Monocrystalline PZT thin films

"More with less" has been the motto behind the hardware miniaturization trend in the microelectronics industry since the 1970s. Active research in the growth of oxide films, including ferroelectrics, which started soon after, followed the same trend. Meanwhile, in ferroelectrics, domain walls have arisen recently as elements having new, potentially useful functional properties. The contribution of this thesis work to the global miniaturization trend is focused at exploring strain engineering techniques to control and possibly redefine limits for imaginable domain patterns and their periodicity in lead zirconate titanate (PZT) monocrystalline thin films. PZT is one of the most heavily used ferroelectrics in microelectronics and piezoelectric applications. In its monocrystalline formit has been much less studied. Here, epitaxial thin films were made and investigated in both, the highly rich titanium region (highly tetragonal with a high spontaneous polarization in bulk PZT ceramics) and compositions near the morphotropic boundary (tetragonal and rhombohedral side, both highly active piezoelectrically in bulk PZT ceramics). In films with rhombohedral bulk compositions, it was found that, compressive misfit strain of 1.5 % result in (001) PZT 60/40 films with tetragonal symmetry. On the other hand, the application of 1 % tensile misfit strain through a PZT 80/20 buffer layer resulted in (001) PZT 60/40 films with rhombohedral symmetry having random domain patterns. All films were grown on PrScO3 (PSO) substrates with SrRuO3 (SRO) bottomelectrodes by the pulsed laser deposition technique (PLD). Re-ordering of these domains in to stripes of 71 ° domain walls was possible through poling with an AFMtip. Monitoring the stability of these patterns revealed that the stripe domain structures relaxed in to herringbone patterns consisting of 180 ° domain walls in amatrix of 109 ° domain walls. This was interpreted in terms of electrostatic effects. Tetragonal PZT films offered the opportunity to study both single domain and polydomain films. In single domain films, the investigation turned to the source of the built-in field that dictates the polarization orientation. The upward oriented c-domain of as-grown 30 nm (001) PZT 50/50 thin films grown on PSO with SRO bottomelectrodes, were made to switch downwards by using (i) substrates with polar surface termination, (ii) 25 % excess PbO in the PLD targets and (iii) doping. While interfacial mechanisms seem to get weaker with film thickness, bulk work function modification through Nb doping appeared to play a significant role in films as thick as 150 nm. Theoretical calculations were conducted in order to predict the stability of c-domains as a function of temperature andmisfit strain in (001) films. The effect of elastic compliances and higher order Landau coefficients on the phase diagram of single domain epitaxial (001) PZT 50/50 thin films was studied by using a single domain theory. It is shown that while values of elastic compliances affect the positions of the phase boundaries including phase transition temperature of the cubic phase; higher order terms can potentially introduce an a1a2-phase previously predicted in the pure PbTiO3 phase diagram. In the literature, it has been demonstrated that, under special treatment conditions, dense domain patterns can be realized in single crystals through poling along non-polar orientations. Triggered by this knowledge, we studied multidomain ...

Macroscopic Theory of Charged Domain Walls in Ferroelectrics

Maxim Gureev

This thesis consists of a theoretical analysis of charged domain walls in ferroelectrics based on Landau theory and the theory of semiconductors. First, the internal structure of a 180-degree charged domain wall is considered. It is shown that different regimes of screening and different dependences for width of charged domain walls on the temperature and parameters of the system are possible, depending on spontaneous polarization and concentration of carriers in the material. In typical perovskites the screening is provided by a degenerate electron gas in the nonlinear screening regime. The corresponding width of charged domain walls is about one order of magnitude larger than the width of neutral domain walls. The formation energies of "head-to-head" walls in an isolated sample under different regimes of screening are derived, neglecting the poling ability of the surface. In the nonlinear regimes of screening, this energy is equal to the energy necessary for the creation of electron-hole pairs in a sufficient amount to screen the spontaneous polarization, which is proportional to the band gap of the ferroelectric. It is shown that if the poling ability of the surface is large enough, either "head-to-head" or "tail-to-tail" configurations can be energetically favorable in comparison with the monodomain state of the ferroelectric. For the case of a surface with weak poling ability the existence of charged domain walls in bulk ferroelectrics is merely a result of domain-growth kinetics. It is shown that, at large enough sample thicknesses, a charged domain wall can be energetically favorable in comparison to other possible states: a multidomain state with antiparallel domains separated by neutral walls and a state with zero polarization. This size effect provides a possible explanation of the reason for experimental observation of 180-degree charged domain walls in bulk lead titanate but not in barium titanate. The results obtained for the case of an isolated ferroelectric sample are compared with the results for a sample with electrodes. It is shown that a charged domain wall in an electroded sample can either be metastable or stable, depending on the work function difference between the electrodes and the ferroelectric and the poling ability of the surface. The interaction of an electric field with charged domain walls in ferroelectrics is also addressed. A general expression for the force acting per unit area of the a charged domain wall carrying free charge is derived. It is shown that in proper ferroelectrics, the free charge carried by the wall dependeds on the size of the adjacent domains. As a result, it is found that the mobility of such a domain wall (with respect to the applied field) is sensitive to the parameters of the domain pattern containing this wall. The problem of the force acting on a planar charged 180° domain wall normal to the polarization direction in a periodic domain pattern in a proper ferroelectric is solved analytically. It is shown that, in the linear regime under small applied field, the forces acting on walls in such a pattern increase with decreasing wall spacing. The direction of the forces coincides with those for the case of the corresponding neutral walls. At the same time, for large enough wall spacings and large enough fields, these forces can be of the opposite sign. It is shown that the domain pattern under consideration is unstable in a defect-free ferroelectric. The poling of a crystal containing such pattern, stabilized by the pinning pressure, is also considered. It is shown that, except for a special situation, the presence of charge domain walls can make poling more difficult. It is demonstrated that the results obtained are also applicable to "zig-zag" walls under the condition that the "zig-zag" amplitude is much smaller than the domain wall spacing. A mechanism that leads to a strong enhancement of the dielectric and piezoelectric properties in ferroelectrics with increasing density of charged domain walls is proposed. It is demonstrated that an incomplete compensation of bound polarization charge at these walls creates a stable built-in depolarizing field across each domain leading to increased electromechanical response. This model clarifies a long-standing unexplained effect of domain wall density on macroscopic properties of domain engineered ferroelectrics. It is demonstrated that lead-free ferroelectrics like barium titanate with dense patterns of charged domain-walls are expected to have strongly enhanced piezoelectric properties, thus suggesting a new route to high-performance, lead-free ferroelectrics. The problem of the screening of ferroelectric bound charge on a ferroelectric-semiconductor interface is considered. It is shown that the screening ability of the interface can be drastically improved due to the presence of a built-in potential in the semiconductor, which depends on the electron affinities and surface state density and can be controlled by a judicious choice of materials.

