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Ferroelectric materials possess a spontaneous polarization that is switchable by an electric field. Robust retention of switched polarization is critical for non-volatile nanoelectronic devices based on ferroelectrics, however, these materials often suffer from polarization relaxation, typically within days to a few weeks. Here we exploit designer-defect-engineered epitaxial BiFeO3 films to demonstrate polarization retention with virtually no degradation in switched nanoscale domains for periods longer than 1 year. This represents a more than 2000% improvement over the best values hitherto reported. Scanning probe microscopy-based dynamic switching measurements reveal a significantly increased activation field for domain wall movement. Atomic resolution scanning transmission electron microscopy indicates that nanoscale defect pockets pervade the entire film thickness. These defects act as highly efficient domain wall pinning centres, resulting in anomalous retention. Our findings demonstrate that defects can be exploited in a positive manner to solve reliability issues in ferroelectric films used in functional devices., The use of ferroelectric materials in memory device applications is held back by low retention times. Here, the authors demonstrate that by intentionally introducing defective nanoregions which increase the activation field for domain wall motion, retention times larger than a year can be achieved.

Controlling ferroic orders (ferroelectricity, ferromagnetism and ferroelasticity) by optical methods is a significant challenge due to the large mismatch in energy scales between the order parameter coupling strengths and the incident photons. Here, we demonstrate an approach to manipulate multiple ferroic orders in an epitaxial mixed-phase BiFeO3 thin film at ambient temperature via laser illumination. Phase-field simulations indicate that a light-driven flexoelectric effect allows the targeted formation of ordered domains. We also achieved precise sequential laser writing and erasure of different domain patterns, which demonstrates a deterministic optical control of multiferroicity at room temperature. As ferroic orders directly influence susceptibility and conductivity in complex materials, our results not only shed light on the optical control of multiple functionalities, but also suggest possible developments for optoelectronics and related applications.

Topological defects in ferroelectric materials have attracted much attention due to the emergence of conductive, ferroic, and magnetic functionalities. However, many topological configurations dynamically evolve during the switching processes, making them a challenge to characterize via traditional techniques. Here, we implement an automated experimentation approach for the exploration of functional properties in BiFeO3 thin films. Specifically, we visualize the ferroelectric domain structures via single frequency piezoresponse force microscopy and implement a computer vision-based algorithm to discover features of interest at which spectroscopic measurements are taken. Subsequently, we employ dimensionality reduction techniques to reveal characteristic polarization behaviors at these features. This approach can be extended to other spectroscopies and modalities to probe only specific features of interest, ultimately enabling dynamical processes in ferroelectrics to be studied.

A nonvolatile highly scalable multilevel memory based on ferroelectric domain walls is demonstrated., Ferroelectric domain walls are atomically sharp topological defects that separate regions of uniform polarization. The discovery of electrical conductivity in specific types of walls gave rise to “domain wall nanoelectronics,” a technology in which the wall (rather than the domain) stores information. This paradigm shift critically hinges on precise nanoengineering of reconfigurable domain walls. Using specially designed nanofabricated electrodes and scanning probe techniques, we demonstrate a prototype nonvolatile ferroelectric domain wall memory, scalable to below 100 nm, whose binary state is defined by the existence or absence of conductive walls. The device can be read out nondestructively at moderate voltages (

Development of magnetoelectric, electromechanical, and photovoltaic devices based on mixed-phase rhombohedral-tetragonal (R-T) BiFeO(3) (BFO) systems is possible only if the control of the engineered R phase variants is realized. Accordingly, we explore the mechanism of a bias induced phase transformation in this system. Single point spectroscopy demonstrates that the T → R transition is activated at lower voltages compared to T → -T polarization switching. With phase field modeling, the transition is shown to be electrically driven. We further demonstrate that symmetry of formed R-phase rosettes can be broken by a proximal probe motion, allowing controlled creation of R variants with defined orientation. This approach opens a pathway to designing next-generation magnetoelectronic and data storage devices in the nanoscale.

