Ludwig Feigl

Controlling domain wall motion in ferroelectric thin films

Domain walls in ferroic materials have attracted significant interest in recent years, in particular because of the unique properties that can be found in their vicinity. 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. Here, using piezoresponse force microscopy, we show the control and manipulation of domain walls in ferroelectric thin films of Pb(Zr,Ti)O₃ with Pt top electrodes. This high-level control presents an excellent opportunity to demonstrate the versatility and flexibility of ferroelectric domain walls. Their position can be controlled by the tuning of voltage pulses, and multiple domain walls can be nucleated and handled in a reproducible fashion. The system is accurately described by analogy to the classical Stefan problem, which has been used previously to describe many diverse systems and is here applied to electric circuits. This study is a step towards the realization of domain wall nanoelectronics utilizing ferroelectric thin films.

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.

Impact of misfit relaxation and a-domain formation on the electrical properties of tetragonal PbZr0.4Ti0.6O3/PbZr0.2Ti0.8O3 thin film heterostructures: Experiment and theoretical approach

Heterostructures consisting of PbZr0.2Ti0.8O3 and PbZr0.4Ti0.6O3 films grown on a SrTiO3 (100) substrate with a SrRuO3 bottom electrode were prepared by pulsed laser deposition. Using the additional interface provided by the ferroelectric bilayer structure and changing the sequence of the layers, the dislocation content and domain patterns were varied. The resulting microstructure was investigated by transmission electron microscopy. Macroscopic ferroelectric measurements have shown a large impact of the formation of dislocations and 90° domains on the ferroelectric polarization and dielectric constant. A thermodynamic analysis using the LANDAU-GINZBURGDEVONSHIRE approach that takes into account the ratio of the thicknesses of the two ferroelectric layers and electrostatic coupling is used to describe the experimental data. a) Electronic mail: lgeske@mpi-halle.de b) Present Adress: Nuclear Science&Engineering, Massachusetts Institute of Technology, Northwest Bldg. 13, Cambridge, MA, USAHeterostructures consisting of PbZr0.2Ti0.8O3 and PbZr0.4Ti0.6O3 epitaxial films on a SrTiO3 (100) substrate with a SrRuO3 bottom electrode were prepared by pulsed laser deposition. By using the additional interface provided by the ferroelectric bilayer structure and changing the sequence of the layers, the content of dislocations and elastic domain types was varied in a controlled manner. The resulting microstructure was investigated by transmission electron microscopy. Macroscopic ferroelectric measurements have shown a large impact of the formation of dislocations and 90° domain walls on the ferroelectric polarization and dielectric constant. A thermodynamic analysis using the Landau–Ginzburg–Devonshire approach that takes into account the ratio of the thicknesses of the two ferroelectric layers and electrostatic coupling is used to shed light on the experimental data.

Compliant ferroelastic domains in epitaxial Pb(Zr,Ti)O3 thin films

Ordered patterns of highly compliant ferroelastic domains have been created by use of tensile strained epitaxial Pb(Zr,Ti)O3 thin films, of very low defect density, grown on DyScO3 substrates. The effect of 180° switching on well-ordered a/c 90° domain patterns is investigated by a combination of transmission electron microscopy, piezoelectric force microscopy, and X-ray diffraction. It is shown that ferroelastic a-domains, having an in-plane polarization, can be created and completely removed on a local level by an out-of-plane electric field. The modifications of the ferroelastic domain pattern can be controlled by varying the parameters used during switching with a piezoresponse force microscope to produce the desired arrangement.

Post-deposition control of ferroelastic stripe domains and internal electric field by thermal treatment

The dependence of the formation of ferroelastic stripe domain patterns on the thermal history is investigated by detailed piezoresponse force microscopy and X-ray diffraction experiments after and during annealing of tensile strained tetragonal Pb(Ti,Zr)O3 epitaxial thin films on DyScO3 substrates. In particular, the ferroelastic pattern is reversibly interchanged between a cross-hatched and a stripe domain pattern if the films are cooled at different rates after annealing above the formation temperature of a-domains. Different types of 180° and non-180° patterns can be created, depending on the thermal treatment. The changes in the 180° domain structure and lattice parameters are attributed to a change of oxygen vacancy concentration, which results in a modification of the internal electric field and unit cell size, causing also a shift of TC. Thermal treatment is done on rhombohedral La:BiFeO3 thin films as well. It is observed that also in these films, appropriate heat treatment modifies the domain pattern and films with a stripe domain pattern can be created, confirming the general validity of the developed model.

