Charge screening strategy for domain pattern control in nano-scale ferroelectric systems.

Strain engineering is a widespread strategy used to enhance performance of devices based on semiconductor thin films. In ferroelectrics strain engineering is used to control the domain pattern: When an epitaxial film is biaxially compressed, e.g. due to lattice mismatch with the substrate, the film displays out-of-plane, often strongly enhanced polarization, while stretching the film on the substrate results in in-plane polarization. However, this strategy is of a limited applicability in nanorods because of the small rod/substrate contact area. Here we demonstrate another strategy, in which the polar axis direction is controlled by charge screening. When charge screening is maintained by bottom and top metallization, the nanorods display an almost pure c-domain configuration (polarization perpendicular to the substrate); when the sidewalls of the nanorods are metallized too, a-domain formation prevails (polarization parallel to the substrate). Simulations of the depolarization fields under various boundary conditions support the experimental observations. The employed approach can be expanded to other low-dimensional nano-scale ferroelectric systems.

Controlled Charging of Ferroelastic Domain Walls in Oxide Ferroelectrics.

Conductive domain walls (DWs) in ferroic oxides as device elements are a highly attractive research topic because of their robust and agile response to electric field. Charged DWs possessing metallic-type conductivity hold the highest promises in this aspect. However, their intricate creation, low stability, and interference with nonconductive DWs hinder their investigation and the progress toward future applications. Here, we find that conversion of the nominally neutral ferroelastic 90° DWs into partially charged DWs in Pb(Zr0.1Ti0.9)O3 thin films enables easy and robust control over the DW conductivity. By employing transmission electron microscopy, conductive atomic force microscopy and phase-field simulation, our study reveals that charging of the ferroelastic DWs is controlled by mutually coupled DW bending, type of doping, polarization orientation and work-function of the adjacent electrodes. Particularly, the doping outweighs other parameters in controlling the DW conductivity. Understanding the interplay of these key parameters not only allows us to control and optimize conductivity of such ferroelastic DWs in the oxide ferroelectrics but also paves the way for utilization of DW-based nanoelectronic devices in the future.

Free-Carrier-Compensated Charged Domain Walls Produced with Super-Bandgap Illumination in Insulating Ferroelectrics.

Charged domain walls in ferroelectrics are movable and electronically conducting interfaces inside insulating materials. A simple and reliable method for their artificial production with ultraviolet (UV) light is described. The UV illumination produces free carriers in ferroelectric bulk, which simultaneously promotes the formation of charged domain walls and provides charge for their compensation.

Néel-like domain walls in ferroelectric Pb(Zr,Ti)O3 single crystals.

In contrast to the flexible rotation of magnetization direction in ferromagnets, the spontaneous polarization in ferroelectric materials is highly confined along the symmetry-allowed directions. Accordingly, chirality at ferroelectric domain walls was treated only at the theoretical level and its real appearance is still a mystery. Here we report a Néel-like domain wall imaged by atom-resolved transmission electron microscopy in Ti-rich ferroelectric Pb(Zr1-xTix)O3 crystals, where nanometre-scale monoclinic order coexists with the tetragonal order. The formation of such domain walls is interpreted in the light of polarization discontinuity and clamping effects at phase boundaries between the nesting domains. Phase-field simulation confirms that the coexistence of both phases as encountered near the morphotropic phase boundary promotes the polarization to rotate in a continuous manner. Our results provide a further insight into the complex domain configuration in ferroelectrics, and establish a foundation towards exploring chiral domain walls in ferroelectrics.

Piezoelectric enhancement under negative pressure.

Enhancement of ferroelectric properties, both spontaneous polarization and Curie temperature under negative pressure had been predicted in the past from first principles and recently confirmed experimentally. In contrast, piezoelectric properties are expected to increase by positive pressure, through polarization rotation. Here we investigate the piezoelectric response of the classical PbTiO3, Pb(Zr,Ti)O3 and BaTiO3 perovskite ferroelectrics under negative pressure from first principles and find significant enhancement. Piezoelectric response is then tested experimentally on free-standing PbTiO3 and Pb(Zr,Ti)O3 nanowires under self-sustained negative pressure, confirming the theoretical prediction. Numerical simulations verify that negative pressure in nanowires is the origin of the enhanced electromechanical properties. The results may be useful in the development of highly performing piezoelectrics, including lead-free ones.

Bent Ferroelectric Domain Walls as Reconfigurable Metallic-Like Channels.