Ferroelectrics are non-centrosymmetric crystalline materials that possess a spontaneous polarization that can be switched by an electric field. The electric-field-dependent optical response of these materials makes them important for optical devices, such as modulators or beam deflectors. In the inexorable drive to miniaturization, the concept of integrated thin film optical devices has led to the incorporation of ferroelectric thin films on single-crystal substrates. These structures have appealing electro-optic modulation characteristics, interesting strain-dependent bandgaps and refractive index, as well as promising possibilities for solar harvesting. Here, we review the work on epitaxial ferroelectric (FE) films for optical applications. We first show that FE thin film materials are attractive for integrated electro-optic modulators and then show that epitaxial strain can be used to enhance the FE and optical functionality of films. Next, we describe some of the photovoltaic functionality of FE thin film materials' systems and conclude the review by highlighting some thin-film devices that exploit the aforementioned optical effects.

We investigate the interfacial chemistry of secondary Fe2O3 phases formed in a BiFeO3 (BFO) layer in BFO/ La0.67Sr0.33MnO3 (LSMO)/SrTiO3 (STO) heterostructures. A combination of high-resolution spherical aberration corrected scanning TEM and spectroscopy results, reveals that specific chemical and crystallographic similarities between Fe2O3 and BFO, enable the BFO layer to form a facile host for Fe2O3.

Flux-mediated epitaxy of self-assembled ferroelectric Bi4Ti3O12 (BIT) nanoplates on SrTiO3 (001) is investigated with vanadium oxide (VOx) as the flux medium. It is shown that precoverage of a SrTiO3 (001) substrate with VOx helps to nucleate thin rutile TiO2 (001) crystallites that act as templates for the subsequent growth of (110)-oriented BIT nanoplates. Piezoresponse force microscopy reveals the presence of 90° domain walls even within a single BIT nanoplate, similar to BIT bulk single crystals.

The property of ferroelectricity was first reported in 1921 in Rochelle salt (sodium potassium tartrate). Once considered a rare phenomenon limited to a comparatively small class of crystals, the discovery of ferroelectricity in the binary oxide compound barium titanate (BaTiO3) in the mid-1940s paved the way for rapid advances in the search of new materials. Since that time, a great many other ferroelectric compounds and solid solutions have been found and there has been broad interest in ferroelectric materials that has continued essentially uninterrupted until the present time. Ferroelectric materials remain subjects of intensive investigation today for three principal reasons. First, the unique dielectric, pyroelectric, piezoelectric, and electro-optic properties exhibited by ferroelectric crystals, ceramics, composites, and thin films can be exploited in great many devices of commercial importance. Second, apart from their many technological applications, ferroelectric materials as a class exhibit a great diversity of phase transitions that make them ideal objects for scientific investigations into the origins and mechanisms of a wide range of structural transformation phenomena. Finally, advances in thin-film deposition and nanoscale fabrication techniques made over the past two decades have created new possibilities for the integration of these materials into the ever expanding array of microelectronic devices. Ferroelectrics form a sub-group of functional (or smart) materials whose physical properties are sensitive to changes in external conditions such as temperature, pressure, and electric fields. Below some critical temperature TC, the dielectric displacement (electric polarization) spontaneously assumes non-zero values in the absence of any externally applied force. The transition between non-ferroelectric and ferroelectric phases is accompanied by a loss of symmetry, characterized by double-well minima below the transition temperature, resulting in a switchable polarization that is accompanied by a hysteresis in the electric field–dielectric displacement response. As a class of materials, ferroelectrics also exhibit unusually large and nonlinear generalized susceptibilities; their dielectric, piezoelectric, elastic, and other properties display critical behavior near the ferroelectric phase transformation temperature. Ferroelectrics may be regarded as high energydensity materials as well that can be configured to store, release or interconvert electrical and mechanical energy in a well-controlled manner. Their exceptionally large piezoelectric compliances, pyroelectric coefficients, and dielectric susceptibilities can be exploited in a variety of microelectronic devices. Important examples of these include piezoelectric sensors and actuators, pyroelectric thermal imaging devices, high-dielectric constant capacitors, electro-optic light valves, and thin-film memories. Because of their strongly non-linear dielectric response ferroelectrics can be utilized in applications such as frequency-agile phase shifters and filters in wireless telecommunications systems. Because the properties of ferroelectric materials that are exploited in technological applications are intimately S. P. Alpay (&) G. A. Rossetti Jr. Materials Science and Engineering Program, Departments of Chemical, Materials, and Biomolecular Engineering, University of Connecticut, Storrs, CT, USA e-mail: p.alpay@ims.uconn.edu

