, wherein the hidden ferroelectricity in the thin PbTiO3 layer is revealed. Atomic scale imaging analysis and electron energy loss spectroscopy reveal the existence of a disordered ferroelectric polarization state without innate tetragonal distortion in the (6PTO/15STO)5 superlattice. The piezoelectric force microscopy analysis confirms that this disordered ferroelectricity can enhance piezoelectric response.

Modulating the Ferromagnet/Molecule Spin Hybridization Using an Artificial Magnetoelectric

Spin‐polarized charge transfer at the interface between a ferromagnetic (FM) metal and a molecule can lead to ferromagnetic coupling and to a high spin polarization at room temperature. The magnetic properties of these interfaces can not only alter those of the ferromagnet but can also stabilize molecular spin chains with interesting opportunities toward quantum computing. With the aim to enhance an organic spintronic device's functionality, external control over this spin polarization may thus be achieved by altering the ferromagnet/molecule interface's magnetic properties. To do so, the magnetoelectric properties of an underlying ferroelectric/ferromagnetic interface are utilized. Switching the ferroelectric polarization state of a PbZr0.2Ti0.8O3 (PZT) bottom layer within a PZT/Co/FePc‐based (Pc ‐ phthalocyanine) device alters the X‐ray magnetic circular dichroism of the Fe site within the phthalocyanine molecular top layer. Thus, how to electrically alter the magnetic properties of an interface with high spin polarization at room temperature is demonstrated. This expands electrical control over spin‐polarized FM/molecule interfaces, which is first demonstrated using ferroelectric molecules, to all molecular classes.

Vertically arrays of Si-LaNiO3/BiFeO3/Au core-shell nano-capacitors for prominent coupled electro-optic effect

The current study delivers the first assessment of well-separated cone shaped n-type epitaxial BiFe0.9Mn0.1O3 (n-BFMO) shells integrated on LaNiO3 (LNO) buffered vertical arrays of p-type nanoporous Si nanowire cores as an absorber for high efficiency and sustainable all- oxide photovoltaic devices. The photovoltaic effect of the LNO/n-BFMO/Au radial heterojunction is investigated by registering the current-voltage curves under AM 1.5 (100 mW cm−2) illumination conditions at various ferroelectric polarization states of BFMO. The as-deposited LNO/BFMO/Au nanowire arrays fabricated by scalable Pulsed Laser deposition are completely self-polarized and produced a remarkably large open circuit photovoltage up to 0.53 V under zero poling condition. The direction of photocurrent and photovoltage is reversibly switchable in response to ferroelectric polarization switching. The short circuit photocurrent “ISC” and open circuit voltage “VOC” depict a hysteresis loop which is more or less identical to that of ferroelectric polarization and thus can be utilized as a nondestructive optical readout of the ferroelectric nonvolatile memories. By introducing special architecture for the BFO based photovoltaic device, a photoconversion efficiency ∼0.3% and External Quantum Efficiency ∼11% are achieved at zero polarization state, which can further be enhanced and modulated to ∼0.6% and 14%, respectively, at the negative poling state of BFMO.

Manipulating the ferroelectric polarization state of BaTiO3 thin films

Controlling the ferroelectric polarization at macroscopic or microscopic levels is crucial in the framework of the development of ferroelectric materials used in yet challenging photo-electrochemical (PEC) cells and spintronic applications. We report here on polarization methods allowing to electrically polarize prototypical samples of BaTiO3 (001) films. Epitaxial single crystalline layers were grown up to a thickness of 25nm by atomic oxygen assisted molecular beam epitaxy on 1at.% Nb doped SrTiO3 (001) single crystals. The samples were both microscopically and macroscopically polarized using Piezoresponse Force Microscopy and electrochemical poling in an electrolyte respectively. In addition we demonstrate the possibility to retrieve a quasi-native mixed ferroelectric polarization state after annealing. These polarization methods may be applied to many other ferroelectric thin films.

Controllable Charge Transfer by Ferroelectric Polarization Mediated Triboelectricity

Next‐generation memory and energy harvesting devices require a higher output performance for charging lectric devices. Generally, it is very limited to control charge transfer through triboelectricity in triboelectric materials. Here, using ferroelectric polarization, it is found that both the amount and direction of charge transfer in triboelectric materials can be controlled. The ferroelectric‐dependent triboelectricity in a ferroelectric co‐polymer film is explored using atomic force microscopy (AFM). Ferroelectric surfaces are rubbed with the AFM tip after poling with positive and negative bias voltages to achieve a triboelectric effect. The surface potential of the positively (negatively) poled area becomes smaller (larger) after rubbing the film surface with the AFM tip. Furthermore, the power output from the triboelectric nanogenerator is dependent on the ferroelectric polarization state. The results indicate that the amount and direction of the charge transfer in triboelectricity can be controlled by the ferroelectric polarization state.

Computational characterization of magneto‐electric composites: the role of ferroelectric pre‐polarization

AbstractThe interaction between electric and magnetic fields enables smart devices which can find applications in sensor technology and data storage. Materials showing magneto‐electric (ME) coupling combine different ferroic characteristics. In the present contribution we focus on composites, which combine ferroelectric and ferromagnetic phases due to strain couplings, such that they generate a strain‐induced ME coupling. We derive a two‐scale homogenization approach for the determination of effective properties in consideration of microscopic morphologies. Furthermore, we demonstrate the strong influence of ferroelectric polarization states on the ME‐coefficient by modeling the switching of remanent polarizations on the microscale. (© 2015 Wiley‐VCH Verlag GmbH & Co. KGaA, Weinheim)

ChemInform Abstract: Ferroelectric‐Based Catalysis: Switchable Surface Chemistry

AbstractReview: 138 refs.