EPFL2012

Properties and phase transitions in lead free piezoelectrics

Naama Klein

Piezoelectric materials are used in a wide range of applications, with lead zirconate titanate (PZT) ceramics being the dominant family of materials, due to high piezoelectric coefficients, dielectric permittivity and coupling factors. The high properties have been found in compositions in the proximity of a morphotropic phase boundary (MPB). The MPB exhibits a weak temperature dependence so that the stability of the properties is attained over a large temperature range. However, environmental problems rising from the toxicity of lead have driven scientists around the world to search for lead free piezoelectric materials. These new materials should exhibit properties comparable to those of PZT and the efforts are at the present concentrated on solid solutions with an MPB. In this thesis an emphasis was made on constructing the phase diagrams and detecting an MPB in two of the most promising families of lead-free solid solutions: bismuth sodium titanate based and potassium sodium niobate based ceramics and single crystals. By investigating the structure of the phases, using X-ray and neutron diffractions and Raman spectroscopy, and measuring the dielectric and piezoelectric properties at temperatures from -266 °C to 550 °C, the phase transitions were assigned and phase diagrams constructed. Both systems (1-x)(Bi0.5Na0.5)TiO3-xBaTiO3 (0 ≤ x ≤ 0.09) (BNBT) and (1-x)(K0.5Na0.5)NbO3-xLiNbO3 (0.02 ≤ x ≤ 0.1) (KNLN), reveal similar MPBs to the one found for PZT. However, in PZT the MPB is pseudovertical up to about 350 °C whereas the MPB in BNBT is vertical only till 75 °C and in KNLN is till -50 °C. This difference in the temperature stability range is crucial in most applications. Other interesting findings in the BNBT phase diagram correspond to the phase sequence at the temperature range from 200 to 300 °C, where ferroelectric phase transforms into a mixed phase (antiferroelectric or paraelectric) with polar regions. As temperature rises, the polar regions gradually disappear. Whereas dielectric and piezoelectric properties of BNBT could compete with those of PZT in some compositions, and fairly simple processing of the ceramics is used, the fabrication of KNLN ceramics has shown to be complex and reproducible properties difficult to obtain. Even after the critical processing conditions were identified in this thesis, and reproducibility was achieved, properties were significantly lower than for PZT. Exceptional values of the thickness coupling coefficients were measured on potassium niobate (KN) and KNLN single crystals rotated away from the Z axis by θ = 45° about the orthorhombic Y axis, [001]C-cut. Monodomain and domain-engineered single crystals cut at this angle showed very high coefficients (kt ≤ 70%), which are extremely stable over a large temperature range and after thinning to thickness below 60 µm. Together with low permittivity, these properties make KN single crystals a good candidate for high-frequency applications. The higher permittivity of KNLN makes it an attractive crystal for applications at lower frequencies. Another possible application of lead free piezoelectrics could be in biological systems. For this reason, preliminary in-vitro biocompatibility study of KNLN and PZT ceramics was conducted, showing similar microscopic attachment of cells to the two ceramics and slightly lower viability values when compared to non coated tissue culture plates. Laboratory attempts to introduce KNLN ceramics as an implant in the mineral of the bone, hydroxyapatite (HA), ended with a strong chemical reaction between the two. However, some piezoelectric response could be measured from the composite of HA and KNLN.

EPFL2009

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).