The scaling of the piezoelectric and dielectric constants with film thickness in ultra-thin ferroelectric heterostructures is investigated. Epitaxial (001) PbZr0.2Ti0.8O3 films ranging in thickness from 5 nm to 30 nm with top and bottom SrRuO3 electrodes were grown onto (001) SrTiO3 substrates via pulsed laser deposition. Piezoelectric and dielectric measurements were performed using an atomic force microscope. The remnant value of the out of plane piezoresponse (d33) decreases from 60 pm/V for the 30 nm film to just 7 pm/V for the 5 nm film. This systematic decline in d33 is accompanied by a corresponding increase in the coercive field. The d33 loops show a systematic increase in tilt towards the applied field axis as function of reducing thickness coupled with a decrease in piezoelectric tunability. The small-signal relative dielectric response in the direction normal to the film-substrate interface decreases from 140 for a 50 nm film to just 60 for a 8 nm film. A similar drop is also observed in the dielectric tunability, from ∼17% to approximately −2% at an electric field of 750 kV/cm with the film thickness decreasing from 50 nm to 8 nm. We show that these observations cannot be explained using a straightforward application of a modified Landau-Devonshire thermodynamic model that incorporates the internal stresses due to the lattice and thermal expansion mismatch between the film and the substrate. We attribute this behavior to degradation in the polarization due to an intrinsic finite size effect.

We present a sensitive method to simultaneously acquire the C(V ) characteristics and piezoresponse of sub-micron size ferroelectric capacitors using an Atomic Force Microscope (AFM). Model Pt/(La0.5,Sr0.5)CoO3/PbZr0.4Ti0.6 O3/(La0.5,Sr0.5)CoO3/La:SrTiO3/Si nanocapacitors were fabricated by focused ion beam milling from 100 μm2 down to 0.04 μm2. With this AFM based capacitance measurement technique we show clear “double-humped” C(V ) for all sizes with no significant change in the peak value of the er down to capacitors with the smallest area of 0.04 μm2. The smallest capacitance measured is only of the order a few femtofarads, demonstrating the high sensitivity of the technique. Simultaneously, the piezoelectric response is recorded for each measurement, thus the technique facilitates simultaneous piezoresponse and dielectric characterization of ferroelectric memory devices.

In this paper we demonstrate a modified focused ion beam (FIB) milling approach integrated with self-alignment to synthesize ferroelectric nanostructures. Nanoscale holes ranging from 500 nm down to 100 nm in diameter are created on 'spun-on' amorphous TiO2 on Pt/TiO2/Si using FIB milling. It is found that spin-coating of PbZr0.4Ti0.6O3 (PZT) sol–gel precursors on these templated surfaces followed by low-temperature (~400 °C) annealing leads to a ferroelectric phase only within the holes. Piezoresponse force microscopy (PFM) was used to confirm that the crystallized phase within the holes were ferroelectric and switchable. The smallest lateral size shown to be ferroelectric is roughly 70–80 nm in diameter. Quantitative PFM of the nanoscale ferroelectric regions yielded a value of 17 pm V−1, in good agreement with previously published data on polycrystalline PZT. Additionally this method facilitates the integration of interlayer dielectrics or 'spun-on' glass during the synthesis without the need for additional lithography and processing.

