Nanoscale Defect Engineering and the Resulting Effects on Domain Wall Dynamics in Ferroelectric Thin Films

Defect engineering is one of the cornerstones of the modern electronics industry. Almost all electronic devices include materials that have been doped by ion bombardment. For materials where crystallinity is essential, such as ferroelectrics, defect type and concentration can vastly influence properties and are often used to optimize device performance. This study shows a method to effectively control the density and position on the nanoscale of defect sites in thin films of Pb(Zr,Ti)O3 via focused ion beam microscopy. This allows for exceptional clarity of observation of the role of defects in nucleation, polarization switching, and domain wall interaction through investigation with piezoresponse force microscopy and transmission electron microscopy, adding insight to accepted but seldom-demonstrated facts on defect-induced effects. This nanoscale defect engineering can be used as a tool to control material properties, and furthermore, a route is demonstrated toward a practical application.

Controlled creation and displacement of charged domain walls in ferroelectric thin films

Arnaud Robert André Crassous, Ludwig Feigl, Silviu Cosmin Sandu, Nava Setter, Tomas Sluka

Charged domain walls in ferroelectric materials are of high interest due to their potential use in nanoelectronic devices. While previous approaches have utilized complex scanning probe techniques or frustrative poling here we show the creation of charged domain walls in ferroelectric thin films during simple polarization switching using either a conductive probe tip or patterned top electrodes. We demonstrate that ferroelectric switching is accompanied - without exception - by the appearance of charged domain walls and that these walls can be displaced and erased reliably. We ascertain from a combination of scanning probe microscopy, transmission electron microscopy and phase field simulations that creation of charged domain walls is a by-product of, and as such is always coupled to, ferroelectric switching. This is due to the (110) orientation of the tetragonal (Pb, Sr) TiO3 thin films and the crucial role played by the limited conduction of the LSMO bottom electrode layer used in this study. This work highlights that charged domain walls, far from being exotic, unstable structures, as might have been assumed previously, can be robust, stable easily-controlled features in ferroelectric thin films.

Nature Publishing Group2016

In Search of Ferroelectricity in Antiferroelectric Lead Zirconate

Kaushik Vaideeswaran

Since the prediction of antiferroelectricity and the subsequent discovery of PbZrO3, the description of antiferroelectric behaviour in materials has been constantly modified to account for the latest properties identified in antiferroelectric materials. Adding to this refinement, the current study considers Lead Zirconate (PbZrO3) as a prototypical antiferroelectric and through it, aims to understand the origin of antiferroelectricity, and the possibility of localised ferroelectric behaviour inside the material. The observation and control of the occurence of such localised ferroelectricity is attempted and the mechanical behaviour of such ferroelectric structures is simulated. Antiferroelectrics are defined as materials which undergo a phase transition from one centrosymmetric phase to another while being accompanied by a dielectric anomaly at the transition. The origin of antiferroelectricity as observed in the prototypical antiferroelectric PbZrO3, involves multiple lattice instabilities with interactions that remained only partially deciphered. The current study provides for an explanation of the lattice dynamics occuring in PbZrO3 through results obtained from a combination of scattering techniques such as Inelastic XRay, Thermal Diffused and Brillouin scattering. The primary structural instability associated with the antiparallel lead displacements is seen to be coupled with the ferroelectric order parameter, and to the order parameter related to the oxygen octahedral rotations. On the appearance of the structural order parameter, these interactions result in the formation of the antiferroelectric phase of PbZrO3 as known previously. The premises for such interactions and their validations are provided, while explaining all aspects associated with the antiferroelectric phase transition occuring in PbZrO3 including the dielectric anomaly. One significance of the scenario with competing order parameters lies in the possible appearance of the subdued order parameters in regions where the primary order parameter is absent. The conditions for the disappearance of this order parameter in the case of PbZrO3 is discussed, and structural domain boundaries are shown to be potential regions for the local observation of otherwise subdued ferroelectricity. Aimed at the observation of localised ferroelectric structures in an otherwise antiferroelectric material, growth of epitaxial thin films of PbZrO3 is undertaken using Pulsed Laser Deposition. The growth parameters are varied to control the crystalline orientation and the defect concentration in the films through the control of the interfacial strain. In this process, a technique for the tunable variation of the epitaxial strain using a single composition buffer layer has been documented. The antiferroelectric films are then utilised for the observation of regions of disrupted symmetry, for the observation of localised ferroelectricity. The distribution of such regions in the thin films, as well as the anomalous interaction of different types of domain walls with crystalline defects are observed and analysed. Alongside, the response of such localised ferroelectric structures to external electric fields is simulated. The conditions necessary for sufficiently large displacements of such structures, large enough for potential observation using Scanning Probe Microscopy techniques, have been listed.

EPFL2015

In Search of Ferroelectricity in Antiferroelectric Lead Zirconate

Kaushik Vaideeswaran

EPFL2015