Impact of high interface density on ferroelectric and structural properties of PbZr(0.2)Ti(0.8)O(3)/PbZr(0.4)Ti(0.6)O(3) epitaxial multilayers

Multilayers consisting of two tetragonal compositions PbZr(0.2)Ti(0.8)O(3) and PbZr(0.4)Ti(0.6)O(3) were deposited onto a SrRuO(3) electrode grown on a vicinal (100) SrTiO(3) substrate. It has been shown by extensive structural investigations comprising transmission electron microscopy in conventional and high resolution mode, reciprocal space mapping and piezoresponse force microscopy that with decreasing layer thickness a transition from a-domains confined to individual layers to a-domains propagating through the whole film takes place. This is caused by the formation of a common strain state of all layers which is responsible for the observed enhancement of the electrical properties. These show a maximum in the product of remanent polarization and dielectric constant at a certain density of interfaces. If the interface density becomes too high the lattice distortion accompanying each interface deteriorates the properties of the multilayer structure.

Superdomain Structure in Epitaxial Tetragonal PZT Thin Films Under Tensile Strain

The a/c domain pattern of tetragonal PbZr0.10Ti0.90O3 thin films under tensile misfit strain is investigated by piezoresponse force microscopy and X-ray diffraction. The results show a hierarchical ordering of the dense a/c domain structure into larger superstructures. The latter exhibit a preferred orientation and occasionally form distinct patterns such as flux closure loops of the net polarization. Additionally, the residual strain is measured with reciprocal space maps and the a-domain fraction is determined by theoretical calculations and correlated with results from rocking curve scans.

Room temperature concurrent formation of ultra-dense arrays of ferroelectric domain walls

Properties of ferroelectric domain walls are attractive for future nano- and optoelectronics. An important element is the potential to electrically erase/rewrite domain walls inside working devices. Dense domain wall patterns, formed upon cooling through the ferroelectric phase transition, were demonstrated. However, room temperature domain wall writing is done with a cantilever tip, one domain stripe at a time, and reduction of the inter-wall distance is limited by the tip diameter. Here, we show, at room temperature, controlled formation of arrays of domain walls with sub-tip-diameter spacing (i.e., inter-wall distance down to approximate to 10 nm). Each array contains 100-200 concurrently formed walls. Array rewriting is confirmed. The method is demonstrated in several materials. Dense domain pattern formation through a continuous electrode, practical for potential device applications, is also demonstrated. A quantitative theory of the phenomenon is provided

Chromium doping of epitaxial PbZr0.2Ti0.8O3 thin films

Epitaxial ferroelectric PbZr0.2Ti0.8O3 thin films were grown by pulsed laser deposition. PbZr0.2Ti0.8O3 was doped with Cr acting as acceptor ion. Microstructural characterization was performed by (high resolution) transmission electron microscopy. The voltage dependence of polarization, dielectric constant, and leakage current were measured with respect to the Cr content. To derive the electronic properties, PZT was considered as a wide-gap semiconductor which allows treating the metal-PZT interface as a Schottky contact. The Cr was found to facilitate the elastic relaxation of the film. Furthermore, the leakage current was increased through a reduction of the Schottky barrier.

Polarization Switching and Domain Wall Motion in Circular and Ring Capacitor Structures in PZT Thin Films

Ferroelectric switching in circular and ring capacitors has been performed using a stroboscopic mode of piezoresponse force microscopy. A simple geometric model incorporating the characteristic domain wall motion is sufficient to describe the switching of circular capacitors but which however breaks down when attempting to describe the ring switching. Analysis of the switching dynamics implies that the domain wall moves faster along the perimeter of both the circle and ring electrode structures.