We investigate photo-induced ferroelectric domain switching in a series of Pb(Zr0.2Ti0.8)O3/La0.7Sr0.3MnO3 (PZT/LSMO) bilayer thin films with varying surface morphologies by piezoresponse force microscopy under light illumination. We demonstrate that reverse poled ferroelectric regions can be almost fully recovered under laser irradiation of the PZT layer and that the recovery process is dependent on the surface morphology on the nanometer scale. The recovery process is well described by the Kolmogorov-Avrami-Ishibashi model, and the evolution speed is controlled by light intensity, sample thickness, and initial write voltage. Our findings shed light on optical control of the domain structure in ferroelectric thin films with different surface morphologies.

Ferroelectrics have been used as memory storage devices, with an upper bound on the total possible memory levels generally dictated by the number of degenerate states allowed by the symmetry of the ferroelectric phase. Here, we introduce a new concept for storage wherein the polarization can be rotated arbitrarily, effectively decoupling it from the crystallographic symmetry of the ferroelectric phase on the mesoscale. By using a Bi5Ti3FeO15-CoFe2O4 film and via Band-Excitation Piezoresponse Force Microscopy, we show the ability to arbitrarily rotate polarization, create a spectrum of switched states, and suggest the reason for polarization rotation is an abundance of sub-50 nm nanodomains. Transmission electron microscopy-based strain mapping confirms significant local strain undulations imparted on the matrix by the CoFe2O4 inclusions, which causes significant local disorder. These experiments point to controlled tuning of polarization rotation in a standard ferroelectric, and hence the potential to greatly extend the attainable densities for ferroelectric memories.

Near-field optical second harmonic microscopy has been applied to imaging of the c/a/c/a polydomain structure of epitaxial PbZrxTi1–xO3 thin films in the 0

We report on the measurement of switchable pulse polarization of micron and submicron ferroelectric capacitors contacted using an atomic force microscope. Fast square pulses with rise time on the order of tens of nanoseconds are used to obtain the switchable polarization (ΔP) of discrete polycrystalline Pb(ZrTi)O3 capacitors of 21.5, 0.69, and 0.19 μm2 prepared by sputtering and reactive ion etching. Our studies show that the switching characteristics of these capacitors are well behaved, indicating that high-speed and high-density ferroelectric memory capacitors are not limited by scaling down the capacitor area.

90° elastic domains were observed after local polings in epitaxial ferroelectric Pb(Zr0.2Ti0.8)O3 thin films via piezoresponse force microscopy. An area of internal stress arises under a conductive atomic force microscope tip due to the opposite signs of the converse piezoelectric effects in the switched domain and the unswitched films surrounding. The formation of 90° domains leads to the relaxation of the internal stress and stabilization of 180° domain after turning off the electric field applied by the tip. The criterion that formulates the necessary condition for realization of the relaxation mechanism is presented as well.

Polarization rotation engineering is a promising path to giant dielectric and electromechanical responses in ferroelectric materials and devices. This work demonstrates robust and reversible in- to out-of-plane polarization rotation in ultrathin (nanoscale) epitaxial (001) tetragonal PbZr0.3Ti0.7O3 (PZT-T)/rhombohedral PbZr0.55Ti0.45O3 (PZT-R) ferroelectric bilayers. An underlying 20 nm thick PZT-R layer reduces the symmetry in a 5 nm thick PZT-T layer by imposing an in-plane tensile strain while simultaneously decoupling the PZT-T layer from the substrate. This prevents clamping and facilitates large-scale polarization rotation switching (≈60 μC cm−2) and an effective d 33 response 500% (≈250 pm V−1) larger than the PZT-R layer alone. Furthermore, this enhancement is stable for more than 107 electrical switching cycles. These bilayers present a simple and highly controllable means to design and optimize rotational polar systems as an alternate to traditional composition-based approaches. The precise control of the subtle interface-driven interactions between the lattice and the external factors that control polarization opens a new door to enhanced—or completely new—functional properties.

