Controlling domain wall motion as a route towards new functionalities in Pb(Zr,Ti)O3 ferroelectric thin films
Dr Leo McGilly, École Polytechnique Fédérale de Lausanne, Switzerland
Ferroelectric domain walls offer the exciting prospect of truly nanoscale reconfigurable circuits owing to their small thickness, typically ~1-5 nm, their inherently mobile nature and the functional properties they exhibit. However, to fully harness their potential as nanoscale functional entities, it is essential to achieve reliable and precise control of their nucleation, location, number and velocity. In this work we demonstrate an ability that allows extensive control of individual and multiple 180° domain walls in PbZr0.1Ti0.9O3 thin films. Instrumental in this implementation are the unique properties of the Pt top electrode deposited by electron-beam induced breakdown of a precursor gas. Additionally this work is accurately described through the framework of an analogy to the classical Stefan problem which has previously been used to describe many diverse systems but is here applied to electric circuits.
Furthermore advances towards readout of domain wall position, via a ‘read-restore’ technique will be presented. This method involves measurement of partial switching currents due to domain wall perturbations from an initial position through sub-switching voltage pulses.
Finally, understanding the interaction of domain walls with defects is crucial to tailoring their properties. Domain wall pinning/depinning has a significant impact on bulk materials properties. For future devices based on individual domain walls and their motion, defects could potentially kill or create functionality. Dr McGilly will show that local nanoscale defect regions can be used to modify imprint and domain wall motion. This adds an additional dimension to the domain-wall-control toolbox.
Professor Gustau Catalan, ICREA and Institut Catala de Nanociencia i Nanotecnologia, Spain.
Among the many interesting features of ferroic domain walls, one that has attracted recent interest is their photovoltaic response, triggered by the observation of above-bandgap photovoltages proportional to the number of domain walls in BiFeO3 films. However, ferroelectrics are known to also be able to display above-bandgap photovoltages even when in a mono-domain state, owing to the so-called ‘bulk photovoltaic effect’. Isolating the physics of the bulk photovoltaic effect is, however, complicated because the roles of shift currents, depolarisation fields, domain walls and even photochemistry are generally entangled.
In an effort to better understand the bulk photovoltaic effect, we have studied the photovoltaic properties of antiferroelectric thin films of PbZrO3 grown on transparent conductive substrates. We find the effect to be independent of depolarisation fields or domains, and depend only on internal symmetry (shift current theory) and on conductivity: the bulk photovoltaic effect is ‘switched on’ only when the antiferroelectric is switched into the polar state, and remains stable even in the absence of external bias thanks to the photorefractive pinning of the polar state. This allows antiferroelectrics to functionally mimic the photovoltaic behaviour of ferroelectrics, thus becoming a second family of materials where the bulk photovoltaic effect is possible.
Quantitatively, the photovoltage is the product of the (intrinsic) shift current times the (extrinsic) resistivity of the film, which explains the large dispersion of photovoltages reported for the same material. This also means that high resistivity films can display giant photovoltages only limited by the breakdown strength of the film; in the particular case of our antiferroelectric films, photovoltages in excess of 100 V can be achieved in the vertical direction. When divided by thickness, the resulting photoelectric fields are >5MV/cm, the largest ever reported for any material.
Domain wall transport and novel device architectures in PZT thin films
Professor Patrycja Paruch, University of Geneva, Switzerland
In ferroelectric materials, domain walls separate regions with different polarisation orientation, and can present novel functional properties quite different from those of the parent phase. The extreme localisation of such properties at these intrinsically nanoscale features makes them potentially useful as active components in future miniaturised electronic devices.
Particularly exciting has been the discovery of domain-wall-specific electrical conduction in many ferroelectric families. Here, I will present our scanned probe microscopy observations of such conduction in Pb(Zr,Ti)O3 (PZT), highlighting the key role of oxygen vacancies and surface adsorbates, whose distribution can be modulated to reversibly control domain wall transport. We also map out the effects of the surface adsorbates on polarisation switching and domain wall velocities under different conditions of relative humidity. Using a ‘pump-probe’ approach, we explore the earliest stages of domain switching, demonstrating unexpectedly long lifetimes for the subcritical nucleus formed under ultrashort high voltage pulses.
In the same ferroelectric samples, we also find an unusual piezoelectric shear response, forbidden by symmetry in the parent phase, which could be technologically important for ferroelectric based surface acoustic wave devices.
Polarisation-enabled electronic transport in ferroelectric films
Professor Alexei Gruverman, University of Nebraska-Lincoln, USA
Variability of the electronic properties of 2D materials and ferroelectrics (FE) offers a wealth of fundamentally important physical phenomena and exciting technological opportunities for the hybrid 2D-FE heterostructures comprised of these materials. Among particularly promising aspects of these heterostructures is a coupling between the electrically-switchable polarisation and electronic transport, which allows realisation of advanced devices with enhanced functional characteristics. This talk will focus on recent advances in realisation of these electronic devices. Specifically, we employ polarisation reversal to modulate (1) the in-plane transport of the interfacial conducting channel in the ferroelectric field effect transistor (FE-FET) devices, and (2) the perpendicular-to-plane tunnelling conductance across the ferroelectric barrier in the ferroelectric tunnel junction (FTJ) devices. We show that interface engineering in the 2D-FE systems provides a possibility of successfully addressing the most serious challenges relevant to device performance, such as ON/OFF ratio, lifetime, operation endurance and reliability.