U. Boettger

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.

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.

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.

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.

An extensive study of the influence of dopants on the ferroelectric properties of HfO2

The ferroelectric properties of hafnium oxide based thin films prepared by chemical solution deposition (CSD) are investigated. In this extensive study, a wealth of strongly different dopants (size and valence) and dopant concentrations is used to induce ferroelectricity in 42 nm thin films. Using the same precursors and preparation conditions for all dopants a good comparability is given. In particular, the dopant size appears to have a crucial impact on the resulting ferroelectric properties. For smaller dopants only a small ferroelectric response is observed whereas for larger dopants the remanent polarization is increased significantly. The crystal phase for varying dopant concentrations and dopant sizes is investigated by grazing incidence X-ray diffractions (GI-XRD). A dominating cubic phase is found for doping concentrations showing the highest remanent polarization. Similar to first CSD studies on Y:HfO2, this is reflected in a prominent wake-up behavior, which is attributed to a phase transition from cubic to orthorhombic during field cycling.

Interface-related thickness dependence of the tunability in BaSrTiO3 thin films

The thickness dependence of the tunability of Ba0.7Sr0.3TiO3 thin films is investigated. The capacitance–voltage curves, revealing the tunability of the films with thickness from 30 to 370nm, show a strong thickness dependence. This is attributed to a bias-independent interface capacity. The interface suppresses the permittivity of the film with increasing influence for decreasing film thickness, whereas the tunability of the bulk of the film remains constant. Calculations are performed from a thermodynamic model based on the Landau–Ginzburg–Devonshire theory leading to the assumption of the bias-independent interface capacity. The bias dependence of the bulk of the films derived from measurement data are in very good agreement with the theoretically derived values.

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.

Ferroelectric and piezoelectric properties of Hf1-xZrxO2 and pure ZrO2 films

Ferroelectric and piezoelectric properties of Hf1-xZrxO2 (HZO) and pure ZrO2 films with a layer thickness of up to 390 nm prepared by chemical solution deposition (CSD) are investigated. The piezoelectric properties are measured using a double-beam laser interferometer (DBLI) and piezoresponse force microscopy. It is shown that for 100 nm thick films, the maximum remanent polarization is found for pure ZrO2 and reduces for the increasing hafnium content. A stable remanent polarization of 8 μC/cm2 is observed for ZrO2 film thicknesses between 195 and 390 nm. A piezoelectric coefficient of 10 pm/V is extracted from unipolar DBLI measurements. The observed thickness limitation for atomic layer deposition deposited HZO based ferroelectrics can be overcome by the CSD deposition technique presented in this work. Thick ZrO2 films are promising candidates for energy related applications such as pyroelectric and piezoelectric energy harvesting and electrocaloric cooling as well as for microelectromechanical systems.

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.