Wafer-scale fabrication of ferroelectric oxide nanoshell tubes as well as ordered nanotube arrays have been accomplished using a simple and convenient fabrication method that allows full tailoring of tube dimensions as well as array pattern and size. Using different silicon and alumina templates, barium titanate and lead zirconate titanate tubes with diameters ranging from 50 nm up to several micrometers meter and lengths of more 100 μm have been fabricated. Ferroelectric switching of submicrometer tubes has been shown using piezoresponse scanning probe microscopy.

A ferroelectric field-effect transistor with a SrRuxTi1−xO3 solid-solution channel layer and a lead zirconate titanate gate oxide has been fabricated. The remnant polarization of the ferroelectric yields two states at 0 V, which produce a relative change in channel resistance (ΔR/R) of 75% and a coercivity of 3 V. The channel has sufficient off-state free carrier concentration to provide sufficient balancing charge for ferroelectric stability. The device was subjected to more than 1010 read-write cycles with no degradation. This nonvolatile device offers the possibility of a nondestructive, current sense memory cell with good retention properties.

We report on the out-of-plane piezoelectric response (d33), measured via piezoresponse scanning force microscopy, of submicron capacitors fabricated from epitaxial PbZrxTi1−xO3 thin films. Investigations on 1 μm2 and smaller capacitors show that the substrate-induced constraint is dramatically reduced by nanostructuring. At zero field, the experimentally measured values of d33 for clamped as well as submicron capacitors are in good agreement with the predictions from thermodynamic theory. The theory also describes very well the field dependence of the piezoresponse of clamped capacitors of key compositions on the tetragonal side of the PbZrxTi1−xO3 phase diagram as well as the behavior of submicron PbZr0.2Ti0.8O3 (hard ferroelectric) capacitors. However, the field-dependent piezoresponse of submicron capacitors in compositions closer to the morphotropic phase boundary (soft ferroelectrics) is different from the behavior predicted by the theoretical calculations.

Use of an epitaxial conducting template has enabled the integration of epitaxial ferroelectric perovskites on silicon. The conducting template layer, LaxSr1−xTiO3 (LSTO), deposited onto (001) silicon wafers by molecular-beam epitaxy is then used to seed {001}-oriented epitaxial perovskite layers. We illustrate the viability of this approach using PbZr0.4Ti0.6O3 (PZT) as the ferroelectric layer contacted with conducting perovskite La0.5Sr0.5CoO3 (LSCO) electrodes. An important innovation that further facilitates this approach is the use of a low-temperature (450 °C) sol–gel process to crystallize the entire ferroelectric stack. Both transmission electron microscopy and x-ray diffraction analysis indicate the LSCO/PZT/LSCO/LSTO/Si heterostructures are epitaxial. The electrical response of ferroelectric capacitors (for pulse widths down to 1 μs) measured via the underlying silicon substrate is identical to measurements made using conventional capacitive coupling method, indicating the viability of this approach.

Direct hysteresis measurements on single submicron structure sizes were performed on epitaxial ferroelectric Pb(Zr,Ti)O3 thin films grown on SrTiO3 with La0.5Sr0.5CoO3 (LSCO) electrodes. The samples were fabricated by focused-ion-beam milling resulting in pad sizes down to 200 nm×200 nm. The influence of parasitic capacitance of the measurement setup was eliminated by applying an enhanced compensation procedure. No size effects were observed in capacitors milled down to 400 nm×400 nm. Thus, a published increase of Pmax1 for decreasing pad size can be explained by the parasitic influence of the setup. Finally, the inaccuracy of increasing coercive voltage due to the coating of the cantilever of the atomic force microscope is discussed.

