Organic non-volatile memories from ferroelectric phase-separated blends

New non-volatile memories are being investigated to keep up with the organic-electronics road map. Ferroelectric polarization is an attractive physical property as the mechanism for non-volatile switching, because the two polarizations can be used as two binary levels. However, in ferroelectric capacitors the read-out of the polarization charge is destructive. The functionality of the targeted memory should be based on resistive switching. In inorganic ferroelectrics conductivity and ferroelectricity cannot be tuned independently. The challenge is to develop a storage medium in which the favourable properties of ferroelectrics such as bistability and non-volatility can be combined with the beneficial properties provided by semiconductors such as conductivity and rectification. Here we present an integrated solution by blending semiconducting and ferroelectric polymers into phase-separated networks. The polarization field of the ferroelectric modulates the injection barrier at the semiconductor-metal contact. The combination of ferroelectric bistability with (semi)conductivity and rectification allows for solution-processed non-volatile memory arrays with a simple cross-bar architecture that can be read out non-destructively. The concept of an electrically tunable injection barrier as presented here is general and can be applied to other electronic devices such as light-emitting diodes with an integrated on/off switch.     

Light-controlled molecular resistive switching ferroelectric heterojunction

Molecular ferroelectrics have attained significant advancement as a promising approach towards the development of next-generation non-volatile memory devices. Herein, the semiconducting-ferroelectric heterojunctions which is composed of molecular ferroelectrics (R)-(−)-3-hydroxlyquinuclidinium chloride together with organic charge transfer complex is reported. The molecular ferroelectric domain provides polarization and bistability while organic charge transfer phase allows photo-induced charge generation and transport for photovoltaic effect. By switching the direction of the polarization in the ferroelectric phase, the heterojunction-based devices show non-volatile resistive switching under external electric field and photocurrent/voltage induced by light excitation, stable fatigue properties and long retention time. Overall, the photovoltaic controlled resistive switching provides a new route for all-organic multiphase non-volatile memories.     

Microscopic mechanism of imprint in hafnium oxide-based ferroelectrics

Hafnia-based ferroelectrics have greatly revived the field of ferroelectric memory(FeRAM),but certain reliability issues must be satisfactorily resolved before ...     

Solution-process coating of vertical ZnO nanowires with ferroelectrics

We have developed a modified misted deposition process by combining substrate and mist heating for the deposition of ferroelectrics on 3D nanostructures. Arrays of vertical ZnO nanowires, sputter coated with Pd bottom electrodes, are used as the substrate. Scanning electron microscopy investigations show that conformal coating of ferroelectric Pb(Zr,Ti)O(3) (PZT) with good step coverage is obtained at deposition temperatures above 140?°C. The substrate heating also eliminates the common 'bundling' problem of the nanowire arrays. On the basis of data on x-ray diffraction, energy dispersive x-ray spectroscopy, and P-E hysteresis of PZT films on flat substrates, we obtain the optimum substrate temperature window to be 180-220?°C, in terms of best step coverage and an evident ferroelectricity. This is a significant step towards the end-goal of fully integrated ZnO nanowires with ferroelectric capacitors, which may be useful for the light-emitting applications of ZnO.     

Ferroelectric polymer gates for non-volatile field effect control of ferromagnetism in (Ga, Mn)As layers

(Ga, Mn)As and other diluted magnetic semiconductors (DMS) attract a great deal of attention for potential spintronic applications because of the possibility of controlling the magnetic properties via electrical gating. Integration of a ferroelectric gate on the DMS channel adds to the system a non-volatile memory functionality and permits nanopatterning via the polarization domain engineering. This topical review is focused on the multiferroic system, where the ferromagnetism in the (Ga, Mn)As DMS channel is controlled by the non-volatile field effect of the spontaneous polarization. Use of ferroelectric polymer gates in such heterostructures offers a viable alternative to the traditional oxide ferroelectrics generally incompatible with DMS. Here we review the proof-of-concept experiments demonstrating the ferroelectric control of ferromagnetism, analyze the performance issues of the ferroelectric gates and discuss prospects for further development of the ferroelectric/DMS heterostructures toward the multiferroic field effect transistor.     

