D. Bolten

The interface screening model as origin of imprint in PbZrxTi1−xO3 thin films. I. Dopant, illumination, and bias dependence

Comprehensive imprint measurements on PbZrxTi1−xO3 (PZT) thin films were carried out. Different models, which were proposed in literature to explain imprint in ferroelectric thin films or a similar aging effect (internal bias) in ferroelectric bulk material, are reviewed. Discrepancies between the experimental results obtained on the PZT films in this work and the prediction of the literature models indicate that these models do not describe the dominant imprint mechanism in PZT thin films. Hence, in this work a model is proposed which suggests imprint to be caused by a strong electric field within a thin surface layer in which the ferroelectric polarization is smaller or even absent compared to the bulk of the film. With the proposed imprint model the influence of important experimental parameters like dopant, illumination, and bias dependence can be qualitatively explained.

Relaxation mechanism of ferroelectric switching in Pb(Zr,Ti)O3 thin films

The polarization reversal process of tetragonal Pb(Zr,Ti)O3 thin films has been intensively studied using conventional hysteresis and rectangle pulse measurements. Decreasing the voltage level of the pulses significantly slows down the polarization switching to the range of milliseconds. The switching current response shows a Curie–von Schweidler behavior over a broad time range. The transient current and the frequency dependence of the P–V loops of these films compared to the properties of ferroelectric single crystals show some similarities but also significant differences. The theoretical models of the classical ferroelectric phase transition and especially the conditions of the pulse measurements in single crystals and thin films are discussed. It leads to the conclusion that it is not the domain wall structure and domain wall motion that determine the polarization reversal but dissipative polarization processes which can take place in both ferroelectric and nonferroelectric high-k dielectric thin films.

Reversible and irreversible processes in donor-doped Pb(Zr,Ti)O3

In this letter, the reversible and irreversible polarization contributions in donor-doped Pb(Zr,Ti)O3 are investigated using conventional capacitance–voltage (C–V) and hysteresis measurements. The dependence of the Rayleigh coefficient in the subcoercive regime on the doping concentration is investigated and shown to be consistent with the assumption of an interaction of the domain walls with randomly distributed defects.

The interface screening model as origin of imprint in PbZrxTi1-xO3 thin films. II. Numerical simulation and verification

In this article, the interface screening model is theoretically discussed which explains imprint in ferroelectric thin films caused by a large electric field within a surface layer with deteriorated ferroelectric properties. During aging this field is gradually screened by electronic charges. Different screening mechanisms such as charge injection from the electrodes into the film as well as charge separation within the surface layer are considered by implementing a numerical simulation based on the different screening mechanisms. A comparison between experimental and simulation results is presented. The best agreement between experiment and simulation is obtained for a Frenkel–Poole type charge separation mechanism within the surface layer. The simulation results indicate relatively shallow trap states (0.35 eV) and a surface layer extension of approximately 5 nm.

Reversible and irreversible polarization processes in ferroelectric ceramics and thin films

In this article, the separation between reversible and irreversible polarization where the reversible polarization component is determined by capacitance–voltage curve measurements, is used to characterize ferroelectric materials. After giving a thorough foundation of the method, it is used to investigate the influence of the composition on the reversible and irreversible polarization contributions in ferroelectric thin films and/or bulk ceramics. The reversible polarization is also monitored during fatigue. A comparison to bulk ceramics suggests that the domain wall motion in ferroelectric thin films is reduced compared to bulk ceramics.

Correlation between switching and fatigue in PbZr0.3Ti0.7O3 thin films

Fast pulse switching experiments with variable width and amplitude of the write pulse were performed on PbZr0.3Ti0.7O3 thin films and the results were correlated to fatigue measurements with varying frequency and amplitude of the fatigue signal. It was found that small amplitudes in combination with a small pulse width of the write pulse does not provide sufficient switching of the ferroelectric film. Furthermore, for the fatigue measurements, it is shown that the degree of switching caused by the fatigue excitation signal strongly influences the fatigue results. In the case of complete switching, the fatigue behavior is found to be independent of the fatigue frequency and only the number of switching cycles is decisive for the polarization decrease.

Reversible and irreversible domain wall contributions to the polarization in ferroelectric thin films

Abstract In this work, standard characterization methods like hysteresis loops and C-V- measurements are used to distinguish the reversible and irreversible parts of the total ferroelectric polarization. Special attention has been given to the frequency dependence of the hysteresis loops-and the C-V-measurements. The frequency dependence of the C-V measurements indicates that the relaxation of the polarization is enhanced in ferroelectric materials compared to paraelectric SrTiO3. The reversible and irreversible contributions are demonstrated for SrBi2Ta2O9 thin films and Pb(Zr,Ti)O3 thin film capacitors. Additionally, a method is described to obtain the static hysteresis curve, which allows to characterize the polarization of a ferroelectric material independent of the measuring frequency, since remanent polarizations or coercive voltages inferred from usual hysteresis loop measurements strongly depend on the measuring frequency.

Lifetime estimation due to imprint failure in ferroelectric SrBi2Ta2O9 thin films

Two different failure modes for a ferroelectric memory cell caused by imprint, the read failure due to the loss of polarization, and the write failure due to the shift of the hysteresis loop are investigated. The quasistatic hysteresis loop allows us to distinguish which failure mode is dominating in a ferroelectric random access memory application and, hence, it can also be used as a powerful tool for lifetime estimation of ferroelectric thin films limited by imprint failure under operating conditions. The experimental results show that the write failure is only decisive for very low voltage operation (Vp<1.25 V), whereas for the Pt/SrBi2Ta2O9/Pt under investigation the read failure is the dominant failure mode for operating voltages exceeding 1.25 V.

Reversible and irreversible piezoelectric and ferroelectric response in ferroelectric ceramics and thin films

Abstract In this article, a novel characterization method, the separation between reversible and irreversible contributions, is applied to the piezoelectric and ferroelectric response of ferroelectric ceramics and thin films. The reversible contributions are determined by the measurement of appropriate small-signal properties, e.g. the piezoelectric coefficient for the piezoelectric response and the small-signal capacitance for the polarization response of the material, and compared to the corresponding large signal properties (i.e. the strain–field dependence and polarization–field dependence, respectively). The comparison between thin films and bulk ceramics indicates that the non-180° domain wall motion in ferroelectric thin films is reduced compared to bulk ceramics.

Interface-related decrease of the permittivity in PbZrxTi1−xO3 thin films

In ferroelectric thin films, a decrease of the permittivity is observed obeying a logarithmic time dependence. In the literature, a similar effect has been reported for ferroelectric single crystals and ceramics, which is referred to as ferroelectric aging, and different models have been proposed to explain this phenomenon. In this letter, ferroelectric aging of PbZrxTi1−xO3 thin films is studied as a function of dopant types and concentrations as well as the temperature. The results clearly show that the traditional models for the aging mechanism of ferroelectric single crystals and ceramics are not applicable. Based on these results, a mechanism is proposed which explains the decrease of the dielectric constant in ferroelectric thin films by the growth of a thin surface layer with suppressed ferroelectric properties in the course of aging.