Scaling of the structural order parameter, polarization, and electrical properties was investigated in model ultrathin epitaxial SrRuO3/PbZr0.2Ti0.8O3/SrRuO3/SrTiO3 heterostructures. High-resolution transmission electron microscopy images revealed the interfaces to be sharp and fully coherent. Synchrotron x-ray studies show that a high tetragonality (c/a similar to 1.058) is maintained down to 50 angstrom thick films, suggesting indirectly that ferroelectricity is fully preserved at such small thicknesses. However, measurement of the switchable polarization (Delta P) using a pulsed probe setup and the out-of-plane piezoelectric response (d(33)) revealed a systematic drop from similar to 140 mu C/cm(2) and 60 pm/V for a 150 angstrom thick film to 11 mu C/cm(2) and 7 pm/V for a 50 angstrom thick film. This apparent contradiction between the structural measurements and the measured switchable polarization is explained by an increasing presence of a strong depolarization field, which creates a pinned 180 degrees polydomain state for the thinnest films. Existence of a polydomain state is demonstrated by piezoresponse force microscopy images of the ultrathin films. These results suggest that the limit for a ferroelectric memory device may be much larger than the fundamental limit for ferroelectricity. (c) 2006 American Institute of Physics.

Voltage-modulated scanning force microscopy (Piezoresponse microscopy) is applied to investigate the domain structure in epitaxial PbZr0.2Ti0.8O3 ferroelectric thin films grown on (001) SrTiO3. By monitoring the vertical and lateral differential signals from the photodetector of the atomic force microscope it is possible to separate out and observe the out-of-plane and in-plane polarization vectors in the thin film individually. The relative orientation of the polarization vectors across a 90degrees domain wall is observed. Nucleation of new reversed 180degrees domains at the 90degrees domain wall is studied and its impact on the rotation of polarization within the a domain is analyzed as a function of reversal time. (C) 2002 American Institute of Physics.

A ferroelectric field effect transistor with an oxide channel layer and a lead zirconate titanate gate oxide has been fabricated. The channel is a strontium ruthenate/titanate solid solution with n type semiconducting behavior, which has sufficient OFF-state free carrier concentration to provide proper balancing charge for ferroelectric stability. The dependence of channel resistance with gate voltage at room temperature yields a hysteresis curve with two state at zero volts with a ΔR/R of 75% and a coercive voltage of 3 volts. The device was subjected to more than 1010 cycles with no degradation and was also operated at 60° C with a only a slight reduction in the switching ratio.

Near field optical second harmonic microscopy has been applied to imaging of the c/a/c/a polydomain structure of epitaxial PbZrxTi1−xO3 thin films in the 0

We present investigations on the resistive switching effect in SrRuO3/PbZr0.2Ti0.8O3/Pt ferroelectric capacitors. Using a conductive atomic force microscope, the out-of-plane piezoelectric response and the capacitive and resistive current were simultaneously measured as a function of applied bias voltage. We observed two independent switching phenomena, one attributed to the ferroelectric switching process and the other to resistive switching. We show that I-V curves alone are not sufficient in ferroelectric materials to clarify the underlying switching mechanism and must be used with sufficient caution. (C) 2008 American Institute of Physics.

In this letter, we report on the study of nanoscale polarization relaxation phenomena in polycrystalline PbZr0.4Ti0.6O3 films. Piezoresponse force microscopy PFM images of the as-grown sample reveal grains with a range of contrast, from fully white to gray to fully black. It is shown that this local change in the contrast magnitude of the piezoresponse from grain to grain can be attributed to the crystallographic orientation within each grain. PFM-based relaxation experiments show that the rate of relaxation is different for each grain, furthermore it is strongly dependent on the tilt of individual crystallographic orientation with respect to the polar axis. Strongly tilted away nonpolar axis grains show a much stronger decay of the polarization compared to polar axis-oriented grains. Therefore, for an ensemble of grains under a common top electrode, the relaxation events would first take place in grains, which are nonpolar axis oriented. © 2005 American Institute of Physics. DOI: 10.1063/1.1977183

We have performed depth profile studies on PZT films of different thickness. Thin epitaxial films of PbZr0.2Ti0.8O3 have been deposited on mono-crystalline (001) LaAlO3 substrate by pulsed laser deposition. Grazing incidence X-ray scattering was used to study the films' structure. GIXS technique was used to analyze the in-plane compression of lattice parameters as a function of depth within the films.