Nava Setter

Piezoelectric properties of Li- and Ta-modified (K0.5Na0.5)NbO3 ceramics

Lead-free, potassium sodium niobate piezoelectric ceramics substituted with lithium (K0.5−x∕2,Na0.5−x∕2,Lix)NbO3 or lithium and tantalum (K0.5−x∕2,Na0.5−x∕2,Lix)(Nb1−y,Tay)O3 have been synthesized by traditional solid state sintering. The compositions chosen are among those recently reported to show high piezoelectric properties [Y. Saito, H. Takao, T. Tani, T. Nonoyama, K. Takatori, T. Homma, T. Nagaya, and M. Nakamura, Nature (London) 42, 84 (2004); Y. Guo, K. Kakimoto, and H. Ohsato, Appl. Phys. Lett. 85, 4121 (2004); Mater. Lett. 59, 241 (2005)]. We show that high densities and piezoelectric properties can be obtained for all compositions by pressureless sintering in air, without cold isostatic pressing, and without any sintering aid or special powder treatment. Resonance and converse piezoelectric (strain-field) measurements show a thickness coupling coefficient kt of 53% and converse piezoelectric coefficient d33 around 200pm∕V for the Li-substituted ceramics, and a kt of 52% and d33 over 300pm∕V fo...

Dielectric and Structural Characteristics of Ba- and Sr-based Complex Perovskites as a Function of Tolerance Factor

The temperature coefficient of the dielectric permittivity (τe) of nonferroelectric complex perovskites is of importance in the application of these cermaics to microwave filters and resonators. Recent work has directly related changes in the τe of complex perovskites to the onset of structural phase transitions which involve tilting of the octahedra. It can be argued that the onset of octahedral tilting is controlled by the tolerance factor (t). Thus, a relationship between τe and t is postulated. This relationship is discussed and transmission electron microscopy is used to demonstrate examples of the structural modifications which cause the anomalies in τe at given values of t.

Polarization fatigue in ferroelectric films: Basic experimental findings, phenomenological scenarios, and microscopic features

The reduction in switchable polarization of ferroelectric thin films due to electrical stress (polarization fatigue) is a major problem in ferroelectric nonvolatile memories. There is a large body of available experimental data and a number of existing models which address this issue, however the origin of this phenomena is still not properly understood. This work synthesizes the current experimental data, models, and approaches in order to draw conclusions on the relative importance of different macro- and microscopic scenarios of fatigue. Special attention is paid to the role of oxygen vacancy migration and electron injection into the film and it is concluded that the latter plays the predominant role. Experiments and problems for theoretical investigations, which can contribute to the further elucidation of polarization fatigue mechanisms in ferroelectric thin films, are suggested.

Interferometric measurements of electric field-induced displacements in piezoelectric thin films

Interferometric measurements of electric field‐induced displacements in piezoelectric thin films using single‐beam and double‐beam optical detection schemes are reported. It is shown that vibrational response measured with a single‐beam interferometer includes a large contribution of the bending motion of substrate. Therefore, it is difficult to apply single‐beam technique for piezoelectric measurements in thin films. To suppress the bending effect a high‐resolution double‐beam interferometer is proposed. The sensitivity of the interferometer is significantly improved in comparison with previously reported system. The interferometer is shown to resolve small displacements without using a lock‐in technique. An example of the interferometric capabilities is demonstrated with experimental results on electric field, frequency, and time dependences of piezoelectric response for quartz and Pb(Zr,Ti)O3 thin film.

Effect of Structural-Changes in Complex Perovskites on the Temperature-Coefficient of the Relative Permittivity

The dielectric behavior and structure of the Sr(Zn1/3Nb2/3)O3‐Ba(Zn1/3Nb2/3)O3 solid solution have been investigated with the intention of understanding the relationship between the structural changes and the temperature coefficient of the relative permittivity τe. A correlation between the value of τe and the occurrence of O‐octahedra tilts has been established. The occurrence of ferroelastic domains and their influence on τe has also been investigated. It is proposed that the results obtained can be used to generalize about the structure‐property relationships in compounds of the same class.

