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

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 mu m with a predominant mixed electronic and ionic screening scenario, hinting to a possible exploitation of charged domain walls in agile electronics and sensing devices

Physics and applications of charged domain walls

The charged domain wall is an ultrathin (typically nanosized) interface between two domains; it carries bound charge owing to a change of normal component of spontaneous polarization on crossing the wall. In contrast to hetero-interfaces between different materials, charged domain walls (CDWs) can be created, displaced, erased, and recreated again in the bulk of a material. Screening of the bound charge with free carriers is often necessary for stability of CDWs, which can result in giant two-dimensional conductivity along the wall. Usually in nominally insulating ferroelectrics, the concentration of free carriers at the walls can approach metallic values. Thus, CDWs can be viewed as ultrathin reconfigurable strongly conductive sheets embedded into the bulk of an insulating material. This feature is highly attractive for future nanoelectronics. The last decade was marked by a surge of research interest in CDWs. It resulted in numerous breakthroughs in controllable and reproducible fabrication of CDWs in different materials, in investigation of CDW properties and charge compensation mechanisms, in discovery of light-induced effects, and, finally, in detection of giant two-dimensional conductivity. The present review is aiming at a concise presentation of the main physical ideas behind CDWs and a brief overview of the most important theoretical and experimental findings in the field

Identification of microscopic domain wall motion from temperature dependence of nonlinear dielectric response

It is known that the permittivity of ferroelectric polydomain films and single crystals in weak electric fields is strongly enhanced by the reversible movement of pinned domain walls. Two mechanisms of the movement exist: first, the bending of free segments of the domain wall and second the planar movement of the domain wall as a whole. In this work, we theoretically demonstrate that it is possible to determine the dominant mechanism of the reversible domain wall movement by means of a temperature measurement of a nonlinear macroscopic dielectric response. In addition, we demonstrate that using this approach, it is possible to obtain quantitative information on the microscopic distribution of the pinning centers. Thus, we suggest that this concept may serve as a simple and useful characterisation tool in the process of development of high-permittivity materials. Published by AIP Publishing

dietz 621. FORTRAN Beschreibung

SIGLETechnische Informationsbibliothek Hannover: AC 5756. / FIZ - Fachinformationszzentrum Karlsruhe / TIB - Technische InformationsbibliothekDEGerman

Static negative capacitance of a ferroelectric nano-domain nucleus

Miniaturization of conventional field effect transistors (FETs) approaches the fundamental limits beyond which opening and closing the transistor channel require higher gate voltage swing and cause higher power dissipation and heating. This problem could be eliminated by placing a ferroelectric layer between the FET gate electrode and the channel, which effectively amplifies the gate voltage. The original idea of using a bulk ferroelectric negative capacitor suffers however from irreversible multi-domain ferroelectric switching, which does not allow us to stabilize static negative capacitance, while a recent reversible solution with super-lattices may be difficult to integrate onto FET. Here, we introduce a solution which provides static negative capacitance from a nano-domain nucleus. Phase-field simulations confirm the robustness of this concept, the conveniently achievable small effective negative capacitance and the potentially high compatibility of such a negative nano-capacitor with FET technology. Published by AIP Publishing

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(1-3) 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(2,4,5). The intentional creation of stable charged domain walls has so far only been reported in BaTiO3 single crystals(6), 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 mu m 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(7) one step forward

Phase field simulations of ferroelastic toughening: The influence of phase boundaries and domain structures

Limited reliability of ferroelectric-based actuators restricts their use in high-performance applications, where stress-induced cracking of ferroelectric ceramics often leads to fatal failure. The main limiting factors are the relatively small fracture toughness and the brittle nature of ferroelectrics. However, ferroelectrics naturally exhibit fracture toughening (so called ferroelastic toughening) due to stress induced reorientation of non-180 degrees domains that inhibits crack propagation. Here we present a phase-field study of ferroelastic toughening based on Landau-Ginzburg-Devonshire theory. The primary qualitative factors that control the magnitude of ferroelastic toughening are identified and discussed. (c) 2012 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved

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