Thin-film-specific elastic effects in ferroelectric domain structures

An essential feature of ferroelectric thin films is the presence in them of domain structures. In order to efficiently implement ferroelectric films into potential new ferroelectrics-based devices, it is of high interest to understand the behaviour of domain structures. Some properties of ferroelectric domain structures, such as internal structure of domain walls, orientation of domain walls, which were investigated and formulated for bulk systems, are typically believed to be applicable to thin films as well. In this work, importance of thin-film-specific effects on domain structure formation is discussed. It is shown that in thin films nonferroelastic domain walls may interact by elastic means on distances far exceeding the domain wall width, which is sometimes considered as the maximal range of their interaction. Theoretical study of films of practical interest shows that the force of elastic repulsion between the walls can be strong enough to be important during the polarization switching and for control of nonferroelastic domain structures. Further, this work discusses mechanisms of ferroelastic domain wall orientation specific to thin films. Typically, when a ferroelastic wall orients violating the condition of mechanical compatibility of the domains, macroscopic elastic fields appear leading to increase of the energy of the system restraining the deviation of the wall from the compatible state. It is shown that in thin films this increase of the energy can be small enough to allow competition with other contributions to the energy, such as domain wall self-energy, leading to large deviations of the wall orientation from its compatible state. Another investigated mechanism of ferroelastic domain wall orientation is interference of elastic fields generated by an incompatible wall with the film-substrate misfit stresses. A system is investigated where such interference, which leads to reduction of elastic energy of the structure, results in deviation of the wall from its compatible state, violating the domain wall electric neutrality as well. Using this result, the concept of elasticity-driven generation of charged conductive domain walls is developed; the concept is supported by reports of experimental observation of metallic-like domain wall conduction in Pb(Zr,Ti)O3 films. A theoretical description is provided for ferroelastic structures with narrow domain inclusions into dominant domain state. It is obtained that in sparse structures the domain periodicity follows linear dependence on the film thickness, not the classical Kittel-like square root dependence. Equilibrium parameters of the domain structure, such as periodicity of the structure, width of the narrow inclusions, are obtained; the results for Pb(Zr,Ti)O3 films are compared with experimental observations of the films. Finally, antiphase boundary with local ferroelectricity in antiferroelectric is investigated. The mechanical response of such boundary on external electric field is modelled. It is shown that the electric field gradient can be used as the driving force for displacement of the boundary. Conditions for displacement of the boundary sufficient for Scanning Probe Microscopy observations are formulated.

Comparison of Lead Zirconate Titanate Thin Films for Microelectromechanical Energy Harvester With Interdigitated and Parallel Plate Electrodes

Davide Balma, Nachiappan Chidambaram, Andrea Mazzalai, Paul Muralt

Lead zirconate titanate (PZT) thin films on insulator-buffered silicon substrates with interdigitated electrodes (IDEs) have the potential to harvest more energy than parallel plate electrode (PPE) structures because the former exploit the longitudinal piezoelectric effect, which is about twice as high as the transverse piezoelectric effect used by PPE structures. In this work, both options are compared with respect to dielectric, ferroelectric, and piezoelectric properties, leakage currents, and figure of merit (FOM) for energy harvesting. The test samples were silicon beams with {100} PZT thin films in the case of the PPE geometry, and random PZT thin films for the IDE geometry. Both films were obtained by an identical sol-gel route. Almost the same dielectric constants were derived when the conformal mapping method was applied for the IDE capacitor to correct for the IDE geometry. The dielectric loss was smaller in the IDE case. The ferroelectric loops showed a higher saturation polarization, a higher coercive field, and less back-switching for the IDE case. The leakage current density of the IDE structure was measured to be about 4 orders of magnitude lower than that of the PPE structure. The best FOM of the IDE structures was 20% superior to that of the PPE structures while also having a voltage response that was ten times higher (12.9 mV/mu strain).