Use of ferroelectric domain-walls in future electronics requires that they are stable, rewritable conducting channels. Here we demonstrate nonthermally activated metallic-like conduction in nominally uncharged, bent, rewritable ferroelectric-ferroelastic domain-walls of the ubiquitous ferroelectric Pb(Zr,Ti)O3 using scanning force microscopy down to a temperature of 4 K. New walls created at 4 K by pressure exhibit similar robust and intrinsic conductivity. Atomic resolution electron energy-loss spectroscopy confirms the conductivity confinement at the wall. This work provides a new concept in "domain-wall nanoelectronics".

Formation of charged ferroelectric domain walls with controlled periodicity.

Charged domain walls in proper ferroelectrics were shown recently to possess metallic-like conductivity. Unlike conventional heterointerfaces, these walls can be displaced inside a dielectric by an electric field, which is of interest for future electronic circuitry. In addition, theory predicts that charged domain walls may influence the electromechanical response of ferroelectrics, with strong enhancement upon increased charged domain wall density. The existence of charged domain walls in proper ferroelectrics is disfavoured by their high formation energy and methods of their preparation in predefined patterns are unknown. Here we develop the theoretical background for the formation of charged domain walls in proper ferroelectrics using energy considerations and outline favourable conditions for their engineering. We experimentally demonstrate, in BaTiO3 single crystals the controlled build-up of high density charged domain wall patterns, down to a spacing of 7 µm with a predominant mixed electronic and ionic screening scenario, hinting to a possible exploitation of charged domain walls in agile electronics and sensing devices.

Negative-pressure-induced enhancement in a freestanding ferroelectric.

Ferroelectrics are widespread in technology, being used in electronics and communications, medical diagnostics and industrial automation. However, extension of their operational temperature range and useful properties is desired. Recent developments have exploited ultrathin epitaxial films on lattice-mismatched substrates, imposing tensile or compressive biaxial strain, to enhance ferroelectric properties. Much larger hydrostatic compression can be achieved by diamond anvil cells, but hydrostatic tensile stress is regarded as unachievable. Theory and ab initio treatments predict enhanced properties for perovskite ferroelectrics under hydrostatic tensile stress. Here we report negative-pressure-driven enhancement of the tetragonality, Curie temperature and spontaneous polarization in freestanding PbTiO3 nanowires, driven by stress that develops during transformation of the material from a lower-density crystal structure to the perovskite phase. This study suggests a simple route to obtain negative pressure in other materials, potentially extending their exploitable properties beyond their present levels.

Polarization charge as a reconfigurable quasi-dopant in ferroelectric thin films.

Impurity elements used as dopants are essential to semiconductor technology for controlling the concentration of charge carriers. Their location in the semiconductor crystal is determined during the fabrication process and remains fixed. However, another possibility exists whereby the concentration of charge carriers is modified using polarization charge as a quasi-dopant, which implies the possibility to write, displace, erase and re-create channels having a metallic-type conductivity inside a wide-bandgap semiconductor matrix. Polarization-charge doping is achieved in ferroelectrics by the creation of charged domain walls. The intentional creation of stable charged domain walls has so far only been reported in BaTiO3 single crystals, with a process that involves cooling the material through its phase transition under a strong electric bias, but this is not a viable technology when real-time reconfigurability is sought in working devices. Here, we demonstrate a technique allowing the creation and nanoscale manipulation of charged domain walls and their action as a real-time doping activator in ferroelectric thin films. Stable individual and multiple conductive channels with various lengths from 3 µm to 100 nm were created, erased and recreated in another location, and their high metallic-type conductivity was verified. This takes the idea of hardware reconfigurable electronics one step forward.

Controlled stripes of ultrafine ferroelectric domains.

In the pursuit of ferroic-based (nano)electronics, it is essential to minutely control domain patterns and domain switching. The ability to control domain width, orientation and position is a prerequisite for circuitry based on fine domains. Here, we develop the underlying theory towards growth of ultra-fine domain patterns, substantiate the theory by numerical modelling of practical situations and implement the gained understanding using the most widely applied ferroelectric, Pb(Zr,Ti)O3, demonstrating controlled stripes of 10 nm wide domains that extend in one direction along tens of micrometres. The observed electrical conductivity along these thin domains embedded in the otherwise insulating film confirms their potential for electronic applications.

Ferroelectric translational antiphase boundaries in nonpolar materials.

