E. L. Colla

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.

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.

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...

Discrimination between bulk and interface scenarios for the suppression of the switchable polarization (fatigue) in Pb(Zr,Ti)O3 thin films capacitors with Pt electrodes

A simple and reliable method which allows one to distinguish between the two major microscopic scenarios for the suppression of the switching polarization (Prs), i.e., pinning of ferroelectric domain walls (DWs) through the Pb(Zr,Ti)O3 film (PZT) (bulk scenario) and inhibition of the growth of opposite domains due to the nucleus suppression at the electrode interfaces (interface scenario), is proposed. In addition, a new electric treatment able to significantly suppress Prs in Pt–PZT–Pt ferroelectric capacitors (FECAPs) of thicknesses above 1.4 μm, was discovered and studied. It consists of the application of an external alternating electric field (Ee) which cycles the polarization at very low frequency (1.7 mHz). After only 10–20 cycles, Prs can be suppressed by a factor of 10. The same FECAP, when subjected to Ee at higher frequency (30 kHz), endures at least 108 switches, before attaining an equivalent Prs suppression (hereafter called fatigue). The fatigued states obtained with the two different proce...

Dielectric spectroscopy of Ba(B1/2’B1/2‘)O3 complex perovskite ceramics: Correlations between ionic parameters and microwave dielectric properties. I. Infrared reflectivity study (1012–1014 Hz)

An attempt has been undertaken to find a correlation between ionic parameters of ceramic materials and their complex permittivity at microwave frequencies. Ten Ba(B1/2’B1/2‘)O3 complex perovskite compounds (B’=Y3+, In3+, Nd3+, Gd3+; B‘=Nb5+, Ta5+ and B’=Mg2+, Cd2+, B‘=W6+) are compared in order to study the effect of ionic radii, mass, and valence state on dielectric properties. Fourier transform infrared reflectivity spectra in the 30–4000 cm−1 range were measured and evaluated by means of Kramers–Kronig analysis and classical oscillator fit. The data were extrapolated below the measured frequency range to estimate the intrinsic microwave losses. The correlations between loss, permittivity, ionic size, mass, and effective charge and polar‐phonon mode parameters were investigated. Ionic size was revealed to be the most important parameter, determining the tolerance factor of the structure packing and through this controlling the phonon frequencies and dampings as well as the extrapolated low‐frequency int...

High resolution study of domain nucleation and growth during polarization switching in Pb(Zr,Ti)O3 ferroelectric thin film capacitors

The domain nucleation and growth during polarization switching in Pb(Zr,Ti)O3 (PZT) ferroelectric thin film capacitors with Pt top (TE) and bottom electrodes (BE) were studied by means of atomic force microscopy (AFM). The experimental configuration used in this study differs from that conventionally used (AFM tip/PZT/BE) where the AFM tip acts as a positionable TE. A small ac voltage was applied between the electrodes with a step by step increasing dc bias voltage. The induced piezoelectric vibration was detected by the AFM tip, its amplitude and phase determined with the lock-in amplifier. The phase difference between the applied ac voltage and the piezoelectric signal as a function of the x-y position was nearly locked at 0 or 180, representing film regions with parallel (in-phase) and antiparallel (antiphase) polarization direction, respectively. The polarization reversal was induced by application of a step by step increasing dc bias field opposite to the polarization of the prepoled sample. At each ...

FATIGUE OF PIEZOELECTRIC PROPERTIES IN PB(ZR,TI)O3 FILMS

Piezoelectric properties of Pb(Zr,Ti)O3 thin films are investigated as a function of the number of bipolar (switching) and unipolar (nonswitching) voltage pulses. The longitudinal piezoelectric coefficient d33 decreases with bipolar fatigue reflecting the decrease of switchable polarization. Simultaneously, a strong vertical shift of piezoelectric hysteresis loops is observed, which is considered as the buildup of fixed internal polarization due to the pinning of ferroelectric domains in a preferred orientation. Piezoelectric fatigue induced by unipolar (nonswitching) pulses is considerably smaller than the fatigue under bipolar conditions and can be described by the internal bias field which shifts piezoelectric hysteresis loops along the field axis.

Ferroelectric domains and piezoelectricity in monocrystalline Pb(Zr,Ti)O3 nanowires

Monocrystalline lead zirconate titanate nanowires were grown by a polymer assisted solvothermal technique. X-ray and electron diffractions confirmed tetragonal perovskite structure and a [001] orientation along the wire axis, respectively. Raman scattering was used to analyze the structure and composition of single wires. Ferroelectric/ferroelastic domain walls were imaged by transmission electron microscopy, showing some domains with polarization directions along the wire axis and some perpendicular to it. The domain walls disappeared upon heating above the ferroelectric phase transition at 460°C. Ferroelectric switching, as well as piezoelectric activity and hysteresis, were shown locally using piezoelectric force microscopy.

Fatigued state of the Pt-PZT-Pt system

Abstract The fatigued state of Pt-PZT-Pt ferroelectric capacitors (FECAP) was investigated by means of piezoelectric coefficient, polarisation charge and permittivity measurements. The suppression of switching polarisation Pr S appears to be the result of freezing of ferroelectric domains without affecting the lattice dielectric properties. The frozen polarised domains show a preferential orientation which is related to the electrode interface asymmetry. The fatigue mechanism in thin films is not assisted by a growing passive layer and is characterised by a substantial reversible character and adaptation to the used fatiguing field (field self-adjusting). With consideration of the frozen asymmetry and of the newly discovered slow cycling fatiguing effect in thicker samples, the fatigue mechanism was interpreted as inhibition of the nucleation at the top electrode interface. Since it is unlikely that the domain walls (DW) cross the grain boundaries, it is suggested that the effective suppression of Pr S oc...