A Molecular Polycrystalline Ferroelectric with Record‐High Phase Transition Temperature

An outstanding advantage of inorganic ceramic ferroelectrics is their usability in the polycrystalline ceramic or thin film forms, which has dominated applications in the ferroelectric, dielectric, and piezoelectric fields. Although the history of ferroelectrics began with the molecular ferroelectric Rochelle salt in 1921, so far there have been very few molecular ferroelectrics, with lightweight, flexible, low‐cost, and biocompatible superior properties compared to inorganic ceramic ferroelectrics, that can be applied in the polycrystalline form. Here, a multiaxial molecular ferroelectric, guanidinium perchlorate ([C(NH₂)₃]ClO₄), with a record‐high phase transition temperature of 454 K is presented. It is the rectangular polarization–electric field (P –E ) hysteresis loops recorded on the powder and thin film samples (with respective large P _r of 5.1 and 8.1 µC cm^?2) that confirm the ferroelectricity of [C(NH₂)₃]ClO₄ in the polycrystalline states. Intriguingly, after poling, the piezoelectric coefficient (d ₃₃) of the powder sample shows a significant increase from 0 to 10 pC N^?1, comparable to that of LiNbO₃ single crystal (8 pC N^?1). This is the first time that such a phenomenon has been observed in molecular ferroelectrics, indicating the great potential of molecular ferroelectrics being used in the polycrystalline form like inorganic ferroelectrics, as well as being viable alternatives or supplements to conventional ceramic ferroelectrics.     

A Monte Carlo simulation on domain pattern and ferroelectric behaviors of relaxor ferroelectrics

The domain configuration and ferroelectric property of mode relaxor ferroelectrics (RFEs) are investigated by performing a two-dimensional Monte Carlo simulation based on the Ginzburg-Landau theory on ferroelectric phase transitions and the defect model as an approach to the electric dipole configuration in relaxor ferroelectrics. The evolution of domain pattern and domain wall configuration with lattice defect concentration and temperature is simulated, predicting a typical two-phase coexisted microstructure consisting of ferroelectric regions embedded in the matrix of a paraelectric phase. The diffusive ferroelectric transitions in terms of the spontaneous polarization hysteresis and dielectric susceptibility as a function of temperature and defect concentration are successfully revealed by the simulation, demonstrating the applicability of the defect model and the simulation algorithm. A qualitative consistency between the simulated results and the properties of proton-irradiated ferroelectric copolymer is presented.     

Progress in ferroelectric memory technology

The application of ferroelectric films to semiconductor memories is discussed. Design and testing considerations are examined. A nonvolatile memory using the polarization hysteresis of ferroelectrics is described. Test results for PZT films on GaAs substrate are presented. The data show that nonvolatile memories are feasible and that commercial applications are within reach in the near future.     

The origin of ferroelectricity in magnetoelectric YMnO3

Understanding the ferroelectrocity in magnetic ferroelectric oxides is of both fundamental and technological importance. Here, we identify the nature of the ferroelectric phase transition in the hexagonal manganite, YMnO(3), using a combination of single-crystal X-ray diffraction, thorough structure analysis and first-principles density-functional calculations. The ferroelectric phase is characterized by a buckling of the layered MnO(5) polyhedra, accompanied by displacements of the Y ions, which lead to a net electric polarization. Our calculations show that the mechanism is driven entirely by electrostatic and size effects, rather than the usual changes in chemical bonding associated with ferroelectric phase transitions in perovskite oxides. As a result, the usual indicators of structural instability, such as anomalies in Born effective charges on the active ions, do not hold. In contrast to the chemically stabilized ferroelectrics, this mechanism for ferroelectricity permits the coexistence of magnetism and ferroelectricity, and so suggests an avenue for designing novel magnetic ferroelectrics.     

Thin film ferroelectrics for capacitor applications

Pulsed laser deposition (PLD) has been used to fabricate simple thin film capacitor structures with a variety of ferroelectric materials. Thin film capacitors using the conventional ferroelectric material BaxSr1-xTiO3(BSTO) have been made across the entire compositional series. Electrical characterization shows that in thin film form these ferroelectrics display Curie point behaviour which is largely independent of composition. This contrasts sharply with bulk behaviour. The thin film fabrication and characterization of relaxor ferroelectric ceramics, such as Pb(Mg1/3Nb2/3)O3 (PMN) and Pb(Zn1/3Nb2/3)O3-BaTiO3(PZN-BT), is also reported. © 1998 Chapman & Hall     

The domain switching and structural characteristics of PLZT bulk ceramics and thin films chemically prepared from the same acetate precursor solutions

Lead lanthanum zirconate titanate (PLZT) ferroelectrics were produced in bulk ceramic and thin-film form from the same acetate precursor solutions in order to compare their electrical and physical properties. Bulk ceramics were hot pressed from chemically coprecipitated powders, and thin films were fabricated by spin coating on silver foil and platinum-coated silicon wafer substrates. A number of PLZT compositions were investigated, including ferroelectric memory materials near the morphotropic phase boundary with 2% La, memory and slim-loop ferroelectric x/65/35 (La/Zr/Ti) compositions with up to 12% La, as well as some antiferroelectric thin-film materials. Internal film stress from thermal expansion mismatch between films and substrates was found to contribute to differences in electrical properties and Curie temperatures between the thin film and bulk materials, as were interface layers between the films and substrates, mechanical clamping from the substrates and grain size.     