Ieee-Inst Electrical Electronics Engineers Inc2013

Positional order in lead scandium tantalate (PST) as a "tool" for the investigation of relaxor ferroelectric behavior in thin films

Kyle Brinkman

Relaxor ferroelectric thin films exhibit a drastic reduction in the dielectric constant and associated properties in the thin film form, even for thicknesses in the micron range, which are essentially infinity for the size effects typically investigated in conventional ferroelectrics. In order to investigate this phenomenon, the material system Lead Scandium Tantalum (PST) was fabricated by a Sol-Gel technique using mixed acetate and alkoxide precursors. The positional order of the "B" site species was tailored through different annealing procedures to produce ordered (conventional ferroelectric) and disordered (relaxor ferroelectric) PST thin films. With regards to processing, it was found that the dehydration of scandium acetate by acetic acid anhydride, followed by precursor modification via the solvent 2-Methoxyethanol was necessary for a stable "B" site Sc/Ta complex and subsequent stoichiometric thin films. Excess lead was required in amounts greater than 20% to compensate for lead loss during annealing and avoid the formation of secondary phases. Disordered films with measurable properties were only achieved at low annealing temperatures near 700°C. Higher temperatures up to 1000°C resulted in "B" site ordering, and at annealing temperatures above 1000°C significant film degradation was observed. The ordering of PST thin films was limited to low degrees of order (S=0.22) on Pt/Si substrates after annealing at 800°C for 20 minutes. Higher degrees of order were achieved on single crystal substrates by extended time annealing in a controlled atmosphere. An ordering degree of S=0.91 observed for PST films on sapphire substrates annealed at 850°C for 35 hours. Highly ordered films showed the development of a ferroelectric state, and had dielectric properties comparable with ordered ceramic samples. Disordered films fabricated on numerous substrates and under many processing conditions consistently displayed dielectric constant values reduced by an order of magnitude as compared to ceramic or single crystals. A direct comparison of ordered and disordered PST on a common substrate showed that the dielectric constant increased with an increase in "B" site order in thin films, while in ceramics the opposite effect was observed (a decrease in the dielectric constant with an increase in order). In terms of lattice dynamics, ordered PST thin films unexpectedly showed that the transverse optic (TO1) mode softened to the Burns temperature, with the simultaneous appearance of a central mode (CM) attributed to the appearance of polar regions in the paraelectric phase. Higher processing temperatures were seen to increase the dielectric strength of the CM. Ordered and disordered samples displayed drastically different behavior at low frequencies (below 1MHz), while possessing essentially the same lattice dynamics. It was shown by comparing the "In-Plane" and "Out-of-Plane" dielectric response that the commonly observed low values of dielectric constant in relaxor thin films cannot be explained by a passive layer, at least in nominally thick films near 1 micron in thickness. The changes in the dielectric constant introduced as a result of thermal induced stress were small and relatively insignificant as compared to the order of magnitude reduction in the dielectric constant of thin films as compared to ceramics. A study on the effects of grain size from dielectric measurements showed no detectable change in the value of the dielectric constant by reducing the grain size from 325 to 140 nm. In addition, even epitaxial films fabricated on STO and MgO substrates displayed a reduced dielectric response. Self-polarization was observed in PST thin films and the quantitative impact on the dielectric response was estimated to be near a 10% reduction in the dielectric constant at temperatures close to the dielectric maximum. A hypothesis was proposed to explain the reduced dielectric response in relaxor thin films emphasizing that the quantitative contribution of polar regions may be different. Disordered relaxor PST thin films were annealed at 700°C resulting in a Low-Temperature disordered state, while ceramic samples were annealed at temperatures in excess of 1500°C leading to a High-Temperature disordered state. Both states of disorder evolved to the same long range ordered ferroelectric state after annealing at temperatures near 900°C.

EPFL2004

Positional order in lead scandium tantalate (PST) as a "tool" for the investigation of relaxor ferroelectric behavior in thin films

Kyle Brinkman

EPFL2004