Orientation of rapid thermally annealed lead zirconate titanate thin films on (111) Pt substrates

The nucleation, growth, and orientation of lead zirconate titanate thin films prepared from organometallic precursor solutions by spin coating on (111) oriented platinum substrates and crystallized by rapid thermal annealing was investigated. The effects of pyrolysis temperature, post-pyrolysis thermal treatments, and excess lead addition are reported. The use of post-pyrolysis oxygen anneals at temperatures in the regime of 350-450 degrees C was found to strongly affect the kinetics of subsequent amorphous-pyrochlore-perovskite crystallization by rapid thermal annealing. The use of such post-pyrolysis anneals allowed films of reproducible microstructure and textures [both (100) and (111)] to be prepared by rapid thermal annealing. It is proposed that such anneals and pyrolysis temperature affect the oxygen concentration/average Pb valence in the amorphous films prior to annealing. Such changes in the Pb valence state then affect the stability of the transient pyrochlore phase and thus the kinetics of perovskite crystallization.

{1 0 0}-Textured, piezoelectric Pb(Zrx, Ti1−x)O3 thin films for MEMS: integration, deposition and properties

Pb(Zr-x, Ti1-x)O-3 (PZT) piezoelectric thin films are of major interest in MEMS technology for their ability to provide electro-mechanical coupling. In this work, the effective transverse piezoelectric coefficient e(31,f) of sol-gel processed films was investigated as a function of composition, film texture and film thickness. Dense, textured and crack-free PZT films have been obtained on silicon substrates up to a thickness of 4 mum. Crystallization anneals have been performed for every 0.25 mum. Nucleation on the previous perovskite layer combined with directional growth leads to a gradient of the compositional parameter x of +/-20% (at x = 0.53 average composition). Best properties have been achieved with {100}-textured film of x = 0.53 composition. Large remanent e(31,f) values of -11 to -12 C/m(2) have been obtained in the whole thickness range of 1-4 mum. These values are superior to values of undoped bulk ceramics, but smaller than in current, optimized (doped) bulk PZT

Investigation of Pt/Ti bilayer metallization on silicon for ferroelectric thin film integration

The stabilities of Pt/Ti bilayer metallizations in an oxidizing atmosphere have been investigated with several thicknesses of interfacial Ti‐bonding layers. Reactions in the Pt/Ti/SiO2/Si interface were examined as a function of various annealing conditions in the temperature range 200–800 °C by using Rutherford backscattering spectrometry, Auger electron spectroscopy, x‐ray diffraction, and transmission electron microscopy. Thermal treatment in oxygen was found to cause rapid oxidation of the Ti layer, accompanied by the migration of Ti into the Pt film. Diffusion of oxygen through the Pt grain boundaries was mainly responsible for the adverse reactions at the interface and loss of mechanical integrity. Thin Ti (10 nm) layers resulted in the depletion of the interfacial bonding layer causing serious adhesion problems, whereas thicker Ti films (100 nm) caused the formation of TiO2−x in the Pt‐grain boundaries, ultimately encapsulating the Pt surface with an insulating TiO2 layer. Improved stability and ad...

Principle of ferroelectric domain imaging using atomic force microscope

The contrast mechanisms of domain imaging experiments assisted by atomic force microscope (AFM) have been investigated by model experiments on nonpiezoelectric (silicon oxide) and piezoelectric [Pb(Zr,Ti)O3] thin films. The first step was to identify the electrostatic charge effects between the tip, the cantilever, and the sample surface. The second step was to explore the tip–sample piezoelectric force interaction. The static deflection of the cantilever was measured as a function of dc bias voltage (Vdc) applied to the bottom electrode (n-type Si wafers) for noncontact and contact modes. In addition, a small ac voltage (Vac sin ωt) was applied to the tip to measure the amplitude (Aω) and phase (Φω) of the first harmonic (ω) signal as a function of Vdc. By changing from the noncontact to the contact mode, a repulsive contribution to the static deflection was found in addition to the attractive one and a 180° phase shift in Φω was observed. These results imply that in the contact mode the cantilever buckl...

The Spontaneous Relaxor-Ferroelectric Transition of Pb(Sc0.5ta0.5)O3

A zero‐field spontaneous relaxor‐ferroelectric transition is reported in Pb(Sc0.5Ta0.5)O3 (PST). This behavior is different from that of other relaxors, where such transitions occur only under the field. A highly disordered PST that has the wide relaxation spectrum typical of relaxors is shown to transform spontaneously into a macroscopic ferroelectric state. Introduction of defects (lead vacancies) into the material impedes the transition resulting in the usual relaxor behavior. Dielectric properties of PST, with and without defects, are analyzed. For the interpretation of the observed properties, a model invoking an additional nonpolar phase is proposed. This model does not imply a freezing in the system. At the low‐frequency limit, it is possible to account for the Vogel–Fulcher (VF) law for the temperature of the maximum of the dielectric constant, using only the commonly accepted assumption of an exponentially wide relaxation time spectrum that shrinks on heating. The presented approach interprets th...