Ferroelectric materials are heavily used in electro-mechanics and electronics. Inside the ferroelectric, domain walls separate regions in which the spontaneous polarization is differently oriented. Properties of ferroelectric domain walls can differ from those of the domains themselves, leading to new exploitable phenomena. Even more exciting is that a non-ferroelectric material may have domain boundaries that are ferroelectric. Many materials possess translational antiphase boundaries. Such boundaries could be interesting entities to carry information if they were ferroelectric. Here we show first that antiphase boundaries in antiferroelectrics may possess ferroelectricity. We then identify these boundaries in the classical antiferroelectric lead zirconate and evidence their polarity by electron microscopy using negative spherical-aberration imaging technique. Ab initio modelling confirms the polar bi-stable nature of the walls. Ferroelectric antiphase boundaries could make high-density non-volatile memory; in comparison with the magnetic domain wall memory, they do not require current for operation and are an order of magnitude thinner.

Free-electron gas at charged domain walls in insulating BaTiO3.

Hetero interfaces between metal-oxides display pronounced phenomena such as semiconductor-metal transitions, magnetoresistance, the quantum hall effect and superconductivity. Similar effects at compositionally homogeneous interfaces including ferroic domain walls are expected. Unlike hetero interfaces, domain walls can be created, displaced, annihilated and recreated inside a functioning device. Theory predicts the existence of 'strongly' charged domain walls that break polarization continuity, but are stable and conduct steadily through a quasi-two-dimensional electron gas. Here we show this phenomenon experimentally in charged domain walls of the prototypical ferroelectric BaTiO3. Their steady metallic-type conductivity, 10(9) times that of the parent matrix, evidence the presence of stable degenerate electron gas, thus adding mobility to functional interfaces.

Enhanced electromechanical response of ferroelectrics due to charged domain walls.

While commonly used piezoelectric materials contain lead, non-hazardous, high-performance piezoelectrics are yet to be discovered. Charged domain walls in ferroelectrics are considered inactive with regards to the piezoelectric response and, therefore, are largely ignored in this search. Here we demonstrate a mechanism that leads to a strong enhancement of the dielectric and piezoelectric properties in ferroelectrics with increasing density of charged domain walls. We show that an incomplete compensation of bound polarization charge at these walls creates a stable built-in depolarizing field across each domain leading to increased electromechanical response. Our model clarifies a long-standing unexplained effect of domain wall density on macroscopic properties of domain-engineered ferroelectrics. We show that non-toxic ferroelectrics like BaTiO(3) with dense patterns of charged domain walls are expected to have strongly enhanced piezoelectric properties, thus suggesting a new route to high-performance, lead-free ferroelectrics.

Cold-field switching in PVDF-TrFE ferroelectric polymer nanomesas.

Polarization reversal in ferroelectric nanomesas of polyvinylidene fluoride with trifluoroethylene has been probed by ultrahigh vacuum piezoresponse force microscopy in a wide temperature range from 89 to 326 K. In dramatic contrast to the macroscopic data, the piezoresponse force microscopy local switching was nonthermally activated and, at the same time, occurring at electric fields significantly lower than the intrinsic switching threshold. A "cold-field" defect-mediated extrinsic switching is shown to be an adequate scenario describing this peculiar switching behavior. The extrinsic character of the observed polarization reversal suggests that there is no fundamental bar for lowering the coercive field in ferroelectric polymer nanostructures, which is of importance for their applications in functional electronics.

Thermally induced cooperative molecular reorientation and nanoscale polarization switching behaviors of ultrathin poly(vinylidene fluoride-trifluoroethylene) films.

Ultrathin films of the ferroelectric polymer poly(vinylidene fluoride-trifluoroethylene) [P(VDF-TrFE)] have recently attracted intensive research interest due to their potential applications in emerging organic devices. As special geometry confinement systems, many aspects about their processing, microstructure, and performance are far from being well understood. Here, the cooperative molecular orientation, macroscopic ferroelectric properties, and nanoscale polarization switching behaviors of thermally crystallized ultrathin P(VDF-TrFE) films were investigated. With increasing annealing temperature, the films showed a distinct granule toward layered needle-network (LNN) morphology transition with deteriorated ferroelectricity at a critical point (T(cr)) around 140 °C. Accompanying this is that the polymer backbone first lay more parallel relative to the substrate, and then exactly at T(cr) it showed an abrupt standing-up reorientation. Interestingly, the polarization axis simultaneously showed just opposite orientation and reorientation. Nanoscale polarization switching characterization by using piezoresponse force microscopy and local ferroelectric hysteresis loops revealed a varied molecular orientation in the same needle grain and a polarization reversal constraint effect by the inhomogeneous LNN structure. On the basis of these observations, a tilted-chain lamellae structural model was proposed for the LNN film. The lying down of the polarization axis and the polarization reversal constrain effect well explain the inferior performance of the LNN film despite its higher crystallinity than that of the granular film. The results may shed some light on the understanding of the intercorrelation among the thermal crystallization, microstructure, and macroscopic performance of ultrathin polymer films.