Defect-Enhanced Polarization Switching in the Improper Ferroelectric LuFeO3

Results of switching behavior of the improper ferroelectric LuFeO₃ are presented. Using a model set of films prepared under controlled chemical and growth-rate conditions, it is shown that defects can reduce the quasi-static switching voltage by up to 40% in qualitative agreement with first-principles calculations. Switching studies show that the coercive field has a stronger frequency dispersion for the improper ferroelectrics compared to a proper ferroelectric such as PbTiO₃. It is concluded that the primary structural order parameter controls the switching dynamics of such improper ferroelectrics.     

Individually addressable epitaxial ferroelectric nanocapacitor arrays with near Tb inch-2 density

Ferroelectric materials have emerged in recent years as an alternative to magnetic and dielectric materials for nonvolatile data-storage applications. Lithography is widely used to reduce the size of data-storage elements in ultrahigh-density memory devices. However, ferroelectric materials tend to be oxides with complex structures that are easily damaged by existing lithographic techniques, so an alternative approach is needed to fabricate ultrahigh-density ferroelectric memories. Here we report a high-temperature deposition process that can fabricate arrays of individually addressable metal/ferroelectric/metal nanocapacitors with a density of 176 Gb inch(-2). The use of an ultrathin anodic alumina membrane as a lift-off mask makes it possible to deposit the memory elements at temperatures as high as 650 degrees C, which results in excellent ferroelectric properties.     

Self-assembled nanoscale ferroelectrics

Multifunctional ferroelectric materials offer a wide range of useful properties, from switchable polarization that can be applied in memory devices to piezoelectric and pyroelectric properties used in actuators, transducers and thermal sensors. At the nanometer scale, however, material properties are expected to be different from those in bulk. Fundamental problems such as the super-paraelectric limit, the influence of the free surface, and of interfacial and bulk defects on ferroelectric switching, etc., arise when scaling down ferroelectrics to nanometer sizes. In order to study these size effects, fabrication methods of high quality nanoscale ferroelectric crystals have to be developed. The present paper briefly reviews self-patterning and self-assembly fabrication methods, including chemical routes, morphological instability of ultrathin films, microemulsion, and self-assembly lift-off, employed up to the date to fabricate ferroelectric structures with lateral sizes in the range of few tens of nanometers.     

Key integration technologies for nanoscale FRAMs

We discuss key technologies of 180-nm node ferroelectric memories, whose process integration is becoming extremely complex when device dimension shrinks into a nano scale. This is because process technology in ferroelectric integration does not extend to conventional shrink technology due to many difficulties of coping with metal-insulator-metal (MIM) capacitors. The key integration technologies in ferroelectric random access memory (FRAM) comprise: etching technology to have less plasma damage; stack technology for the preparation of robust ferroelectrics; capping technology to encapsulate cell capacitors; and vertical conjunction technology to connect cell capacitors to the plate line. What has been achieved from these novel approaches is not only to have a peak-to-peak value of 675 mV in bit-line potential but also to ensure a sensing margin of 300 mV in opposite-state retention, even after 1000 hour suffering at 150 degrees C.     

