Peter J. Schorn

Fatigue effect in ferroelectric PbZr1−xTixO3 thin films

PbZr1−xTixO3 (PZT) is one preferred ferroelectric material being used in nonvolatile ferroelectric random access memory devices. The use of oxide electrodes like IrO2 or SrRuO3 (SRO) is necessary to suppress the serious loss of polarization due to bipolar voltage cycling. Although, there are a number of models under discussion, the origin of the fatigue phenomenon is still not completely understood. In this paper, the fatigue effect of ferroelectric Pb(Zr0.40,Ti0.60)O3 thin films has been studied in detail. To achieve a deeper understanding of the effect, several PZT samples with different electrode materials were investigated. After determining the dependence of the single fatigue parameters, a simulation approach was made to analyze the fatigue effect qualitatively. A sample with SRO electrodes was measured up to 1013cycles and no fatigue of the switchable polarization was observed.

Structural investigations of Pt/TiOx electrode stacks for ferroelectric thin film devices

Effects of the thermal treatment and the fabrication process of Pb(Zr0.3Ti0.7)O3 (PZT) thin films using chemical solution deposition on Pt∕TiOx electrode stacks were investigated using complementary analytical techniques including atomic force microscopy (AFM), x-ray photoelectron spectroscopy, high-resolution transmission electron microscopy, and grazing incidence x-ray reflectivity of synchrotron radiation. The surface and interface structures of the Pt∕TiOx electrode stacks with different thermal treatments, and the PZT∕Pt∕TiOx sample were examined. The propagation of Pt hillocks on the bare Pt∕TiOx electrode stacks upon the annealing was observed. AFM observations also revealed that the upper surface of the Pt bottom electrode under PZT thin film became rougher than that of the bare Pt electrode with the same thermal history. Global structural information including the density, surface or interface root-mean-square roughness, and thickness of each constituent layer in the samples were determined using...

Non-Linear Imprint Behavior of PZT Thin Films

Ferroelectric thin film capacitors are promising candidates for non-volatile ferroelectric Random Access Memories (FeRAMs) as they exhibit a switchable polarization. There are three important failure-mechanisms influencing the performance of these capacitors and disturbing the long-term stability and reliability under operation conditions fatigue, retention, and imprint. The imprint effect of lead zirconate titanate (PZT) thin films was investigated in this paper. Establishing and maintaining a polarization state leads to a shift of the hysteresis loop on the voltage axis and also to a loss of polarization. The voltage shift as well as the loss of polarization can cause a failure of the ferroelectric memory cell (read and write failure). The experimental results obtained on PZT films are discussed in view of the predictions of imprint models proposed in the literature.

Effects of ferroelectric fatigue on the piezoelectric properties (d33) of tetragonal lead zirconate titanate thin films

The fatigue of the electromechanical properties of tetragonal PbZrx, Ti1−xO3 thin films is investigated. The decrease of electromechanical small-signal response is compared to the fatigue of the electric properties and examined in detail. Property fatigue is attributed mainly to switching-failure of the unit cells.

Probing fatigue in ferroelectric thin films with subnanometer depth resolution

The authors report the study of polarization fatigue in Pb(Zr,Ti)O3 (PZT) ferroelectric thin films using in situ high-resolution grazing incidence x-ray specular reflectivity of synchrotron radiation. The results demonstrate that there is no formation of a region of different electron densities in the film growth direction with subnanometer depth resolution during fatigue. The upper bounds on the theoretically predicted interfacial accumulation of oxygen vacancies at the interfaces between PZT and Pt electrodes are determined by the comparison of experimental results and theoretical simulations.

Monte Carlo simulations of imprint behavior in ferroelectrics

In this letter, Monte Carlo simulation methods were used to investigate the influence of the defect orientation and concentration on the hysteresis loop in ferroelectric thin films. The hysteresis loops were calculated by an existing Monte Carlo model. For a certain type of defect orientation, the simulations revealed an asymmetric hysteresis loop behavior, similar to hysteresis curves recorded by imprint measurements. Though these results may not directly offer a new explanation for the imprint mechanism in ferroelectric thin films, they still provide insight information about the often observed phenomenon of imprinted hysteresis loops of as-prepared thin-film capacitors.

Influence of Asymmetric Oxide Electrode Structures on the Interface Capacity and the Failure Mechanisms in PZT Thin Films

The influence of symmetric and asymmetric electrodes including SRO and IrO2 within the electrode structure on the imprint and fatigue behavior in sputtered and CSD derived PbZrxTi1 − xO2 (PZT) thin films is investigated. It is found that SRO buffer layers are needed within top and bottom electrode to improve the fatigue behavior. However, for improvement of the imprint behavior, only one SRO buffer layer within either top or bottom electrode is sufficient, whereas IrO2 reveals no improvement of the imprint behavior. Furthermore the direction dependence of the imprint behavior is examined. To examine the influence of the interface capacity on the fatigue mechanism in more detail, the capacity is measured for different PZT film thicknesses during fatigue treatment. It is shown that there is hardly any difference in interface capacity during fatigue.

Density inhomogeneity in ferroelectric thin films

Structural investigations of Pb(Zr,Ti)O3 (PZT) ferroelectric thin films derived by chemical solution deposition on Pt∕TiOx electrode stacks were performed using grazing incidence x-ray specular reflectivity of synchrotron radiation and transmission electron microscopy. A density inhomogeneity, i.e., a sublayer structure, in the PZT thin films was observed; the upper PZT sublayer had a lower density and the lower sublayer had a higher density. The influence of the density inhomogeneity, as a possible extrinsic contribution to size effects in ferroelectric thin films, was discussed.

Effects of Thermal Annealing on Lead Zirconate Titanate Thin Film Capacitors with Platinum Electrodes

The effects of thermal annealing on Pb(Zi 0.3 Ti 0.7 )O 3 ferroelectric thin film capacitors derived by chemical solution deposition with Pt electrodes were investigated. Sample-averaged structural information of the capacitors with different thermal histories was obtained from fitting the X-ray specular reflectivity using a homogeneous stratified multilayered structure model of Pt/PZT/Pt/TiO x /SiO 2 . Electrical measurements showed a remarkable remanent polarization reduction of up to 8.8% and an increase of the dielectric constant at low biases with increasing annealing time from 5 to 10 min. However, no evident change to the fatigue endurance was found except for the reduced initial remanent polarization. The effects of the thermal annealing on the electrical properties of the ferroelectric capacitors are discussed in correlation with the structural changes in the ferroelectric thin films.

NEW RESULTS ON FATIGUE AND IMPRINT EFFECT

ABSTRACT A ferroelectric thin film capacitor is the key element in non-volatile ferroelectric Random Access Memories (FeRAMs). There are three important failure-mechanisms influencing the performance of these capacitors: fatigue, retention, and imprint. In this contribution the fatigue and imprint effect are investigated in detail. The material under investigation is lead zirconate titanate (PZT) which was deposited as ferroelectric thin film. Several fatigue measurements were performed with different cycling frequencies, different applied voltage amplitudes, and at varying hysteresis frequencies to investigate dependencies of these parameters on the fatigue effect. An equation is introduced in order to fit the measured behavior of polarization over time and to assign physical parameters to the numerical fitting values. This equation is used to fit the single fatigue curves and the obtained parameters are discussed. Finally, various imprint measurements were performed. Here, the imprint effect was investigated with respect to varying doping concentrations of Nd and Fe and to the initial hysteresis state.