Improved screening ability of ferroelectric-semiconductor interface.

Recent progress in integrating ferroelectrics directly on silicon opens the exciting possibility of implementing ferroelectric-semiconductor devices. One of the major problems for such integration is the instability of the ferroelectric state in very thin films, which is mainly controlled by the screening ability of the ferroelectric-semiconductor interface. We show here that the presence of built-in potential in the semiconductor can strongly influence the screening ability of the interface. The built-in potential depends on the electron affinities and surface states density and can be controlled by choosing the materials carefully.

Tunable thin film bulk acoustic wave resonator based on Ba(x)Sr(1-x)TiO3 thin film.

A tunable membrane-type thin film bulk acoustic wave resonator (TFBAR) based on a Ba(0.3)Sr(0.7)TiO(3)(BST) thin film has been fabricated. The resonance and antiresonance frequencies of the device can be altered by applying a dc bias: both shift down with increasing dc electric field. The resonance and antiresonance frequencies showed a tuning of -2.4% and -0.6%, respectively, at a maximum dc electric field of 615 kV/cm. The electromechanical coupling factor of the device increased up to 4.4%. We demonstrate that the tuning of the TFBAR is nonhysteretic. The Q-factor of the device showed some variation with dc bias and is about 200. The tuning of the TFBAR is caused by the dc bias dependence of the sound velocity and the intrinsic electromechanical coupling factor of the BST layer. We apply our recently developed theory on the electrical tuning of dc bias induced acoustic resonances in paraelectric thin films to successfully model the tuning behavior of the TFBAR. The modeling enabled us to de-embed the intrinsic electromechanical properties of the BST thin film. We show that the mechanical load of our device does not significantly degrade the tuning performance of the BST layer. The performance of the TFBAR is compared with the available data on varactor tuned TFBARs.

Large-scale fabrication of titanium-rich perovskite PZT submicro/nano wires and their electromechanical properties.

We report a 2-step approach to prepare tetragonal perovskite PbZr(0.1)Ti(0.9)O(3) submicro/nano wires in gram scale and with over 95% wire content. Non-perovskite precursor wires were first fabricated by hydrothermal processing. A subsequent annealing in a PbO atmosphere at 600 degrees C converted these wires into perovskite structures which retain the one-dimensional shape. Binding of the perovksite nanowires to a conductive substrate could be achieved by a similar heat treatment of the non-perovskite precursor wires on a flat Pt coated substrate. Taking advantage of the strong mechanical attachment and good electrical contact between the wires and the metallic layer, piezoresponse force microscopy (PFM) was used to measure the local piezoelectric and ferroelectric properties of the individual wires. Enhanced piezoelectric response relative to sputtered epitaxial PbZr(0.2)Ti(0.8)O(3) film and squared hysterisis loop with sharp switching indicate pronounced electro-mechanical and ferroelectric behavior. The 90 degree domain structure of the as-prepared perovskite PZT wires was confirmed by both PFM and transmission electron microscopy investigations.

DC bias-dependent shift of the resonance frequencies in BST thin film membranes.

Direct current (DC) bias-dependent acoustic resonance phenomena have been observed in micromachined tunable thin film capacitors based on Ba(0.3)Sr(0.7)TiO3 (BST) thin films. The antiresonance frequency is only weakly DC bias dependent, and the resonance frequency exhibits a much stronger dependence on the applied DC bias. The resonance frequency shifted by 1.2% for a frequency of about 6.7 GHz and an applied field of 667 KV/cm. At the same time the effective electromechanical coupling constant k(2)(t,eff) increased to 2.0%. The tuning of the resonance frequency depends on the tunability of the film permittivity and on the mechanical load on the piezoactive layer. The experimental observations correlate well with the theoretical predictions derived from the free energy P expansion using Landau theory.

Nonvolatile gate effect in a ferroelectric-semiconductor quantum well.

Field effect transistors with ferroelectric gates would make ideal rewritable nonvolatile memories were it not for the severe problems in integrating the ferroelectric oxide directly on the semiconductor channel. We propose a powerful way to avoid these problems using a gate material that is ferroelectric and semiconducting simultaneously. First, ferroelectricity in semiconductor (Cd,Zn)Te films is proven and studied using modified piezoforce scanning probe microscopy. Then, a rewritable field effect device is demonstrated by local poling of the (Cd,Zn)Te layer of a (Cd,Zn)Te/CdTe quantum well, provoking a reversible, nonvolatile change in the resistance of the 2D electron gas. The results point to a potential new family of nanoscale one-transistor memories.