Ferroelectricity and Antiferroelectricity of Doped Thin HfO2‐Based Films

The recent progress in ferroelectricity and antiferroelectricity in HfO₂‐based thin films is reported. Most ferroelectric thin film research focuses on perovskite structure materials, such as Pb(Zr,Ti)O₃, BaTiO₃, and SrBi₂Ta₂O₉, which are considered to be feasible candidate materials for non‐volatile semiconductor memory devices. However, these conventional ferroelectrics suffer from various problems including poor Si‐compatibility, environmental issues related to Pb, large physical thickness, low resistance to hydrogen, and small bandgap. In 2011, ferroelectricity in Si‐doped HfO₂ thin films was first reported. Various dopants, such as Si, Zr, Al, Y, Gd, Sr, and La can induce ferro­electricity or antiferroelectricity in thin HfO₂ films. They have large remanent polarization of up to 45 μC cm^?2, and their coercive field (≈1–2 MV cm^?1) is larger than conventional ferroelectric films by approximately one order of magnitude. Furthermore, they can be extremely thin (<10 nm) and have a large bandgap (>5 eV). These differences are believed to overcome the barriers of conventional ferroelectrics in memory applications, including ferroelectric field‐effect‐transistors and three‐dimensional capacitors. Moreover, the coupling of electric and thermal properties of the antiferroelectric thin films is expected to be useful for various applications, including energy harvesting/storage, solid‐state‐cooling, and infrared sensors.     

A COD fracture model of ferroelectric ceramics with applications in electric field induced fatigue crack growth

In this paper, Crack Opening Displacement (COD) is introduced to study the fracture and fatigue of ferroelectrics. A fundamental solution for the COD of ferroelectrics is derived considering both the piezoelectric effect and ferroelectric effect. Bases on this solution, a nonlinear COD fracture model of ferroelectrics, which takes into account the effect of domain switching, is developed and accords well with the experimental results. Furthermore, fatigue crack growth in ferroelectrics is analytically investigated using this COD model. Comparison between the experimental results and the predicted electric-field-induced fatigue crack growth shows the applicability of the proposed COD model.     

Facile Control of Ferroelectricity Driven by Ingenious Interaction Engineering

Construction of ferroelectric and optimization of macroscopic polarization has attracted tremendous attention for next generation light weight and flexible devices, which brings fundamental vitality for molecular ferroelectrics. However, effective molecular tailoring toward cations makes ferroelectric synthesis and modification relatively elaborate. Here, the study proposes a facile method to realize triggering and optimization of ferroelectricity. The experimental and theoretical investigation reveals that orientation and alignment of polar cations, dominated factors in molecular ferroelectrics, can be controlled by easily processed anionic modification. In one respect, ferroelectricity is induced by strengthened intermolecular interaction. Moreover, ≈50% of microscopic polarization enhancement (from 8.07 to 11.68 µC cm⁻²) and doubling of equivalent polarization direction (from 4 to 8) are realized in resultant ferroelectric FEtQ2ZnBrI₃ (FEQZBI, FEtQ = N-fluoroethyl-quinuclidine). The work offers a totally novel platform for control of ferroelectricity in organic–inorganic hybrid ferroelectrics and a deep insight of structure–property correlations.     

Interlayer coupling in ferroelectric bilayer and superlattice heterostructures

Ferroelectric multilayers and superlattices have gained interest for dynamic random access memory (DRAM) applications and as active elements in tunable microwave devices in the telecommunications industry. A number of experimental studies have shown that these materials have many peculiar properties which cannot be described by a simple series connection of the individual layers that make up the heterostructures. A thermodynamic analysis is presented to demonstrate that ferroelectric multilayers interact through internal elastic, electrical, and electromechanical fields and the strength of the coupling can be quantitatively described using Landau theory of phase transformations, theory of elasticity, and principles of electrostatics. The theoretical analysis shows that compositional variations across ferroelectric bilayers result in a broken spatial inversion symmetry that can lead to asymmetric thermodynamic potentials favoring one ferroelectric ground state over the other. Furthermore, the thermodynamic modeling indicates that there is a strong electrostatic coupling between the layers that leads to the suppression of ferroelectricity at a critical paraelectric layer thickness for ferroelectric-paraelectric bilayers. This bilayer is expected to have a gigantic dielectric response similar to the dielectric anomaly near Curie-Weiss temperature in homogeneous ferroelectrics at this critical thickness.     

Phase transitions due to polar region structure in disordered ferroelectrics

We show clear experimental evidence for two phase transitions in titanium doped lead magnesium niobate compositional disordered ferroelectrics. One is the diffuse phase transition near the temperature of the dielectric permittivity maximum, which is often called the characteristic of relaxor ferroelectrics (relaxors). Another is a first-order transformation from relaxor ferroelectric to normal ferroelectric, corresponding to a zero-field spontaneous polar micro-macrodomain switching. According to X-ray diffraction (XRD) measurements, thermal analysis and transmission electron microscope (TEM) results, a dynamic behavior of polar microregions is necessary to explain the phenomena, which is more similar to a stress induced martensitic transformations form cubically stabilized perovskite parent-phase. It is also observed that the dynamic behavior of polar regions was influenced by the titanium doping.